Internet Info 1994 March/Internet Info CD-ROM (Walnut Creek) (March 1994).iso/inet/ien/ien-136

Text File | 1988-12-01 | 87KB | 3,659 lines

                      Memory Management Extensions
                   to the SRI Micro Operating System
                         for PDP-11/23/34/35/40










                                IEN  136

                              1st MAY 1980









                              S.R. Wiseman
                              B.H. Davies























                  ROYAL SIGNALS & RADAR ESTABLISHMENT
           North Site, Leigh Sinton Rd., Malvern, Worcs., UK.

                                   1



    CONTENTS
    ________

0.    Introduction

        PART 1 - SYSTEM DESIGN

1.    Requirement

1.2    Overview of solution

1.3    Use of Memory Management Facility


        PART 2 - TARGET MACHINE ASPECTS

2.1    Alterations to MOS
    2.1.1    The Process Control Table
    2.1.2    The Scheduler
    2.1.3    The $CREAP Macro
    2.1.4    Input and Output Routines
    2.1.5    Memory Sizing
    2.1.6    Protection Routines
    2.1.7    System Configuration

2.2    The EMMOS Debugger - MUD
    2.2.1    A Summary of MUD Commands
    2.2.2    MUD Command Syntax
    2.2.3    The Relocation Mechanism
    2.2.4    Addressing Modes
    2.2.5    Command Specifications
    2.2.6    Using MUD

2.3    Linker and Loader Functions
    2.3.1    The Linker
    2.3.2    The Loader



















                                   2

         PART 3 - HOST MACHINE ASPECTS

3.1    Linking Using the RSX-11M Task Builder
    3.1.1    Use of the Task Builder's Task Image File
    3.1.2    The Format of the Task Image File
    3.1.3    The Overlay Description Language
    3.1.4    The EMMOS ODL File
    3.1.5    Installation of the Debugger
    3.1.6    Creating the Task Image File

3.2    The EMMOS RSX Task Image Loader
    3.2.1    Inputs and Outputs
    3.2.2    The Console Load Map
    3.2.3    The Configuration Table
    3.2.4    The Overlay Description Table
    3.2.5    The Process Description Table
    3.2.6    The Load Map
    3.2.7    The Physical Memory Allocation Algorithm
    3.2.8    The Logical Operation of the Loader
    3.2.9    Error Messages
    3.2.10    Tracing
    3.2.11    Changing the System

3.3    Process Creation





Appendix A:    Pitfalls and How to Avoid Them



























                                   3



0.    Introduction
____________________

     This  document describes the requirement, design and implementation
of  memory  management  extensions  to  the Stanford Research Insitute's
Micro  Operating  System (MOS) for PDP-11 minicomputers. The document is
divided  into three parts. Part 1 covers the rationale behind the way in
which  the memory management facility of the PDP-11 is used. The results
of  the  work  fall  naturally  into Parts 2 and 3. Part 2 describes the
modifications  to  MOS  itself and the debugger which are independent of
the  operating  system of the host machine on which the target system is
generated.  Part  3  describes the Linking and Loading modules which are
dependent on the operating system of the host machine. The host machines
used  at  RSRE are PDP -11/34 and 40s running RSX-11M v3.2. Also we have
provision  in  our  standard  MOS  for writing processes in a high level
language, CORAL 66, which requires the use of an auxiliary stack pointed
to by R0. This facility may be excluded from the extended MOS by the use
of a configuration switch.





































                                   4













                PART ONE
                ________

                 SYSTEM DESIGN









































                                   5

1.1    Requirement
___________________

     For  a  number of planned real-time software projects the 28k words
of  code  and  data  space  accessible  without memory management in the
PDP-11  is  inadequate.  Therefore the question arises is it possible to
use  the  memory  management  registers to provide more code and/or data
space  up  to  124k words, with minimal overheads in the form of context
switching and with no modifications to presently written processes?


1.2    Overview of Solution
____________________________


     The  reason  for  the  provision  of  a two state memory management
system  in  the  PDP-11s is not only to allow access to memory above 28k
words  but  also,  by  the  use  of  kernel  and  user modes, to provide
protection between processes and the core resident part of the operating
system. This usually means that the I/O page is non-resident when a user
process  is  running  and  that I/O is handled in kernel mode. Thus in a
real-time  system  there  may  be  unacceptable  overheads  due  to  the
considerable  amount  of  context  switching involved in this protection
mechanism.  However  if one is willing to dispense with interprocess and
operating system protection a simple and elegant solution which produces
no detectable overheads is possible.

     The  basic  concept  of  the  solution  is  that while a process is
running  there  is  no need to have in scope any other process so that a
snapshot  of  the  32k  virtual memory at any one time will look like an
ordinary   MOS  system  configured  for  one  process.  The  only  extra
executable  code involved is in the scheduler for paging out the process
that  has  just finished running and paging in the process that is about
to  be  run.  The I/O page, operating system, handlers and common buffer
area  are  always resident otherwise access to these entities would have
to  be  mediated  by the operating system so that they could be paged in
with  consequent  increase  in overheads. A typical MOS process runs for
something like 1 millisecond and the additional paging overhead is 4 MOV
instructions  taking  about  20  micro  seconds,  a 2% overhead. A major
change  in  the  MOS  layout  occurs  in  the positioning of the stacks.
Because  we  want the permanently resident part of MOS to be as small as
possible  and  never  greater than 8k words the stacks have been removed
from  the  Process  Control  Tables  and are paged in and out with their
processes.


1.3    Use of the Memory Mangement Facility
____________________________________________

     At  any  one  time  only the running process will reside in virtual
memory,  the  others  will  be  suspended  and  so  have  no  need to be
accessible.  Permanently  resident  in virtual memory will be the vector
area,  global  buffer area, operating system including all handlers, and
the  I/O  page.  Because the maximum size of a page is 4K words and it's
position  is  fixed  in virtual memory, the allocation of these pages is
somewhat  restricted.  However  a trade-off between global buffer space,

                                   6

operating  system size and process space is possible to some extent, and
the following configurable options are catered for:-


Options where code and stack space of any process does not exceed 4k words:-


    Option 1
    ________

    20k words of buffer space
    <4k of operating system and handlers


            |-----------------------| 32k
        7    |    I/O        |
            |-----------------------| 28k
        6    |Running process & stack|
            |-----------------------| 24k
        5    | Operating System EMMOS|
            |-----------------------| 20k
        4    | Global Buffers        |
            |-                     -| 16k
        3    |            |
            |-                     -| 12k
        2    |            |
            |-                     -|  8k
        1    |            |
            |-                     -|  4k
        0    |            |
          page    |-----------------------|  0k

    Option 2
    ________

    16k words of buffers
    <8k operating system and handlers
            |-----------------------| 32k
        7    |    I/O        |
            |-----------------------| 28k
        6    |Running process & stack|
            |-----------------------| 24k
        5    | Operating System EMMOS|
            |-                     -| 20k
        4    |            |
            |-----------------------| 16k
        3    | Global buffers        |
            |-                     -| 12k
        2    |            |
            |-                     -|  8k
        1    |            |
            |-                     -|  4k
        0    |            |
          page    |-----------------------|  0k



                                   7


Options where code and stack space of any process does not exceed 8K words:-

    Option 3
    ________

    16k words of buffer space
    < 4k words of operating system and handlers

            |-----------------------| 32k
        7    |    I/O        |
            |-----------------------| 28k
        6    |Running process & stack|
            |-                     -| 24k
        5    |            |
            |-----------------------| 20k
        4    | Operating System EMMOS|
            |-----------------------| 16k
        3    |  Global Buffers       |
            |-                     -| 12k
        2    |            |
            |-                     -|  8k
        1    |            |
            |-                     -|  4k
        0    |            |
          page    |-----------------------|  0k

    Option 4
    ________

    12k words of buffers
    <8k words for operating system and handlers

            |-----------------------| 32k
        7    |    I/O        |
            |-----------------------| 28k
        6    |Running process & stack|
            |-                     -| 24k
        5    |            |
            |-----------------------| 20k
        4    | Operating System EMMOS|
            |-                     -| 16k
        3    |            |
            |-----------------------| 12k
        2    | Global Buffers        |
            |-                     -|  8k
        1    |            |
            |-                     -|  4k
        0    |            |
          page    |-----------------------|  0k







                                   8

     Only  kernel  mode  is  used  by  EMMOS, even user processes run in
kernel mode because no attempt has been made to produce a secure system.
However  with  options  3  and  4  the process space and stack space are
allocated  separate  pages  wherever  possible.  Thus  the debugger or a
procedure  call  may be used to write-protect the code space if desired.
Holes  appear in virtual memory from the end of the code or stack to the
end of the 4k boundary ensuring that a system stack cannot underflow and
the  running  process  cannot  access  another  process' code or stack .
However,  a  process  can  damage  another  process  indirectly  by, for
example, supplying a pointer to data on its stack to another process.

     Because  the  debugger  is  bigger  than 4k, options 1 and 2 do not
allow  use  of  the  debugger.  This  is not a serious drawback as it is
envisaged  that  debugging could take place with options 3 or 4 and that
options  1  or  2  could then be used to give another 4k words of buffer
space.

     Using  EMMOS  requires a stricter discipline when writing processes
than  was  required  with  MOS.  Strictly speaking the code of a process
should  not  contain  any private data space. If a process wants private
data  space  it  should use its stack. Any data space shich is shared by
two  or  more  processes should be obtained , via MOSMEM calls, from the
global  buffer  area. Exceptionally a single incarnation of an assembler
process  could  reserve  space  after  its code for local tables but any
attempt  by another process to access this area will result in disaster!
All high level language processes should be written as closed procedures
with private data area on their stacks.

     Shared  Library  procedures  must either be replicated so that they
are paged in with each process that requires to use them or they must be
linked  with  the  EMMOS  operating  system so that they are permanently
resident

























                                   9

    A typical example of real and virtual memory allocations for
an EMMOS system are shown  below:-
                 128k    |-----------------------|
                    | I/O addresses         |
                 124k    |-----------------------|

                    |            |

                    |            |
                    |            |
                    |-----------------------|
                    | stack (proc3)         |
                    |-----------------------|
                     | stack (proc2)         |
    |-----------------------| 32k    |_______________________|
   7    |    I/O        |    | Codebody2 (proc 2&3)  |
    |-----------------------| 28k    |_______________________|
   6    |Running process & stack|    | stack (proc1)         |
    |-                     -| 24k    |_______________________|
   5    |            |    | Codebody1 (proc1)     |
    |-----------------------| 20k    |-----------------------|
   4    | Operating System EMMOS|    | Operating System EMMOS|
    |-                     -| 16k    |-               -|
   3    |            |    |            |
    |-----------------------| 12k    |-----------------------|
   2    | Global Buffers        |    | GLOBAL buffers        |
    |-                     -|  8k    |-                     -|
   1    |            |    |            |
    |-                     -|  4k    |-                     -|
   0    |            |    |            |
    |-----------------------|  0k    |-----------------------|
Page        Virtual Memory            Physical Memory




     The  linker/loader  system  used  with  EMMOS  should be reasonably
intelligent!  It  is  it's  job to reserve physical memory in accordance
with  the  stack  size requested for each process. In particular it must
evaluate  the  total memory requirements of a process that has more than
one incarnation and determine whether or not some or all of the code has
to  be replicated.This may arise when the stacks of all the incarnations
of  a  process  plus  the  code will not fit into the paging window. The
linker/loader is also responsible for outputting error messages when the
process  and  stack  requirements cannot be satisfied by that particular
configuration of EMMOS.











                                   10










                PART TWO
                ________

                 TARGET MACHINE ASPECTS












































                                   11

    2.1     Alterations to MOS
    ___________________________

     Perhaps  the most vital alteration is that made to the scheduler. A
context  swap  now  involves  saving  the  suspended process' window and
restoring  the  new  process'  window. This however, only involves about
four move instructions extra, for a typical configuration ( 8K processes
).

     The  major  alteration is, however, to the Process Control Tables (
PCTs ) and consequently to the PCT initializer. In particular the stacks
for  a process no longer reside here, hence a process' PCT is relatively
small.  Extra  fields  in  the  PCTs  give  the memory management window
necessary  to 'page-in' the process and the ends of the system and CORAL
stacks.  The  PCT  initializer  now  has  to  cope with setting up these
fields,  which  it  does  by  consulting  the  load map, supplied by the
linker/loader.

     Additions have also been made to the $CREAP macro. The sizes of the
system and CORAL stacks are given to the $CREAP which in turn passes the
information to the linker/loader ( depending on the method used ).

     It  has  also  been  necessary  to  offer  modified synchronous I/O
routines  'SOUT' and 'SIN'. The original versions can be used if the I/O
is  performed  on  buffers  from  the  permanently resident buffer pool,
however  they  will  not  work if the I/O buffer is taken from the local
code or data space. This is because the local area may be paged-out when
the  interrupt  routine  tries  to get the next character, causing it to
pick  up  rubbish  or trap out. The new versions will copy the local I/O
buffer  into  a  global  buffer before initiating the I/O, thus ensuring
that  it  is  always paged-in. This is likely to be the main overhead of
the system.

     Finally,  the  memory  sizing  performed  in  MOSMEM  is  no longer
required.  This  is  because  the  position of the global buffer pool is
fixed by the page allocation scheme.





















                                   12

    2.1.1 The Process Control Table

The following fields have been added to the PCT:

    pct_mmr : ARRAY [ first_page..last_page ] OF window_type;
    pct_r0e, pct_r6e : virtual_byte_addresses;
where
    TYPE window_type = RECORD par, pdr : INTEGER  END;
    TYPE virtual_byte_addresses = 0..#177777;

The following field has been removed from the PCT:

    pct_stk : ARRAY [ 1..stk_len ] OF BYTES

     pct_mmr is an array containing the window required to 'page-in' the
process.  The  bounds  are  not 0..7 because it is known that most pages
remain  constant,  only  pages first_page to last_page are changed since
this  is  the  amount  of  virtual  memory  allocated to a process. Each
element  of  the  array is four bytes long and contains the page address
register  (  par  ) and page descriptor register ( pdr ) values for that
page.

     pct_r0e  and pct_r6e contain the virtual byte addresses of the ends
(  smallest  numbered  address  )  of the R0 ( CORAL ) and R6 ( system )
stacks. These values are used to detect stack overflow.

     Since  the stacks no longer reside in the PCT the field pct_stk has
been removed.

     The  new  fields  in  the PCT are set up directly from the Load Map
produced by the linker/loader.


























                                   13

    2.1.2 The Scheduler

     A  context  swap  now  involves  saving  and  restoring  the memory
mapping,  as  well as the general register values. We can actually avoid
saving  the  mapping  when  a  process  is suspended because normally it
remains  constant.  The protection on a page is the only thing likely to
change and if this is done using the supplied routines then we can still
avoid doing the saving.

     When  the  process  is  made  runnable  it  is  first 'paged-in' by
restoring it's memory mapping. To ensure that the stack pointer and it's
stack are restored together, interrupts are disabled earlier than in the
old version. If an interrupt is allowed to occur between 'paging-in' the
stack  and  restoring  the  stack  pointer  then disaster will obviously
follow.

     Stack  overflow, on both the R0 ( CORAL ) and R6 ( system ) stacks,
is checked for when the process is suspended. The ends of the stacks are
marked  with  overflow detect words, the address of which is held in the
PCT.  It is a simple matter to check if these words are still intact. If
the  system  has  no CORAL processes or library routines then the checks
made  on  the  R0  stack can be omitted by setting a switch in the EMMOS
configuration file.

    2.1.3 The $CREAP Macro

     This  macro  initializes  parts of the PCT, the rest is done at run
time.  Two extra parameters are now required, which specify the size, in
words,  of the R0 ( CORAL ) and R6 ( system ) stacks. To avoid confusion
the parameters should be called by name, the default size is zero words:

eg:    $CREAP    G802,<UX25  >,,DV.IMP+0,DV.IMP+1,R0SIZE=200,R6SIZE=20
    $CREAP    G802,<UX25  >,,DV.IMP+2,DV.IMP+3,R6SIZE=30,R0SIZE=350

     The  macro  converts the size into memory blocks ( 40 octal words )
and  places  the information, along with the process id and name, in the
.PSECT  LDRCON.  This  information is used by the linker/loader in a way
that depends on the methods being used. Note that different incarnations
of  the  same  process  can have different sizes of stacks. In the above
example ( all numbers are octal ) the first process would have

    ceiling( 200/40 ) =  4 memory blocks = 200 words for R0
    ceiling(  20/40 ) =  1 memory block  =  40 words for R6
and the second process would have
    ceiling( 350/40 ) = 10 memory blocks = 400 words for R0
    ceiling(  30/40 ) =  1 memory block  =  40 words for R6

     Note:  "ceiling(  r )" is a function that rounds up real numbers to
the next largest integer.








                                   14

    2.1.4 Input and Output Routines

     The synchronous I/O routines, SIN and SOUT, have to be modified for
use  with  EMMOS.  The  user  interface  is unchanged, but the method of
operation  is slightly different. SOUT now grabs a global buffer, copies
the  string  into it from the user's buffer and then performs the SIO on
the  global buffer, to output the characters. SIN grabs a global buffer,
calls SIO to fill it and then copies the string into the user's buffer.

     This  is  necessary  because  if  the string to be output is in the
process' local area, it will be 'paged-out' with the process. SIO uses a
pointer to the string ( a virtual address ) to get the characters, so if
it  does this when the initiating process is 'paged-out', it will either
pick up rubbish or cause a memory management error. Similarly for SIN.

     The  copying would not be required for a string already in a global
buffer,  because  the  buffer  will  always be 'paged-in', regardless of
which process is running. In this case the MOS versions could be renamed
and used to avoid a loss of efficiency.

     The  same  problems  apply  to  the  asynchronous I/O routine, SIO.
However,  any  copying  from local data space to a global buffer that is
needed,  is  left  up to the user. Checks have been included in the code
that  ensure  both  the  IORB and the data area are in the global buffer
pool  or  the  resident  EMMOS  code.  These checks can be omitted for a
program  known  to  work  by setting a switch in the EMMOS configuration
file.

    2.1.5 Memory Sizing

     This  is  now not carried out. The limits of the global buffer area
are  assumed  to  be  the  ends of the pages allocated to buffers ( less
space  for the vectors if this includes page 0 ). It is assumed that the
hardware has been configured in a sensible manner.























                                   15

    2.1.6 Protection Routines

     Two  routines are supplied, one to inspect the access control field
(  ACF  )  of  a  page,  the  other  to change the ACF. Under the memory
management system each page can be either

    1.    non-resident    -all attempts at access will cause
                 a trap
    2.    read-only    -attempts to write into a location
                 in the page will cause a trap
    3.    (unused)    -same as non-resident
    4.    read/write    -neither reads nor writes are trapped

     The  two routines will only work for one of the process pages. They
both  return an integer result. This equals 'true' if the page specified
is  a process page, else it equals 'false' and the ACF is not changed or
inspected.  The  pre-defined constants, 'non resident', 'read only', 'un
used' and 'read write', should be used to specify and compare the ACFs.

INTEGER PROCEDURE change page protection ( VALUE INTEGER page, access );

     If  the  page specified is a process page then the ACF of that page
is  changed  to  the  given  access.  The  copy of the memory management
registers held in the PCT table for this process, is also updated.

INTEGER PROCEDURE current protection ( VALUE INTEGER page;
                       LOCATION INTEGER pct, pdr );

     If the page specified is a process page then pct and pdr are set to
the  values of the ACF field of the memory management registers for that
page,  taken  from  the  PCT  window  area  and  active  page  registers
respectively.  The  two  different  versions are supplied in case one of
them has been corrupted. Both should always be the same.


    CORAL Constant Values

    true        1
    false        0
    non resident    0
    read only    2
    un used        4
    read write    6














                                   16

cont.

     The  assembler  versions of these routines are completly re-entrant
and run to completion, so it is possible to have one copy of the code in
the  resident  part of the system which is shared between all processes.
The  routine  to  change  the  page  protection is called $CHAPP and the
routine  to  inspect  the current protection is called $CURPP, they have
the same effect as the CORAL versions but are called as follows:



Change Page Protection :
    called by JSR    PC,$CHAPP
    with    R2 = number of the page to be changed
        R3 = the new access of that page
    returns with
        R1 = TRUE or FALSE
        R0 and R2...R6 unchanged

Inspect Current Page Protection :
    called by JSR    PC,$CURPP
    with    R2 = number of page to be examined
    returns with
        R1 = TRUE or FALSE
        R3 = value from PCT
        R4 = value from active PDR
        R0, R5 and R6 unchanged


Assembler Constant Values

    TRUE    1
    FALSE    0
    NONRES    0
    READON     2
    UNUSED    4
    RWRITE    6




















                                   17

    2.1.7 System Configuration

     The  page  allocation  scheme, operating system options and various
buffer addresses, are specified in the configuration file, EMMSYS.SML .

    2.1.7i The Page Allocation Scheme

     The virtual memory is divided up into 8 pages of 4K words, numbered
0..7.  Page  7 is always allocated for the I/O page, the remaining seven
are  split  between  the  global  buffers,  the operating system and the
running  process  (  including  it's  stacks  ).  The order is critical,
buffers  should  be  at  the low numbered end, then the system, then the
process and finally the I/O page. The exact split is given by specifying
the first and last page used by each part.

eg    buffers    0 - 3    emmos    4 - 5    process    6 - 6
or    buffers 0 - 2    emmos    3 - 4    process    5 - 6

     When  decreasing the space allocated to the global buffers, be sure
that  the  workspace  buffers  ( see 2.1.7iii ) are still located before
EMMOS,  otherwise  EMMOS may be loaded on top of one of them before it's
finished   with.   Note:  different  linking/loading  schemes  may  lend
themselves  to  a  different ordering of buffers, system and process, in
which  case  minor  alterations to the PCT initializer may be necessary.
The I/O page must always be page 7.
































                                   18

    2.1.7ii Operating System Options


EMMOS:        EMMOS/MOS selection switch
    = 0    for EMMOS
    = 1    for MOS ( SAVMAP  and SET250 have no meaning )

SAVMAP:        save mapping when process suspended switch
    = 0    when a process is suspended the window is saved as part
        of the process' volatile environment. This allows a
        process to change the window dynamically ( eg the
        protection ). Note, only that part of the window for the
        pages allocated to processes is saved and restored, so
        changing other pages will affect all processes.
    = 1    the window is not saved each time the process is
        suspended. The original value is reloaded when the
        process is run again, erasing any changes made.

    It is recommended that SAVMAP = 1 and the supplied protection
    routines are used to change the page allocation, with the other
    fields left well alone.

SET250:        memory management error trap initialization on/off
    = 0    when the system is initialized the trap is set to a local
        handler.
    = 1    the trap is left untouched, this option should only be
        used when the debugger is used ( MUD will set up it's
        own handler ).

COPYIO:        sin/sout version selection switch
    = 0 ( OR EMMOS = 0 )
        use the versions of sin and sout that copy to/from a
        global buffer before initiating the I/O.
    = 1 ( AND EMMOS = 1 )
        the MOS versions will be used

NOCORL:    CORAL process indicator
    = 0    no processes are written in CORAL, or use CORAL libraries,
        so the R0 stack is not checked for overflow each time a
        process is suspended. The stack is still set up as
        usual. The R6 stack is still checked.
    = 1    There is a process using CORAL, so the R0 stack will
        be used. Both stacks are checked for overflow each
        time the process is suspended.

CHKSIO:    SIO buffer address checking on/off
    =0    Each time SIO is called the IORB and the data addresses
        are checked to ensure that they are in the global buffer
        pool or the resident EMMOS code. An attempt to use a
        local iorb or data area will cause a BUGHLT.
    =1    No checks are performed. The use of a local IORB or data
        area is likely to cause the system to crash.





                                   19

    2.1.7iii Workspace Buffer Addresses

     The  linker/loader and system initializer must agree on the postion
of  the  load  map.  It's  address  is  given  by  MAPADD  in the system
configuration  file. The linker/loader is given this information in some
implementation dependant way.

     Note  that  in  the RSX linker/loader system, the area specified by
MAPADD  is  also  used  by the loader for other workspace buffers during
load  time,  because of this MAPADD must specify an area of store, lying
between  the  end of the loader and the beginning of where EMMOS will be
loaded, of about

    25 * max_number_of_processes ( decimal ) words long

     The   loader   ends   around  032000,  without  removing  redundant
libraries, so even for a max number of 30 processes ( 25 * 30 decimal is
about  1400  octal  ), giving MAPADD = 32000 would mean the tables would
end  way  below  040000. This allows us to load a system with only 8K of
buffers.





































                                   20

    2.2    The EMMOS Debuggger  - MUD
    __________________________________

     MUD (Marvellous Universal Debugger) is split into two parts, RESMUD
and  MUD  proper.  RESMUD is a small program written in MACRO-11 that is
permanently resident in virtual memory. It acts as the interface between
the  operating  system and the debugger, MUD, which is only brought into
virtual memory when it is needed.

     MUD  is  loaded by the loader in the same way as any other overlay.
Normally  ,  however, overlays that are not $CREAPed do not have entries
in  the  load  map,  but  the  loader  creates  a  special entry for the
debugger.  RESMUD uses this entry to find out where MUD has been loaded,
and  transfers  control  to it. For this reason RESMUD must be the first
module loaded, even before MOSPM, if the debugger is to be used.

     If  RESMUD  is not included in the system, then MUD will be ignored
if  it  is  itself included, and the debugger cannot be used. EMMOS will
ignore MUD, unless a process is foolishly creaped with the name MUDSYS!

     The  debugger is able to reference the whole of the 18 bit physical
store   without  the  user  knowingly  changing  the  memory  management
registers.


































                                   21

    2.2.1 A Summary of MUD Commands

     The name of a command is one or two letters long, in upper or lower
case, as follows:

Print Commands:

    pb    print breakpoint locations
                ( see breakpoint commands )
    pr    print the value of a relocation register
                ( see relocation commands )

Spy Commands:

    s    spy on store
    sr    spy on the registers
    sw    spy on the memory management window

Breakpoint Commands:

    b    set a breakpoint
    k    kill a breakpoint
    pb    print breakpoint locations
                ( see print commands )

Enter User Program Commands:

    g    go to a specific address
    gs    go to a specific address single stepping
    c    continue from a breakpoint
    cs    continue from a breakpoint single stepping

Relocation Commands:

    r    set a relocation register
    pr    print the value of a relocation register
                ( see print commands )

Exit Command

    x    leaves MUD and halts

Special Commands

    i    change I/O channel
    e    evaluate an arithmetic expression
    f    find a memory location containing a given value










                                   22

    2.2.2 MUD Command Syntax

     The  syntax  is  fairly  simple  and  is  quite free with spaces. A
command consists of the command name, one or two letters( upper or lower
case ), optionally followed by one or two operands.

    <name>
    <name> <op1>
    <name> <op1> <op2>

     The two operands and the name may be seperated by commas or spaces.
The  operands specify 18 bit addresses or numbers in octal. Their syntax
is:

    <operand> ::= <repeat symbol>
          or= <number> <relocation> <mode>

    <repeat symbol>    ::=  *

    <number>  ::= <18 bit octal number>
          or= null

    <relocation> ::=  .  <relocation register>
             or= null

    <relocation register> ::= <octal digit>
                          or= null

    <mode> ::= V
           or= P
           or= null

     If  the  octal  digit is ommitted, it is assumed to be zero. If the
relocation  dot  is  not  present,  then the operand is assumed to be an
absolute  physical  address.  If  the  octal  digit  is missed out, then
relocation  register  one  is  assumed. Hence the following examples all
mean the same:

    s100.,150.
    s100.1  150.
    s 100.   ,   150.1
    s 100.1   150.1















                                   23

    2.2.3 The Relocation Mechanism

     There  are  seven relocation registers supplied ( r1..7 ) which can
be  used  to  modify  an address in a command. Values are put into these
registers  by  the 'r' command (qv). To specify that an address is to be
relocated  it is followed by a dot and perhaps a digit. The digit is the
name  of  the relocation register to be used and is one by default. When
relocation  is  specified,  the  contents of the relocation register are
added to the address giving the actual address used. For example:

    r2,100    - sets relocation register 2 to 100
    s20.2    - spys on location 20+100=120
    r350    - sets relocation register 1 to 350
    s30.    - spys on location 30+350=400

     Many  commands  allow  an  asterisk  to be used as an operand, this
usually means 'all' in some way, for example:

    k100    - kill the breakpoint at location 100
    k *    - kill all breakpoints





































                                   24

    2.2.4 Addressing Modes

     An  operand  can  refer  to  a  virtual  location  by following the
relocation  indicator  by  a  'V' ( the default mode is 'P', so normally
addresses  are  physical  ). The corresponding physcial location will be
computed using the user's window. Page lengths are ignored.

     For  example,  if  the  base  address of page 5 ( virtual addresses
120000-137777  )  is  120100  and relocation register 2 contains 120000,
then the following are equivalent:

    s120110   - physical address by default
    s120110p  - physical address specified
    s120010v  - pa = 010 + base(5) = 120110
    s110.2    - pa = 110 + 120000  = 120110
    s110.2p   -   . . . . physical specified
    s10.2v    - va = 10 + 120000 = 120010
              - pa = 010 + base(5) = 120110







































                                   25

    2.2.5 Command Specification


pb    print breakpoint locations

    pb    prints out a list of all the breakpoints
        that are set, showing their location as an
        absolute address and as an offset +
        relocation.

pr    print relocation registers

    pr    print out the value of relocation register 1
    pr 'd'    print out the value of relocation register 'd'
    pr *    print out the value of all the relocation
        registers.

r    set a relocation register

    r 'n'    set relocation register 1 to 'n'
    r 'd' 'n'    set relocation register 'd' to 'n'
    r * 'n'    set all relocation registers to 'n'
    r *    note that 'n' is zero by default so that
        r* will set all relocation registers to
        zero. Initially all are zero anyway.

x    exit

    x    leaves MUD and halts. This instruction would only
        be used when running under a monitor operating system.



























                                   26

cont.


s    spy on store

    s 'l'    spy on the store starting at location 'l'.
        The address is displayed followed by the
        contents. The contents can be changed by
        typing a number followed by <return>, in
        which case the new value will be displayed.
        To advance to the next location just type
        space, to retreat to the previous location
        type tab. To finish the spy type <return>.
        Note that typing <return> with no number
        does not change the value in store.

        eg
        mud> r100
        mud> s20.
        000120> 123456  23
        000120> 000023   000122> 077177
        mud>

        this leaves location 122 unchanged, but puts
        23 into location 120.

    s 'l1' 'l2'
        spys on locations 'l1' to 'l2', but does
        not allow you to change any values.
        Effectively this is just a display or dump
        command. 'l1' should be less than or equal
        to 'l2' or nothing will happen. To stop
        the printing hit <escape>. This is handy if
        you specify an enormous amount of printing
        by mistake.

sr    spy on registers

    sr    spys starting at register 0
    sr 'd'    spys starting at register 'd'
        This is similar to spy on store, but you can
        only progress forward one register at a
        time, by typing <space>. After reaching
        register 7, the program counter, you get to
        look at the psw. The only way to spy on the
        psw is to first spy on register 7. If you
        space forward from the psw you get back to
        register 0, to get out of the spy type
        <return>.








                                   27

cont.


sw    spy on the window

    sw    spy on the memory management registers. At present
        you are restricted to looking at the registers for
        the pages allocated to processes, which under EMMOS
        should be the only ones to change.
        The registers are automatically split into their
        composite fields for displaying
        and changing. To go forward to the next field, type
        <space>, to go to the next page, type <tab> and to
        exit type <return>. Numeric fields are changed by
        typing a number followed by <return>, other fields
        are changed by typing one character.
        the expansion direction, ed> is either up=u or down=d
        the access control is either  non resident  = n
                                      read only     = r
                                  or  read/write    = w
        an access control of 'unused' indicated an error
        in the memory management registers. The written into
        bit cannot be examined or changed.

b    set a breakpoint

    b 'l'    puts a breakpoint at location 'l'. Note
        that you are not allowed to place a breakpoint
        on top of another breakpoint. It
        is possible to set breakpoints on top of emt
        instructions, but, once they have been
        reached, it is not possible to
        continue or single step from them.

k    kill a breakpoint

    k 'l'    kills the breakpoint at location 'l'. If
        the program is halted at the breakpoint, it
        is not possible to continue from there, use
        'go' instead.
    k *    kills all breakpoints. If the program had
        halted at a breakpoint it will not be
        possible to continue, use 'go' instead.














                                   28

cont.


g    go to command

    g 'l'    jumps to location 'l' and starts
        executing. 'l' must be a virtual address,
        mapped according to the window. The
        registers are loaded as set.
    gs 'l'    as above, but single stepping. Only one
        instruction is executed before control is
        returned to MUD. When single stepping all
        breakpoints are temporarily removed, so no
        breakpoints as such will be hit.

c    continue command

    c    continue from the breakpoint last reached or at the
        next instruction if previously single stepping.
    cs    as for continue but single stepping
        ( see 'gs' above )

        Note that it is not possible to continue from a
        breakpoint that has just been killed, but you can
        use 'g' instead with 'l' as the contents of the program
        counter, R7.

i    change I/O command

        This command has a special syntax, because it is
    used to redirect the input and output of MUD from one
    terminal to another.

    i    change I/O to the console
    iv 'n'    change I/O to vdu 'n'
    is    sink the output. All output will be thrown away,
        including echoing. Input remains on the same channel.

f    find a value

    f 'v'    searches the physical memory, stating at
        location zero, for a word containing the value
        'v'. Every so often a message will say
        "still looking" if one has'nt been found
        yet. To stop the search early hit <escape>
        to break in.
    f 'v' 'l'
        As above but the search starts at 'l'
        instead of zero.








                                   29

cont.


e    evaluate an expression

        This command evaluates an 18 bit expession, using octal
    or decimal numbers or the contents of a relocation register. There
    is one accumulator, acc, which holds an 18 bit number. The operations
    that can be performed on it are:

    ^ val    Load the acc with 'val'
    + val    Add 'val' into the acc
    - val    Subtract 'val' from the acc
    P    Convert the least significant 16 bits of acc into an
        18 bit virtual address
    V    Treat the value in acc as an 18 bit physical address and
        convert it into a 16 bit virtual address.

    The values are given by:

    # octal number    - an 18 bit octal number, the # can be omitted
    $ decimal number- a 15 bit decimal number, ie positive only
    R num        - the 18 bit value of relocation register 'num',
              num is 1 by default

        Several commands can appear on one line, seperated by commas
    or spaces if it is necessary to avoid ambiguity. The value in acc is
    displayed when the end of the line is reached.

        The value in acc is zero the first time the 'e' command is
    used. Subsequently the value is that left in it after the last use
    of 'e'.

























                                   30

    2.2.6    Using MUD

     If  RESMUD  is  linked  in  before  EMMPM  and MUD is present as an
overlay,  then  the  debugger  will  be entered once the system has been
loaded. If not the debugger cannot be used.

     When  control  is passed from MUD to the user program, by using the
go  or  continue  commands,  the  system error traps are set so that any
error  will  be  handled  by MUD. This remains so until the user program
changes  them.  Note  that  while  MUD  is  running the traps are set to
different handlers which detect whether the debugger itself is in error.
If  so  check  that  RESMUD  has  been  assembled  with the current page
configuration.

     We can return to the debugger at any time via the console emulator.
Find  the  address of the global symbol $MUDIN, it is part of RESMUD and
so  is  permanently  resident,  it's  virtual  and  physical  address is
therefore  the  same.  Use the console emulator to jump to this location
and MUD will restart.






































                                   31

    2.3    Linker and Loader Functions
    ___________________________________

     In  general  a  system's  linker  and  loader perform two logically
distinct   functions,  though  in  practice  the  border  between  these
functions  may  be  less clear. In a virtual memory system it is usually
the  operating  system  that allocates physical memory, but in a monitor
system  this  may be done by the linker. There are a number of functions
that  the  linker and loader are required to perform on an EMMOS system.
Some  of  these  functions  require a degree of intelligence that is not
usually  found in either linkers or loaders. These intelligent functions
may  be  incorporated in either the linker or the loader, or distributed
between  them. When presenting these functions we will describe them all
as being part of the linker, for ease of understanding. The loader then,
just copies an output image file from the linker, word for word into the
128K  physical  memory.  However,  linkers  are complex pieces of system
software, that are difficult to write or modify. Therefore, in practice,
one usually takes the best of the functions that the linker has to offer
and  writes  a  loader  program  which  incorporates those remaining.The
functions that the linker/loader must perform are listed below:

    1.    Resolve all global references and relocate the
        object code.
    2.    Allocate physical memory for the system, buffers,
        process code and stacks. This takes into account
        the page configuration scheme.
    3.    Construct a Load Map which specifies where processes
        are in physical memory and where the ends of their
        stacks are in virtual memory. This is passed on to the
        loaded system.

    2.3.1 The Linker

     We  can consider the input to the linker to be a set of Relocatable
Binary  (  RLB  )  modules, with some system of reference resolving that
need  not  concern  us.  A  process'  body is made up of one or more RLB
modules,  with  sharing allowed. EMMOS is similarly composed. To specify
how  the  system and process bodies are constructed we supply the linker
with  a  Link  Control  File,  which specifies which modules make up the
system  and process bodies, and the order in which they are to be linked
to  form  the body. We also need a way of uniquely identifying a process
body, so that when we create a process we can specify which code body to
use.

     Here we give an example of what this file could look like. The line
headed  'SYSTEM' would define the construction of the resident operating
system  part  of  EMMOS  and  lines  headed 'BODY' would each describe a
process body, it's name given in brackets:

    SYSTEM:        EMMPM-EMMTTY-..........-EMMMEM;
    BODY(X25):    X25CODE;
    BODY(PRINTR):    IOCODE-IOLIB;
    END:




                                   32

cont.

     The EMMOS linker must also allocate physical memory for the process
code  bodies and stacks. In order to do this it needs to know which code
bodies  will be incarnated as which processes and how much stack to give
those processes.T This information could go in the Link Control File. As
an  example of how it could be represented, we consider the example body
description given above, and create some processes with those bodies:

    PROC(X25-A):    "X25"   ,SYSTEM=200
    PROC(X25-B):    "X25"    ,SYSTEM=200
    PROC(PRINTR):    "PRINTR",SYSTEM=100,CORAL=400
    END:

     Here  we  describe three processes, called X25-A, X25-B and PRINTR.
The  first two are both incarnations of the body "X25" and both have 200
words  for their system stack and no CORAL stack. It's not necessary for
them  to  have  the  same size stacks, though some implementations could
impose  this as a restriction with no great hardship. The third process,
"PRINTR",  is  the one and only incarnation of the body "PRINTR". It has
100 words for it's system stack and 400 words for it's CORAL stack.

     The  linker  still  requires  information on the page configuration
before it can allocate physical memory. This information is not strictly
necessary. We could allocate the code and stacks for each process as one
lump  of  physical  store  and if it is too large for the configuration,
then  the  system  will fail at run time. By knowing the page allocation
scheme  however,  the  linker  can be quite subtle and avoid unnecessary
duplication of code and could even arrange for the code and stacks to be
on separate pages if the sizes are right. ( For more details see 3.2.7/8
).   To  complete  our  example  we  shall  give  an  idea  of  how  the
configuration could be specified in the Link Control File:

    PROCS 5..6,    BUFFERS 0..2,    EMMOS 3..4;

     Note that page 7 must always be allocated for the I/O page and that
the  section  using  page 0 will lose the first 1000 octal bytes for the
vector area.

     In  our  example,  if the body "X25" was less than 4K words then it
would  fit  on one memory page. Since the stacks for "X25-A" and "X25-B"
are  quite  small, they too fit on one page. The linker should then only
put one copy of the body into physical memory and arrange the mapping of
both  the processes such that page 5 maps onto it. Page 6 would map onto
the  processes  stack areas, which would of course be different physical
memory locations.

     Once the linker has finished allocating physical memory it needs to
be  able to tell the EMMOS initializer what the memory management window
for each process is and where it's stacks are. It does this by including
a  table  with  this  information in, in the Load Image File. This table
must  be  loaded at a place in memory where the initializer can find it,
say somewhere in the global buffer pool.




                                   33

    2.3.2 The Loader

     The  Loader  takes  the  Load  Image  file  as  input. This file is
produced  by  the  linker  and  contains  absolute  binary  data with an
indication  of  where in physical memory to load it. The loader need not
know  anything  about the processes or the system it is loading. However
there needs to be some way of telling where the system's entry point is.
Note  that  since  the  entry  point  will  be  a  virtual address it is
necessary  to  include  an  initial  memory  mapping  as  part  of  it's
definition.















































                                   34














                PART THREE
                __________

               HOST MACHINE ASPECTS








































                                   35

    3.1     Linking Using the RSX Task Builder
    ___________________________________________

     The  RSX  task  builder( TKB ) is used as the linker. It is because
not  all  the functions we require are supplied by this linker, that our
loader  is  more  complicated  then  usual.  The Task Builder is used to
perform  the  relocating  and  global  reference  resolving parts of the
required  actions.  The  physical memory allocation, process incarnation
and the production of the Load Map are all actions that are performed by
our  loader.  This  does  mean  that the loader is complicated, but less
overall  effort  is required to write it than to completely re-write the
linker.

    3.1.1 Use of the Task Builder

     The task builder is able to handle overlays in several ways. It can
produce  disk resident overlays, in which "paged-out" segments reside on
disk,  or  memory  resident  overlays,  in  which all segments reside in
physical memory but not all are "paged-in" into virtual memory. TKB will
also  allow  your  task  to be some combination of the two. Overlays are
paged in and out by special calls to RSX which TKB automatically inserts
into  the  object code. It is possible to instruct TKB to leave out this
code,  allowing  the  user  to  handle  his  own  paging.  Overlays  are
characterized by being relocated at the same place in memory. it is this
feature  that EMMOS exploits to get all the process' bodies relocated at
the  same  base  address. We do however require that the linker does not
put any auto loading code in for the overlays.

     Amongst  the various tables hidden in the task image, is the window
description  table,  this does not appear for disk resident overlays, so
we  chose  to  use memory resident overlays throughout. Also, when using
memory resident overlays, each layer of the overlay tree is relocated on
a  page  boundary,  which  is another requirement for EMMOS. There is an
option,  when  running a task under RSX, of allowing it to be swapped in
and out of physical memory ( checkpointed ). A checkpointable task has a
large  space in it's task image file, so as EMMOS has no need of it, the
task  must  be  built not checkpointable ( /-CP switch ). We will assume
then  that the task is not checkpointable, with manually loaded ( though
we shall not include the calls to do it ) memory resident overlays.


















                                   36

     The first snag is met when we try to get information about the size
of  the  overlays  and  their position within the task image. The tables
that  contain  this information follow on directly after the code of the
root  segment,  but  there  is no indication as to how long the root is,
except  in  these  tables.  Normally their position is known because the
overlay  loading  mechanism  references  them,  but we've done away with
this.  Ideally  the root segment would be the EMMOS system, but there is
no  clean way of finding out how long it is, so we chose to not allocate
page  0  to  the  system.  Instead the root segment does not contain any
code,   only   some   tables   of  known  length  used  by  the  loader.
Unfortunately,  because TKB will relocate the next overlay layer on page
1,  we  have  used up 4K of virtual space just for the interrupt vectors
etc.  To  avoid  this  we allocate the global buffer pool space to pages
0,1....  because  no  code  is  loaded  for  it.  The problem remains to
persuade  TKB  to  relocate EMMOS at the end of the buffers. We could do
this  by  extending  the root segment to occupy all these pages and then
just ignore it all when loading, but this would mean the task image file
would  be  enormous.  Instead we use dummy overlay layers. Although each
layer  contains  only  one  word,  it  is  sufficient to persuade TKB to
relocate the next layer at the next page boundary.

     It  is  because  of  such  devious  means  of  getting  the overlay
relocated  to  the  desired base, that all the calculations performed by
TKB  to  produce  physical memory load addresses are useless. Instead of
wasting  large areas of physical memory the loader does the calculations
again. This also means that we can handle the process' stacks nicely.

     Finally  there is the problem of uniquely identifying each overlay,
so  a  process can specify which is to be it's code body. TKB gives each
overlay  a  name,  which is the first six characters of the first object
file's name in the overlay, however this may not be unique. We have then
two  options;  use  the  name  given  by TKB or use a position numbering
scheme. Despite it's drawbacks of non-uniqueness, the former was chosen.
Names  are easier to understand then numbers, and the possible inclusion
of  the  debugger  may  well upset the numbering, though it is a special
case.  The  use  of six character names for the overlays led us to adopt
the  <name> field of the $CREAP macro to specify which code body to use.
Other schemes may require another field in the macro call.



















                                   37

    3.1.2 The Format of the Task Image File

     The  information required to complete the Overlay Description Table
is  scattered  across  the  first few blocks of the task image file. The
format  is  dependant  on various switches and the form of the ODL file.
There  follows  a brief description of the parts relevant to the loader,
assuming it was built in the required manner.


BLOCK 0:

         Contains no useful information, except perhaps the creation
    date of the task image.

BLOCK 1:

    No useful information.

BLOCK 2:

         This  is  the  start  of  the task header. The word at byte
    offset  72  octal  is  the  "header  guard  word  pointer". This
    contains  the  byte offset, within the next block, of the "guard
    word", this immediatly preceeds the code of the root segment and
    is how the loader finds the root.

BLOCK 3:

         This contains the start of the root segment. The first word
    of  the  root follows the "guard word". The root segment is used
    to  transmit  configuration  details and the process description
    table  to  the  loader.  Immediatly following these tables is 29
    words  of  rubbish ( code used to load the overlays under RSX ),
    the  segment  descriptor table, one word of rubbish and then the
    window  descriptor  table.  These  two tables are used by RSX to
    define the positions of the memory resident overlays in physical
    memory,  EMMOS  uses  them  to construct the Overlay Description
    Table, as follows:

    segment descriptor table -
    each entry is 9 words long, there is one entry for each overlay and
    the root
        word offset 1    the virtual base address
        word offset 6&7    the segment's name in radix 50 format.

    window descriptor table -
    each entry is 10 words long, there is one entry for each overlay
        word offset 2    overlay's size in memory blocks (64bytes)


NEXT BLOCK BOUNDARY:

         The  code  of the first overlay begins at the first word of
    the   next  disk  block.  Subsequent  overlays  start  on  block
    boundaries following on in order.


                                   38

    3.1.3 The Overlay Description Language

     The  Task  Builder  takes  a  series of object files and links them
together  to produce a Task Image. This is the exact image of what is to
be  loaded at run time ( under RSX ). EMMOS exploits the memory resident
overlay capabilities of the Task Builder in order to relocate the system
kernel  and  processes'  bodies in the desired place. The arrangement of
the  overlays  is  described in the Overlay Description Language ( ODL )
file, the structure of which is as follows:

    module1 - module2 - ............ - moduleN

     specifies  that  the  N modules are to be concatenated in the given
order

    ( module1, module2, ............ , moduleN )

     specifies  that  the  N  modules will overlay each other in virtual
memory.  The  modules  not paged-in will be resident on disk or, if they
are  memory resident overlays ( indicated by a ! before the open bracket
), in physical memory.

     These concatenation and overlay operations are combined to describe
the overlay tree structure, eg:

    A - B - !( C - D - !( E , F ) , G - !( H - !( I , J ) , K ) , L )
this specifies the following tree
    E   F    I   J
    |___|    |___|
      D        H     K
      |        |_____|
      C           G          L
      |___________|__________|
           B
           |
           A

     At  any  one  time  during  runtime,  the memory will contain those
modules lying on one of the paths from the root to a leaf.

     For  ease  of  writing, it is possible to give parts of the overlay
tree  a name, using the factor command ( .FCTR ). so a complete ODL file
for the above example could be:

    .ROOT        A - B - !( OV1 , OV2 , L )
    OV1:    .FCTR    C - D - OV11
    OV2:    .FCTR    G - !( OV22 , K )
    OV11:    .FCTR    !( E , F )
    OV22:    .FCTR    H - !( I , J )
    .END







                                   39

    3.1.4 The EMMOS ODL File


    .ROOT        ROOT - !( OV1 - !( OV2 - !( EMMOS - !( PROCS ) ) ) )
    EMMOS:    .FCTR    EMMPM - FIRST - ......................
    PROCS:    .FCTR    BODY1 , BODY2 , ..............., BODYn

     The  factor  EMMOS  describes all the system modules, which are all
concatenated.  The  factor  PROCS  describes  each of the different code
bodies  for  the processes. Normally only one code body is required at a
time  so  the  modules  overlay  each  other,  but  it  is  possible  to
concatenate processes which are to access common routines.

     For  example,  suppose  we have four code bodies BODY1, ... , BODY4
which  are each $CREAPed only once to give processes PROC1, ... , PROC4.
If these processes are completely independant then we write:

    PROCS:    .FCTR    BODY1 , BODY2 , BODY3 , BODY4

     Suppose  we have a library module, LIB, that is shared by PROC1 and
PROC2, then we could write:

    PROCS:    .FCTR    BODY1 - LIB , BODY2 - LIB , BODY3 , BODY4

     This  would  cause  two  copies of LIB to be in physical memory. We
could however write:

    PROCS:    .FCTR    BODY1 - BODY2 - LIB , BODY3 , BODY4

     which  would cause one copy of LIB to be put in the task image, but
PROC1  and  PROC2  would  now  be  paged  in  and  out of virtual memory
together, which may not be desirable.

     Note that if the total size of BODY1, BODY2, LIB and the stacks for
PROC1  and  PROC2  exceeds  the  space  allocated to processes, then the
loader  will  duplicate  some or all of the code in an attempt to fit it
in. It may be then, that more physical memory would be used in trying to
share LIB than if we specified duplication in the first place.



















                                   40

cont.

     The modules ROOT, OV1, OV2, ..... OVn are required by the system to
ensure that the operating system and the process bodies are relocated in
the correct place. ROOT also transmits information from the $CREAP macro
calls to the loader. The Task Builder relocates the first "layer" of the
overlay  tree  (  the  root  )  at location zero. Successsive layers are
relocated  at  the  start of the next page boundary ( a page is 4K words
long  ).  So,  because  each  of  these  modules is quite small, ROOT is
relocated  at  0,  OV1  at  20000,  OV2  at 40000 etc. If we require the
operating system to be relocated at 60000, we put it in the next layer (
as  in  the  above example ). Suppose we want EMMOS to start on page 4 (
location 100000 ), and it is over one page long so the processes will be
on page 6 ( location 140000 ), we would need to pack out pages 0..3 thus

.ROOT    ROOT - !( OV1 - !( OV2 - !( OV3 - !( EMMOS - !( PROCS ) ) ) ) )

     For  technical  reasons  it  was not possible to have the operating
system  at  the low numbered end of memory, so that is where the buffers
have  been  put.  Dummy  overlays  are  used  to pad out the buffer area
because  they  cause  the task image to be much smaller than if we had a
.BLKW . The latter would cause it to contain about 12K of zeroes!



































                                   41

    3.1.5 Installation of the Debugger

     The  EMMOS  debugger,  MUD, is in two parts, a resident part RESMUD
and  an overlay part MUD. To be able to use the debugger both parts must
be included in the ODL file. RESMUD must appear before EMMOS but MUD can
be put anywhere amongst the PROCS, eg:

.ROOT    ROOT - !( OV1 - !( OV2 - !( RESMUD - EMMOS - !( PROCS , MUD ) ) ) )

    3.1.6 Creating the Task Image File

     Once  the form of the ODL file has been worked out the task builder
is run under RSX-11 by:

    TKB    EMMOS , EMMOS /NOSP = EMMOS /MP

     The  first  output  file  is  for the Task Image ( .TSK ) file, the
second  is  the task map ( .MAP ) file, which is spooled by default. The
MP switch specifies that the input file is an .ODL file.

     The  .MAP  shows  the  structure  of  the  task image and gives the
virtual  addresses of the modules, global symbols and .PSECTs. This link
map  should  be  used  in  conjunction with the loader's load map, which
specifies  physical  addresses  of  code and stacks, when debugging. The
.TSK file now needs to be copied to the RT-11 format disk which contains
the loader using FLX. The system is now ready for loading and running.































                                   42

    3.2    The EMMOS RSX Task Image Loader
    _______________________________________

     This section describes the operation of the loader for the Extended
Memory  MOS  system.  The  loader itself is loaded from a floppy disk in
RT-11 format, using a fairly standard bootstrap loader. The EMMOS loader
then loads the system from an RSX-11 Task Builder Image file ( stored in
RT-11  format  )  on the same floppy disk. Control is then passed to the
operating system or to the EMMOS debugger, MUD, if it is loaded.
















































                                   43

    3.2.1 Inputs and Outputs

     The input to the loader is an RSX-11 Task Builder Image file stored
on a floppy disk in RT-11 format. This file contains:

    1.    The Configuration Table     ( see 3.2.3 )
    2.    The Overlay Description Table    ( see 3.2.4 )
    3.    The Process Description Table    ( see 3.2.5 )
    4.    Absolute binary code for the operating system
            and processes.

     The  loader  produces  a  table,  the  load map, which it places in
memory  and  displays  on the console, that describes the positioning of
all the processes in physical memory.

     The  address of the entry point to the loaded system must be stored
in  the  first  word  of  the  system.  This  is set by MOSPM or, if the
debugger is included, RESMUD.







































                                   44

    3.2.2 The Console Load Map

     This  is  a human readable form of the LOAD MAP that is supplied to
the  loaded  system.  There  is one line for each process describing the
position of it's code and stacks.

     The  first  column  gives  the process id ( pid ), the next two the
limits of the code and the fourth the limits of the two stacks combined.

     If  none  of  the pages containing code, contain any stack then the
third  column  will  read  "unshared",  that  is,  there is no page that
contains  both  stack  and  code. However, if there is only one page for
processes then this must contain all the code and all the stack, in this
case the third column will read "page is shared".

     If  the code is longer than one page, and it is necessary to have a
page  containing  both stack and code, then the third column will be the
limits of the piece of code that is sharing with the stacks. This may be
a copied piece of code.

     The limits are given in terms of physical memory blocks, so a limit
of

        001200 - 001250

     for  the  code  would  mean that the code starts at physical memory
location 120000 and ends somewhere between 125000 and 125077.

     If  the task image does not contain an overlay for a process' body,
then the load map entry for that process will be of the form:

    pid  -  ABCDEF has'nt got a body

     If  one  or  more  processes  lack  bodies  then  the  load will be
abandoned, once the load map has been printed, with the message:

    "process requires body"   (see ERRORS)

     Note:  the  printing  of  the  Load Map can be abandoned by hitting
<escape>.

















                                   45

    3.2.3 The Configuration Table

     This  table  resides  in  the  root  segment  of  the  task  image,
immediately before the Process Description Table. It is used to tell the
loader  the  page allocation scheme and the maximum number of processes.
The  information is passed to it at run time so that the loader need not
be re-compiled for every change in the configuration.

    The format is:

    VAR config_table :RECORD
                max_number_of_processes,
                load_map_address,
                first_mos_page, last_mos_page,
                first_process_page, last_process_page
                    : INTEGER
            END;








































                                   46

    3.2.4 The Overlay Description Table

     This  table  is  constructed  by  the  loader from the task image's
window and segment descriptor tables, that follow the root segment. Each
overlay named by the task image ( except the dummy ones and MOS ) has an
entry of the form:

    TYPE overlay_description = RECORD
                    name : ascii;
                    size : mem_blocks;
                    disk_address : INTEGER
                END;

    TYPE ascii = ARRAY[ 1:6 ] OF CHAR;

    TYPE mem_blocks = 0..number_of_process_pages * #200 -1;

The table is given by

    odt : ARRAY[ 1..number_of_overlays ] OF overlay_description;


odt[ ov ].size    contains the size of the overlay segment ( that is of
the code ) in 32 word memory blocks.
odt[ ov ].name is the overlay's name, taken from the overlay description
language file at build time.
odt[ ov ].disk_address is the number of the disk block where the overlay starts.

     Note that the disk address field is not used in the current version
of  the  loader. It remains as a left-over from a version that would not
support down line loading.


























                                   47

    3.2.5 The Process Description Table

     This table resides in the root segment of the task image, following
the  Configuration  Table.  It is produced by the $CREAPs used to create
the  processes.  It  is  used  to  tell  the  loader which processes are
required,  which  overlay  will  supply  the  code and the size of their
stacks. There is one entry in the table per process, each of the form:

    TYPE process_description = RECORD
                    pid : INTEGER;
                    name : ascii;
                    r0_size, r6_size : mem_blocks
                END;

    TYPE ascii = ARRAY [ 1..6 ] OF CHAR;

    TYPE mem_blocks = 0..number_of_process_pages * #200 - 1;

The table is given by

    VAR pdt : ARRAY [ 1..number_of_processes ] OF process_description;


pdt[ i ] is the entry for the ith process,
so pdt[ i ].pid = i for all i IN 1..number_of_processes.
The name field contains the six character name of the overlay which is the
code for the process. This is taken from the second parameter of the $CREAP
macro call.
pdt[ i ].r0_size and pdt[ i ].r6_size contain the required sizes, in 32
word memory blocks, of the stacks.
Note that the macro converts the sizes from words to memory blocks before
placing the values in the table, using
    size_in_blocks := ceiling( size_in_words / 32 ).
























                                   48

    3.2.6 The Load Map

     The  load map describes the loaded system to the initializer. It is
placed  in  memory  by  the  loader at an agreed address before it gives
control to the system. There is one entry in the table for each process,
and  a special one for the debugger if it is included in the task image.
The form of a table entry is:

    TYPE load_description = RECORD
                    pid,
                    r0_low, r0_hi,
                    r6_low, r6_hi : INTEGER;
                    window : ARRAY
                        [ first_page..last_page ]
                        OF window_type
                END;

    TYPE window_type = RECORD par, pdr : INTEGER END;

The map is given by:

    map : ARRAY [ 1..number_of_processes + 1 ] OF load_description


     map[i] is the entry for the ith process, so map[i].pid = i, for all
i IN 1..number_of_processes. map[i].pid = -1 for the debugger entry. Any
unused entries have pid = 0.

     r0_low  and  r0_hi contain the virtual byte addresses of the lowest
and  highest  numbered  words in the R0 ( CORAL ) stack for the process.
Similarly for r6_low, r6_hi.

     window describes the mapping from virtual memory onto this process.
There  are  two  words  for  each  page allocated to a process, the page
address register and the page descriptor register.






















                                   49

    3.2.7 The Physical Memory Allocation Algorithm

     The  loader should attempt to arrange the loading of the Load Image
into  physical  memory  in a way that will reduce the amount of physical
memory  required.  The  total  amount  of  virtual memory allocated to a
process  for  it's  code body and stacks will be fixed ( = num_pp memory
pages  ), but for each process the sizes of it's code and the total size
of it's two stacks will vary.

     If  at  all possible the loader will arrange for the stacks and the
code to be on different pages in virtual memory, allowing the process to
put read-only protection on it's code. This is only possible if:

    pages( code_size ) + pages( stack_size ) <= num_pp

     where  pages(  i  )  = the size i rounded up into a whole number of
memory pages. Of course if pages( code_size + stack_size ) > num_pp then
not  enough virtual memory is availiable for this process. However if we
have:

    pages( code_size + stack_size ) = num_pp

     then some of the code and some of the stack space must share one of
the  pages. The shared page cannot then have read-only protection placed
on  it.  If  a process has already been created with this code body then
physical  memory  following  the code body will have been allocated as a
stack.  We  cannot  then  have  the stack for the next process following
directly  on  from  the code, so it may be necessary to copy part of the
code  body and arrange that the memory mapping maps the two parts of the
code onto consecutive virtual memory locations.



























                                   50

E.G.

     Suppose  we have two pages for a process, num_pp = 2, and we have a
code  body which is $CREAPed twice, as PROC1 with stacks S1 and as PROC2
with stacks S2. If the code body is larger than one page then the second
page must be shared between the stacks and some of the code.

     First the loader copies the code body into physical memory, then it
allocates   the   stacks  for  PROC1,  the  window  for  PROC1  will  be
(a..b,b..d).

    --a---------------b----c------d----f---------------
      :    code size       :  S1  :        ....physical memory
    ---------------------------------------------------
      |<- 4K words -->|<-- 4K words -->|

     Now  for PROC2. If pages( code size + S1 + S2 ) <= num_pp, then the
loader  can  allocate  the stack following on directly from S1, thus the
window for PROC2 will be (a..b,b..e).

    ---a--------------b----c------d--e-f---------------
       :   code size       :  S1  :S2:
    ---------------------------------------------------

     Note  that  PROC2's  window  allows  it to ( illegally ) access the
stack  of  S1. If however pages( code size + S1 + S2 ) > num_pp then the
loader  must  take  a copy of the piece of code that overlaps the second
page,  (copy  from  b..c  to  d..g), thus the window for PROC2 will be (
a..b,d..h ), and the copy will be mapped into the same virtual locations
as the original.

    ---a--------------b----c------d--e-fg--h------------
       :   code size       :  S1  :copy :S2:
    ----------------------------------------------------

     Now  PROC1's stack is not in the window of PROC2 so neither process
can use the other's stack.

     The  loader  will  try  and fit as many stacks as possible into the
page  without  copying  a piece of code, but it wont try to optimize the
ordering.  If  the  code  size is smaller than a page and the stacks are
bigger then the whole of the code will be copied.















                                   51

    3.2.8 The Logical Operation of the Loader


    get process description table from task image;
    construct overlay description table;
    load EMMOS;
    FOR each overlay
    DO
        load the code;
        FOR each process
        DO
        IF name of process = name of overlay
        THEN
            IF size of code in pages + size of stacks in pages
            <= number of pages for processes
            THEN
            code and stacks are on completely different pages;
            set up the window for the process;
            leave room for the stacks in physical memory

            ELSF (size of code + stacks) in pages
            = number of pages for processes
            THEN
            one page contains stack and code;
            IF stack will fit on the end of the last
               incarnation's window
            THEN
                leave room for the stacks after last stack;
                set up the process' window
            ELSE
                make a copy of the piece of code that must
                share a page with some stack;
                leave room for the stacks after the copied code;
                set up the window for the process
            FI
            ELSE
            process is too large
            FI
        FI
        OD
    OD
















                                   52

    3.2.9 Error Messages

    i. System Traps

         These  indicate  something  has gone drastically wrong with
    the  loader,  hopefully they will only occur after modifications
    have been made and not thoroughly tested. They may also occur if
    the  loaded  system  goes  wrong  before  it  has  set  the trap
    locations to its own trap handlers.

cpu error: pc = nnnnnn   ps = mmmmmm
reserved instruction: pc = nnnnnn   ps = mmmmmm

         These  messages  occur  when a trap to #004 and #010 occur.
    The contents of the program counter and processor staus word, as
    placed on the stack by the trap, are displayed.


memory management trap 250
nnnnnn at mmmmmm

         This  message  occurs  when  something  goes wrong with the
    memory  management. The contents of the two status registers are
    displayed.  nnnnnn  =  SR0  which  is  further  expanded in full
    following  the  error  message  ,  and mmmmmm = SR2, the virtual
    address   associated  with  the  error.  Following  the  written
    expansion  of  SR0,  the  contents  of  the  kernel  mode memory
    management registers are displayed.


power failed, start again

         the  power  to the processor failed, re-boot the system. NB
    this has not been tested!!!!























                                   53

    ii. Loader Errors and Messages


test: eof

    The end of the task builder image file has been reached prematurely.

test: get has failed

    An error occurred in the transfer of data from the floppy disk. Try
    again.

test: rewind

    The loader requires the input task image file to be rewound. This
    is only an error if the file is not on a rewindable device, eg down
    line loading. It is caused by an error in the loader.

cant find 'task file name'

    The file is not on the floppy disk, check the file name specified
    in the loader is the same as you copied to the disk, and the disk
    is in the correct drive ( drive 0 ). Remember that the name specified
    in the loader must include any blanks necessary to pack the name to
    10 chars, in the form ABCDEF.EXT

too short!

    The file is less than 4 blocks long, so cannot possibly be the
    correct task image.

first dummy overlay not found

    the dummy overlay that is used to pack out the buffer area is not
    present in the task image. Check the file is a task image and that
    the ODL file correctly specifies the dummy overlays.

mos is larger than nn pages long

    the resident part of EMMOS is too large, more than the nn pages
    specified in the configuration file. Make sure that the processes
    have been put in as overlays and not in the resident  part.

window and segment descriptors of unequal size

    The loader has lost it's place in the task image, or the ODL file
    does not specify the overlays in the correct format.

ABCDEF is too large

    The named overlay is too large for the configuration, even without
    stacks. Check that the overlay names are seperated by commas in the ODL
    file.




                                   54

    Errors and Messages - cont.



process nn too large

    The process with pid = nn has stack and code too large for the
    configuration. Check that stack sizes have been given in words in
    the $CREAP macro call.

ABCDEF has'nt been made a process

    The named overlay does'nt appear in any of the $CREAPs. The overlay
    is loaded but otherwise ignored, this is not a fatal error.

process nn: code duplicated

    The loader was forced to make a copy of some of the process' code
    because the code had already been $CREAPed and the stack and code
    needs to share a page. This is not an error.

debugger has been loaded  nnnnnn - mmmmmm

    The overlay part of the debugger has been loaded, starting at
    memory block nnnnnn and finishing in block mmmmmm. This is not an
    error.

ABCDEF has incorrect base address = nnnnnn

    The named overlay has been relocated at nnnnnn by the Task Builder.
    This does not agree with the virtual page allocation scheme given.
    Check that the ODL file is correct, that the right number of dummy
    packing overlays are present and that all overlays are not too
    large for the configuration.

debugger is too large

    The debugger takes up more space than is allowed for processes. it
    is still loaded into physical memory, but only those pages normally
    allowed for processes will be paged into virtual memory, this will
    almost certainly cause MUD to crash. Use the console emulator to
    enter the system instead of MUD. Alternatively, remove the debugger
    or allow more pages for processes. This is not a fatal error.














                                   55

    Errors and Messages - cont.




the debugger cannot be used

    Either the resident part or the overlay part of the debugger are
    missing and hence the debugger cannot be used. If the debugger is
    needed then include the missing name in the ODL file. This is not a
    fatal error.

non-unique overlay  ABCDEF

    The ODL file describes a configuration such that two overlays have
    the same name. The loader requires all overlays to have unique
    names so it can unambiguously select the code body for a process.
    Try re-ordering the overlay description in the ODL file. If the
    effending overlay is a common library, put it's name after the code
    body, otherwise the file may need to be copied to one of a
    different name.

process requires body

    at least one process has specified a body name that does not
    appear in the Task Image as an overlay. An indication of which
    process is causing the problem can be found from the Console Load
    Map (qv). Make sure the name specified in the $CREAP macro call
    is that of one of the object files, check in the task map for
    the names actually used. If the debugger is included, make sure
    it, and all other overlays, are separated by commas in the ODL
    file.

























                                   56

    3.2.10 Tracing

     The  loader and it's libraries are liberally scattered with code to
produce  vast  amounts of output, that describes in detail what is being
processed.  These  pieces  of  code are normally excluded by surrounding
them  in  comment  brackets.  The  start  of  a  piece  of trace code is
indicated  by  {trace  and the end is followed by ecart}. To include the
trace  statements  change  {trace to {trace} and ecart} to {ecart}. This
will make it easy to restore them later.

     The   output  produced  should  be  self  explanatory,  except  for
PROCEDURE  waiting; which displays the message 'waiting' and waits until
any  character  is  typed in at the keyboard. All this does is slow down
the output, since <cntrl-s> and<cntrl-q> are not implemented.

    3.2.11 Changing The System

     The   loader   need  not  be  re-compiled  every  time  the  system
configuration  is changed. This is because the configuration information
is placed in the root segment of the task image file at link time.





































                                   57

    3.3    Process Creation
    ________________________

     Processes  are  created at compile time by calling the $CREAP macro
in  the  system  configuration  module.  The  $CREAP  macro  defines the
attributes  of a process; it's entry point, the name of the overlay that
contains  it's code body, the devices associated with it and the size of
the CORAL and System stacks.

     Note, the name given is the name of the overlay in which the code (
and therefore the entry point ) is found. This name is that of the first
module  given  in  the overlay subtree. E.g. If we have a library module
LIB  and  a  process  body  module  BODY1, then in the ODL file we could
write:

    PROCS:    .FCTR    BODY1 - LIB , .........
in which case the $CREAP call would be
    $CREAP    $ENTRY , <BODY1> , ............

or alternatively
    PROCS    .FCTR    LIB - BODY1 , .........
in which case the $CREAP call would need to be
    $CREAP    $ENTRY , <LIB   > , ...........

     For  this reason all the overlay names must be unique, othewise the
loader  would not know which code body to give a process. Now suppose we
have  another  process body module, BODY2, which also uses LIB. We could
not write:

    PROCS:    .FCTR    LIB - BODY1 , LIB - BODY2 , .........
since both overlays would be called LIB. Instead write either

    PROCS:    .FCTR    BODY1 - LIB , BODY2 - LIB , .........
with
    $CREAP    $ENT1 , <BODY1 > , ......
    $CREAP    $ENT2 , <BODY2 > , ......

or
    PROCS:    .FCTR    LIB - BODY1 - BODY2 , ......
with
    $CREAP    $ENT1 , <LIB   > , ......
    $CREAP    $ENT2 , <LIB   > , ......
or some other combination.

     Remember  that  the name must be of exactly six characters. Pad out
with blanks if necessary.

     The stack sizes are specified using named parameters, these are the
only  extra  parameters of $CREAP due to the EMMOS extensions. The sizes
are  given in words, but are automatically rounded up into memory blocks
(  32  words  ).  A  zero  length R0 stack is acceptable for a non-CORAL
process,  but the R6 stack should always have some space. If the size is
not specified it is set to 32 words by default. Note the stack sizes are
given  seperately  for  each  process,  not  for  each code body, so two
incarnations  of  the same body could have different stack sizes, though
it would be a bit odd.

                                   58














                APPENDICES
                __________










































                                   59

    APPENDIX A    Pitfalls.




A.1 Summary

     Processes  must  ensure that all communication is performed via the
global buffer pool.

     Shared  overlaid  library  procedures must not use the CORAL global
vector for their entry.

     CORAL  code  bodies  incarnated  more  than  once should not preset
variables.

     All  local  variables  must be taken from the stack, in code bodies
incarnated more than once.

     All  IORBs  and  their  data buffers must be located in permanently
resident memory (ref 2.1.4).

    A.2.1    Use of local space for inter-process communication

     Obviously  processes  with  different  code bodies cannot use their
local  static space ( ie not the stack ) for communication, but at first
sight it would appear that different incarnations of the same body could
communicate  using  variables  in the body. Although sometimes possible,
this  is  extremely  configuration  (  and  even loader ) dependant. The
problem  arises when there is not enough room to fit all the code body (
which  includes  local static data ) onto process pages of it's own. The
loader  may  need  to take a copy of some of the code, which may include
local  data. In this case there would be more than one copy of the local
data,  so if two processes use it for communicating, they would actually
be using two different locations.

     Therefore,   processes   must  ensure  that  all  communication  is
performed via the global buffer pool.



















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A.2.2 Use of shared, overlaid CORAL libraries.

     All  CORAL  procedures,  common  variables  etc.,  must  use unique
indices  to  the global vector, to define themselves ( unless some trick
is being performed ). This remains true even if the procedure is defined
in  an  overlay,  because  there is only one global vector. If a library
module  containing  some  CORAL  procedures only appears in one overlay,
then  the  global vector will contain the virtual addresses of the entry
points.  However,  the  procedures will only be in virtual memory when a
process  which  is  an  incarnation  of that overlay is running, so only
these processes can use the procedures.

     This  is  ok,  the problem arises when two or more overlays use the
library  module. If the overlays are such that when relocated, the entry
points  of the procedures are at different virtual locations, the global
vector  mechanism  wont  work. The entry point recorded will probably be
that  for the last overlay processes by the linker, so the first overlay
will crash when it tries to call the procedure.

     To  prevent this all copies of the library module must be relocated
at  the  same  virtual  locations,  either  by placing it on permanently
resident EMMOS pages or putting it first in the overlay, though this may
cause  problems  with the overlay naming convention. A more satisfactory
solution  in  many  ways,  would be to not use the global vector for the
procedure  entry.  The  offending procedures would use genuine names for
their  entry  points,  not  vector indices. This is achieved in CORAL by
using a large index, outside the bounds of the global vector.

A.2.3 Use of static variables

     Any  data  space allocated in a code body, which is incarnated more
than  once, will be used by more than one incarnation, unless the loader
causes  a  copy to be made. The processes cannot then use this space for
private  local  variables,  because  another  may  use the same physical
memory  for  it's  private  local  variables. Neither can it be used for
shared  variables, because the loader may have copied them ( see 'use of
local  space  for inter process communication' ). Constants are ok since
all  incarnations will require them to remain constant. Beware of string
constants  though,  although  they  really  should remain constant, some
unhealthy  procedures actually change some parts of them. Such practices
should be banned.

     In  particular,  note  that  the  local variables can be created in
devious  ways,  in  CORAL  a  preset variable becomes an OWN variable by
being  allocated  not  on  the  stack,  but  in  the local static space.
Presetting  and  variables  in  the  outermost block must be avoided for
multi-incarnate processes.

Always take local variables from the stack.








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            Don't Panic !











































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