HPAVC/HPAVC CD-ROM.iso/ASM_TUTR.ZIP/ASMTUTR3.DOC

Text File | 1984-11-27 | 21KB | 372 lines

  PROGPREF SEGMENT   AT 0      ;Really a DSECT mapping the program prefix
           ORG   PROGPREF+6
  MEMSIZE  DW   ?              ;Size of available memory
  PROGPREF ENDS
Really, no matter whether the AT value represents truth or fiction, it is
your responsibility, not the assembler's, to get set up a segment register
so that you can really reach the storage in question.   So, you can't say
         MOV  AL,EQUIP
unless you first say something like
         MOV  AX,BIOSAREA   ;BIOSAREA becomes a symbol with value 40H
         MOV  ES,AX
         ASSUME ES:BIOSAREA
Enough about SEGMENT.  The END statement is simple.  It goes at the end of
every assembly.  When you are assembling a subroutine, you just say
         END
but when you are assembling the main routine of a program you say
        END label
where 'label' is the place where execution is to begin.
Another pseudo-op illustrated in the program is ASSUME.  ASSUME is like the
USING statement in 370 assembler.  However, ASSUME can ONLY refer to seg-
ment registers.  The assembler uses ASSUME information to decide whether to
assemble segment override prefixes and to check that the data you are try-
ing to access is really accessible.  In this case, we can reassure the
assembler that both the CS and DS registers will address the section called
HELLO at execution time.  Actually, the SS and ES registers will too, but
the assembler never needs to make use of this information.
I guess I have explained everything in the program except that ORG
pseudo-op.  ORG means the same thing as it does in many assembly languages.
It tells the assembler to move its location counter to some particular
address.  In this case, we have asked the assembler to start assembling
code hex 100 bytes from the start of the section called HELLO instead of at
the very beginning.  This simply reflects the way COM programs are loaded.
When a COM program is loaded by the system, the system sets up all four
segment registers to address the same 64K of storage.  The first 100 hex
bytes of that storage contains what is called the program prefix; this area
is described in appendix E of the DOS manual.  Your COM program physically
begins after this.  Execution begins with the first physical byte of your
program; that is why the JMP instruction is there.
Wait a minute, you say, why the JMP instruction at all?  Why not put the
data at the end?  Well, in a simple program like this I probably could have
gotten away with that.  However, I have the habit of putting data first and
would encourage you to do the same because of the way the assembler has of
assembling different instructions depending on the nature of the operand.
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Unfortunately, sometimes the different choices of instruction which can
assemble from a single opcode have different lengths.  If the assembler has
already seen the data when it gets to the instructions it has a good chance
of reserving the right number of bytes on the first pass.  If the data is
at the end, the assembler may not have enough information on the first pass
to reserve the right number of bytes for the instruction.  Sometimes the
assembler will complain about this, something like "Forward reference is
illegal" but at other times, it will make some default assumption.  On the
second pass, if the assumption turned out to be wrong, it will report what
is called a "Phase error," a very nasty error to track down.  So get in the
habit of putting data and equated symbols ahead of code.
OK.  Maybe you understand the program now.  Let's walk through the steps
involved in making it into a real COM file.
1.  The file should be created with the name HELLO.ASM (actually the name
    is arbitrary but the extension .ASM is conventional and useful)
2.
      ASM   HELLO,,;
    (this is just one example of invoking the assembler; it uses the small
    assembler ASM, it produces an object file and a listing file with the
    same name as the source file.  I am not going exhaustively into how to
    invoke the assembler, which the manual goes into pretty well.  I guess
    this is the first time I mentioned that there are really two
    assemblers; the small assembler ASM will run in a 64K machine and
    doesn't support macros.  I used to use it all the time; now that I have
    a bigger machine and a lot of macro libraries I use the full function
    assembler MASM.  You get both when you buy the package).
3.  If you issue DIR at this point, you will discover that you have
    acquired HELLO.OBJ (the object code resulting from the assembly) and
    HELLO.LST (a listing file).  I guess I can digress for a second here
    concerning the listing file.  It contains TAB characters.  I have found
    there are two good ways to get it printed and one bad way.  The bad way
    is to use LPT1: as the direct target of the listing file or to try
    copying the LST file to LPT1 without first setting the tabs on the
    printer.  The two good ways are to either
    a.  direct it to the console and activate the printer with CTRL-PRTSC.
        In this case, DOS will expand the tabs for you.
    b.  direct to LPT1: but first send the right escape sequence to LPT1 to
        set the tabs every eight columns.  I have found that on some early
        serial numbers of the IBM PC printer, tabs don't work quite right,
        which forces you to the first option.
4.
          LINK  HELLO;
    (again, there are lots of linker options but this is the simplest.  It
    takes HELLO.OBJ and makes HELLO.EXE).  HELLO.EXE?  I thought we were
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    making a COM program, not an EXE program.  Right.  HELLO.EXE isn't
    really executable; its just that the linker doesn't know about COM pro-
    grams.  That requires another utility.  You don't have this utility if
    you are using DOS 1.0; you have it if you are using DOS 1.1 or DOS 2.0.
    Oh, by the way, the linker will warn you that you have no stack
    segment.  Don't worry about it.
5.
          EXE2BIN  HELLO HELLO.COM
    This is the final step.  It produces the actual program you will exe-
    cute.  Note that you have to spell out HELLO.COM; for a nominally
    rational but actually perverse reason, EXE2BIN uses the default exten-
    sion BIN instead of COM for its output file.  At this point, you might
    want to erase HELLO.EXE; it looks a lot more useful than it is.
    Chances are you won't need to recreate HELLO.COM unless you change the
    source and then you are going to have to redo the whole thing.
6.
          HELLO
    You type hello, that invokes the program, it says
          HELLO YOURSELF!!!
    (oops, what did I do wrong....?)

What about subroutines?
_______________________
What about subroutines?
What about subroutines?
What about subroutines?
I started with a simple COM program because I actually think they are easi-
er to create than subroutines to be called from high level languages, but
maybe its really the latter you are interested in.  Here, I think you
should get comfortable with the assembler FIRST with little exercises like
the one above and also another one which I will finish up with.
Next you are ready to look at the interface information for your particular
language.  You usually find this in some sort of an appendix.  For example,
the BASIC manual has Appendix C on Machine Language Subroutines.  The
PASCAL manual buries the information a little more deeply:  the interface
to a separately compiled routine can be found in the Chapter on Procedures
and Functions, in a subsection called Internal Calling Conventions.
Each language is slightly different, but here are what I think are some
common issues in subroutine construction.
1.  NEAR versus FAR?  Most of the time, your language will probably call
    your assembler routine as a FAR routine.  In this case, you need to
    make sure the assembler will generate the right kind of return.  You do
    this with a PROC...ENDP statement pair.  The PROC statement is probably

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    a good idea for a NEAR routine too even though it is not strictly
    required:
              FAR linkage:        |            NEAR linkage:
                                  |
    ARBITRARY SEGMENT             |  SPECIFIC  SEGMENT  PUBLIC
              PUBLIC THENAME      |            PUBLIC THENAME
              ASSUME CS:ARBITRARY |            ASSUME CS:SPECIFIC,DS:SPECIFIC
    THENAME   PROC FAR            |            ASSUME ES:SPECIFIC,SS:SPECIFIC
              ..... code and data |  THENAME   PROC NEAR
    THENAME   ENDP                |            ..... code and data ....
    ARBITRARY ENDS                |  THENAME   ENDP
              END                 |  SPECIFIC  ENDS
                                  |            END
    With FAR linkage, it doesn't really matter what you call the segment.
    you must declare the name by which you will be called in a PUBLIC pseu-
    do-op and also show that it is a FAR procedure.  Only CS will be ini-
    tialized to your segment when you are called.  Generally, the other
    segment registers will continue to point to the caller's segments.
    With NEAR linkage, you are executing in the same segment as the caller.
    Therefore, you must give the segment a specific name as instructed by
    the language manual.  However, you may be able to count on all segment
    registers pointing to your own segment (sometimes the situation can be
    more complicated but I cannot really go into all of the details).  You
    should be aware that the code you write will not be the only thing in
    the segment and will be physically relocated within the segment by the
    linker.  However, all OFFSET references will be relocated and will be
    correct at execution time.
2.  Parameters passed on the stack.  Usually, high level languages pass
    parameters to subroutines by pushing words onto the stack prior to
    calling you.  What may differ from language to language is the nature
    of what is pushed (OFFSET only or OFFSET and SEGMENT) and the order in
    which it is pushed (left to right, right to left within the CALL state-
    ment).  However, you will need to study the examples to figure out how
    to retrieve the parameters from the stack.  A useful fact to exploit is
    the fact that a reference involving the BP register defaults to a ref-
    erence to the stack segment.  So, the following strategy can work:
      ARGS     STRUC
               DW   3 DUP(?)  ;Saved BP and return address
      ARG3     DW   ?
      ARG2     DW   ?
      ARG1     DW   ?
      ARGS     ENDS
           ...........
               PUSH BP                 ;save BP register
               MOV  BP,SP              ;Use BP to address stack
               MOV   ...,[BP].ARG2     ;retrieve second argument
               (etc.)

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    This example uses something called a structure, which is only available
    in the large assembler; furthermore, it uses it without allocating it,
    which is not a well-documented option.  However, I find the above
    approach generally pleasing.  The STRUC is like a DSECT in that it
    establishes labels as being offset a certain distance from an arbitrary
    point; these labels are then used in the body of code by beginning them
    with a period; the construction ".ARG2" means, basically, " +
    (ARG2-ARGS)."
    What you are doing here is using BP to address the stack, accounting
    for the word where you saved the caller's BP and also for the two words
    which were pushed by the CALL instruction.
3.  How big is the stack?  BASIC only gives you an eight word stack to play
    with.  On the other hand, it doesn't require you to save any registers
    except the segment registers.  Other languages give you a liberal
    stack, which makes things a lot easier.  If you have to create a new
    stack segment for yourself, the easiest thing is to place the stack at
    the end of your program and:
         CLI                      ;suppress interrupts while changing the stack
         MOV  SSAVE,SS            ;save old SS in local storage (old SP
                                  ; already saved in BP)
         MOV  SP,CS               ;switch
         MOV  SS,SP               ;the
         MOV  SP,OFFSET STACKTOP  ;stack
         STI                      ;(maybe)
    Later, you can reverse these steps before returning to the caller.  At
    the end of your program, you place the stack itself:
             DW   128 DUP(?)          ;stack of 128 words (liberal)
    STACKTOP LABEL WORD
4.  Make sure you save and restore those registers required by the caller.
5.  Be sure to get the right kind of addressibility.  In the FAR call exam-
    ple, only CS addresses your segment.  If you are careful with your
    ASSUME statements the assembler will keep track of this fact and gener-
    ate CS prefixes when you make data references; however, you might want
    to do something like
                    MOV AX,CS      ;get current segment address
                    MOV DS,AX      ;To DS
                    ASSUME DS:THISSEG
    Be sure you keep your ASSUMEs in synch with reality.



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Learning about BIOS and the hardware
____________________________________
Learning about BIOS and the hardware
Learning about BIOS and the hardware
Learning about BIOS and the hardware
You can't do everything with DOS calls.  You may need to learn something
about the BIOS and about the hardware itself.  In this, the Technical Ref-
erence is a very good thing to look at.
The first thing you look at in the Technical Reference, unless you are
really determined to master the whole ball of wax, is the BIOS listings
presented in Appendix A. Glory be:  here is the whole 8K of ROM which deals
with low level hardware support layed out with comments and everything.
In fact, if you are just interested in learning what BIOS can do for you,
you just need to read the header comments at the beginning of each section
of the listing.
BIOS services are invoked by means of the INT instruction; the BIOS occu-
pies interrupts 10H through 1FH and also interrupt 5H; actually, of these
seventeen interrupts, five are used for user exit points or data pointers,
leaving twelve actual services.
In most cases, a service deals with a particular hardware interface; for
example, BIOS interrupt 10H deals with the screen.  As with DOS function
calls, many BIOS services can be passed a function code in the AH register
and possible other arguments.
I am not going to summarize the most useful BIOS features here; you will
see some examples in the next sample program we will look at.
The other thing you might want to get into with the Tech reference is the
description of some hardware options, particularly the asynch adapter,
which are not well supported in the BIOS.  The writeup on the asynch adapt-
er is pretty complete.
Actually, the Tech reference itself is pretty complete and very nice as far
as it goes.  One thing which is missing from the Tech reference is informa-
tion on the programmable peripheral chips on the system board.  These
include
      the 8259 interrupt controller
      the 8253 timer
      the 8237 DMA controller and
      the 8255 peripheral interface
To make your library absolutely complete, you should order the INTEL data
sheets for these beasts.
I should say, though, that the only I ever found I needed to know about was
the interrupt controller.  If you happen to have the 8086 Family User's
Manual, the big book put out by INTEL, which is one of the things people
sometimes buy to learn about 8086 architecture, there is an appendix there
which gives an adequate description of the 8259.


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A final example
_______________
A final example
A final example
A final example
I leave you with a more substantial example of code which illustrates some
good elementary techniques; I won't claim its style is perfect, but I think
it is adequate.  I think this is a much more useful example than what you
will get with the assembler:
          PAGE 61,132
          TITLE SETSCRN -- Establish correct monitor use at boot time
;
;         This program is a variation on many which toggle the equipment flags
;         to support the use of either video option (monochrome or color).
;         The thing about this one is it prompts the user in such a way that he
;         can select the use of the monitor he is currently looking at (or which
;         is currently connected or turned on) without really having to know
;         which is which.  SETSCRN is a good program to put first in an
;         AUTOEXEC.BAT file.
;
;         This program is highly dependent on the hardware and BIOS of the IBMPC
;         and is hardly portable, except to very exact clones.  For this reason,
;         BIOS calls are used in lieu of DOS function calls where both provide
;         equal function.
;
OK.  That's the first page of the program.  Notice the PAGE statement,
which you can use to tell the assembler how to format the listing.  You
give it lines per page and characters per line.  I have mine setup to print
on the host lineprinter; I routinely upload my listings at 9600 baud and
print them on the host; it is faster than using the PC printer.
There is also a TITLE statement.  This simply provides a nice title for
each page of your listing.  Now for the second page:
          SUBTTL -- Provide .COM type environment and Data
          PAGE
;
;         First, describe the one BIOS byte we are interested in
;
BIOSDATA  SEGMENT   AT 40H    ;Describe where BIOS keeps his data
          ORG       10H       ;Skip parts we are not interested in
EQUIP     DB        ?         ;Equipment flag location
MONO      EQU       00110000B ;These bits on if monochrome
COLOR     EQU       11101111B ;Mask to make BIOS think of the color board
BIOSDATA  ENDS                ;End of interesting part
;
;         Next, describe some values for interrupts and functions
;
DOS       EQU       21H       ;DOS Function Handler INT code
PRTMSG    EQU       09H       ;Function code to print a message
KBD       EQU       16H       ;BIOS keyboard services INT code
GETKEY    EQU       00H       ;Function code to read a character
SCREEN    EQU       10H       ;BIOS Screen services INT code
MONOINIT  EQU       02H       ;Value to initialize monochrome screen
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;COLORINIT EQU       03H       ;Value to initialize color screen (80x25)
COLORINIT EQU      01H       ;Value to initialize color screen (40X25)
;
;         Now, describe our own segment
;
SETSCRN   SEGMENT             ;Set operating segment for CODE and DATA
;
          ASSUME CS:SETSCRN,DS:SETSCRN,ES:SETSCRN,SS:SETSCRN    ;All segments
;
          ORG       100H      ;Begin assembly at standard .COM offset
;
MAIN      PROC      NEAR      ;COM files use NEAR linkage
          JMP       BEGIN     ;And, it is helpful to put the data first, but
;                             ;then you must branch around it.
;
;         Data used in SETSCRN
;
CHANGELOC   DD      EQUIP     ;Location of the EQUIP, recorded as far pointer
MONOPROMPT  DB      'Please press the plus ( + ) key.$'    ;User sees on mono
COLORPROMPT DB      'Please press the minus ( - ) key.$'   ;User sees on color
Several things are illustrated on this page.  First, in addition to titles,
the assembler supports subtitles:  hence the SUBTTL pseudo-op.  Second, the
PAGE pseudo-op can be used to go to a new page in the listing.  You see an
example here of the DSECT-style segment in the "SEGMENT AT 40H".  Here, our
our interest is in correctly describing the location of some data in the
BIOS work area which really is located at segment 40H.
You will also see illustrated the EQU instruction, which just gives a sym-
bolic name to a number.  I don't make a fetish of giving a name to every
single number in a program.  I do feel strongly, though, that interrupts