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An Introduction to C-128 CP/M

CPM1.TXT rev 1a 96-11-01

THIS DOCUMENT IS COPYRIGHT (C) 1988,
1996 BY HERNE DATA SYSTEMS LTD.  THE
MATERIAL CONTAINED HEREIN MAY BE
FREELY USED FOR PERSONAL INFORMATION
ONLY.  IF YOU REPRODUCE IT, THIS
COPYRIGHT NOTICE MUST NOT BE REMOVED.
THIS MATERIAL MAY NOT BE EXPLOITED
FOR COMMERCIAL PURPOSES.

Herne Data Systems Ltd.,
PO Box 250, Tiverton, ON N0G 2T0
CANADA.
Voice/fax [phone],
e-mail [e-mail]
internet: http://www.herne.com


An Introduction to C-128 CP/M

What is CP/M?
CP/M stands for 'Control
Program/Monitor'.  It is not a
programming language, the way BASIC or
Fortran is, but a micro computer
operating system, much like the KERNAL
used in C-128 native mode and other
6502 based Commodore computers. CP/M
was developed in the 1970's by Digital
Research Inc.  (DRI) for Intel 8080
microprocessor based computers that
allows a given program to run on a
wide variety hardware configurations
with little or no changes.  It was
later extended to run on Zilog Z80
based computers as well as others.
(The Z80 used in the C-128 is
'upwardly compatible' with the 8080.
That is, its machine language
instruction set includes everything
that the 8080 has and more.)  With
CP/M, a software developer has a
uniform set of basic operating system
functions to work with (such as screen
output, keyboard input & disk access
routines).  To the end user, these
system functions remained fixed, no
matter what computer the program is
actually running on, thus providing a
very high degree of portability. (You
need only develop one version of a
program which operates under the CP/M
system instead of having a different
version for brand x, y, and z
computers.) For those who may be
interested, there is also a 16-bit
version of CP/M designed to run on
8086/88 type computers called CP/M-86,
a multi user version called MP/M and a
16/32 bit version for the MC68000 chip
series called CP/M-68k.

CP/M is divided up into four main
components:  the BIOS (Basic Input
Output System); the BDOS (Basic Disk
Operating System), the CCP (Console
Command Processor) and the TPA
(Transient Program Area).  Each of
these parts will be discussed in later
chapters. Of the four parts, only the
BIOS is hardware dependent.  It acts
as the messenger between the other
parts of the system and the actual
hardware. That is, a separate custom
BIOS must be created for each system
that runs CP/M.

CP/M on the C-128
The version of CP/M used on the C-128
is termed 'CP/M Plus' or 'CP/M version
3'.  This is the latest 8-bit CP/M
version, and is also used on other
recent CP/M based computers such as
the Epson CF:X-10, and TRS-80 Model 4.

The minimum configuration for running
CP/M on the C-128 is the C- 128, one
disk drive, and a 40 or 80 column
monitor.  For speed and ease of use, a
fast disk drive (i.e. the 1571 or
1581) is recommended along with the 80
column monitor.  For disk intensive
programs (many CP/M programs are), a
1700 or 1750 RAM expansion unit is
highly recommended as it can be used
as a super fast RAM disk.  A printer
(device 4 or 5) along with a modem
will round out your system.  (Use of
the modem requires CP/M release dated
DEC 85 or later.)

To start up CP/M on the C-128 or 128
D, put the CP/M BOOT disk in disk
drive device 8 and turn on the drive
and then the computer (in that order).
If the computer and drive are already
on, insert the boot disk, then press
the reset button on the C-128 or type
in BOOT followed by pressing <return>,

After a few moments and some sign-on
status message screens, you will see a
prompt on the screen in the form of
the letter A followed by a 'greater
than' sign & a flashing cursor  A>

Congratulations!  You have just
started up CP/M successfully and are
ready to enter your first CP/M
command.  Just for fun, type in the
letters DIR then press <return>.
A 'directory' or 'catalog' of the
contents of the boot disk should be
displayed on the screen.

The Status Line The bottom line of the
screen display (i.e. the 25th line) on
either the 40 column or 80 column
screen is used by CP/M as a status
line to display various system
messages. These are outlined below.

Disk Status
C-128 CP/M has an optional disk status
window displayed at the bottom right
corner of the screen.  The format of
the status window is as follows:

O Dtt ss

where:

 O  = Operation, either R or W
       indicating a Read or Write
 D  = the logical drive (A,B,C,D, etc)
 tt = the track number currently being
       read or written
 ss = the sector number currently
       being read or written

Normally,  the track and sector
numbers are separated by a space. If
the disk is formatted in MFM, and the
track and sector are separated by a
'-', then the second side of the disk
is currently being accessed. You can
toggle this status window ON/OFF by
the <CONTOL>-<RUN/STOP> key
combination.

Note that window is also erased
temporarily, i.e. until the next disk
access, when you (or your application
program) perform a screen clear.

The disk status window is not used for
RAM disk operations (i.e. 1700/1750
RAM expansion unit).  You may also
note that, due to a bug in the fast
bus GCR sector translation routine,
the sector numbers displayed during
reading & writing of both single and
double sided C-128 native GCR type
CP/M disks are incorrect when using a
1571 drive.  Correct numbers are,
however, displayed when using a 1541
drive to access these disks.

MFM Disk Type Selection
C-128 CP/M supports a number of
industry standard MFM disk types (such
as KayPro, Osborne, IBM, Epson, etc.)
when used with a 1571 disk drive.
However, CP/M sometimes cannot
automatically distinguish between two
very similar disk types such as KayPro
II & KayPro IV.  For example, although
the KayPro II is a single sided disk
& the KayPro IV is a double sided one,
CP/M can confuse the two because both
have ten sectors of 512 bytes per
track, numbered 0 to 9 on side 0.
CP/M looks at the physical format of
side 0 to determine the foreign disk
type.  When two or more matches are
found in its internal parameter table,
you are asked to manually chose which
one you want.

When CP/M cannot uniquely identify the
foreign disk format being accessed,
the program will pause momentarily and
the disk type window will open up at
the center of the bottom line of the
screen to display the name of the
first match, such as KayPro II, in
reverse video.  At this point there
are several things that you can do.

First, if you are sure that the disk
is of the type displayed,
simply press <return> or <enter>.
This will select the currently
displayed entry.  If you change disks,
you will have to go through the
selection process over again.

Second, if you are going to be using
this disk format repeatedly and do not
wish to re-select each time you change
disks, you can 'lock in' the selected
type by pressing <control>-<return>.
This action has the effect of
performing a burst mode 'forced login'
on the drive each time you change
disks.  If you insert a new type of
disk while locked in, you will go back
to the selection window.

Note: The forced login commands are
different for the 1571 & 1581 drives
therefore, <control>-<return> should
not be used with a 1581 drive. If you
do use it on a 1581 and change disks,
the results may be unpredictable, even
resulting in system crash

The third option is used to display
and select the alternate disk choices.
Use the cursor left & right keys in
the cursor control keypad (above the
main keyboard) to scroll through the
list of choices.  In the example used
above, the display should change
between KayPro II & KayPro IV.  When
the correct entry is highlighted, you
can use either of the two methods
outlined above for making selections.

Note that because you are in a
suspended program loop, nothing else
can be done until you make a selection

PAUSE
The  key  labelled NO SCROLL executes
a PAUSE function.  Although this key
appears to have the same effect as
<control>-s (stop) & <control>-CF:
(start) for pausing to read screen
dumps, it is not actually related in
terms of the internal workings of the
operating system.  Using NO SCROLL
sends the program into an endless loop
while displaying the message 'PAUSE'
on the status line.  Because of this,
NO SCROLL cannot be used to pause a
display before pressing <control>-p to
toggle the printer, or <control>-c to
abort execution of a program.

To resume execution after a PAUSE
function, press NO SCROLL again (for
AUG or DEC release only) or RETURN
(for MAY release only).

Disk Drives
C-128 CP/M supports a variety of
different disk drives (virtually any
serial bus drive is either supported
directly or can be easily adapted).
However, since CP/M tends to be a disk
intensive operating system (i.e. it
makes heavy use of temporary disk
files, overlay files, working files,
etc.), a fast bus drive (i.e. a 1571
or 1581) is highly recommended for
maximum disk access speed.

CP/M in general can support up to 16
disk drives, usually labelled A: to P:
On the C-128 implementation, normally
only a maximum of 6 drives are
actually supported.  In the default
system configuration, these are:

DRIVE     DEVICE
A:       serial port device 8 drive 0
B:       serial port device 9 drive 0
C:       serial port device 10 drive 0
D:       serial port device 11 drive 0
E:       serial port device 8, drive 0
      (see 'Virtual Disk Drive' below)
M:       17xx RAM expansion unit
      (see 'RAM Disk' below)

Virtual Disk Drive
The  physical disk drive A can be used
as logical drive A or as the 'virtual'
or 'phantom' logical  disk drive E.
Whenever  you access drive E (for
example by doing DIR E:, or PIP
E:=A:FILE1), the system will display a
prompt on the status line asking you
to insert the diskette for drive E
(respond by putting whatever diskette
you want into physical drive A, then
type <return>). The next time you
access logical drive A, the system
will prompt you to insert the A
diskette in drive A.  Again, insert
the desired diskette & type <return>.
Note that CP/M does not actually
check to see if you in fact changed
the disk.  It will accept whatever is
in the drive when you press <return>
as being the correct disk.

RAM Disk
When the 1700 (128k byte) or 1750
(512k byte) RAM expansion unit (REU)
is connected to the C-128 in CP/M
mode, it becomes an ultra fast RAM
disk, normally assigned to CP/M drive
M:. That is, any CP/M command or
transient utility can access the REU
as drive M:, just like it can any
other 'disk drive' connected to the
system. Note that this is totally
separate from the virtual disk drive
discussed above.

Unlike a physical disk drive, the
contents of the RAM disk are not
preserved when the C-128 is turned
off.  However, the contents are
preserved when the reset button is
pressed momentarily to re-boot the
system, even if you enter C-128 native
mode or C-64 mode before returning to
CP/M mode (providing, of course, you
do not alter the contents either
accidentally or intentionally by
running some other program while in
one of these other operating modes).

Note that in order to use the RAM
disk, you must have the C-128 CP/M
release dated 6 DEC 85 or later.

The Screen
C-128 CP/M supports both the 40 column
and 80 column display screens.  The
screen which is initially active is
determined by the position of the
40/80 switch on boot up.  After
booting, the screen can be selected
using the DEVICE utility (see Chapter
3) or equivalent.  The screen can be
changed during the execution of a user
program by setting the CONOUT
redirection flag in the system control
block (see Chapter 9).

The 40 column screen is configured as
a window on a larger 80 column
'virtual' screen.  This allows
commands and programs to think that
they are dealing with an 80 column
wide screen (the default width for
most CP/M programs).

Use <control>-<left arrow> and
<control>-<right arrow> to scroll the
window across the virtual screen (see
Arrow Keys, below).


        CONTINUED IN PART 2