Extracted from "CP/M2 ON NORTH STAR DISK DOUBLE DENSITY - QUAD
CAPACITY USER'S NOTES BY LIFEBOAT ASSOCIATES"
There are two methods of making changes in your I/O. The SAVEUSER
program is providad as a quick and easy way of making short patches.
The SYSGEN procedure is a comprehensive method of making system
changes.
PATCHING WITH SAVEUSER
Running the SAVEUSER program at any time will write a portion of
the then running CP/M system onto the disk in drive A. The specific
portion written consists of the entire USER AREA (512 bytes or two
pages starting at BIOS+700 hex) and the MODE AREA which is the 128
bytes immediately before the USER AREA. In the distribution 24K
system the MODE AREA starts at 5980 hex and the USER AREA starts at
5A00 hex and ends at 5BFF hex.
Normal changes in the MODE AREA would be modifying the CONFG
byte at 59F8 hex for two sided or fast stepping drives or making a
permanent change in the MODE BYTE. Complete definitions of these are
in Appendix A. Any changes in the USER AREA would be for the purpose
of making a quick patch to your CONSOLE, LIST, PUNCH or READER
routines. These patches could be made with DDT and if they prove
satisfactory, permanently saved on the system disk with SAVEUSER.
To run the program, simply type "SAVEUSER". It will sign on and
ask for confirmation that you really wish to write onto the disk in
drive A. A <CR> response will begin the write. You may also abort
without writing anything by entering control C.
SYSGEN PROCEDURE.
The SYSGEN PROCEDURE is an advanced method of changing your
system configuration. It should only be used when your system is
running properly and you have become completely familiar with the
features of CP/M. This procedure uses the ED.COM editor, the ASM.COM
assembler, the DDT.COM debugger, and the SYSGEN.COM program.
Familiarize yourself with the use of these programs before attempting
to change your system configuration in this way.
This procedure uses the facilities of CP/M to create an assembly
language file containing your customized I/O. Use as a starting
point USER.ASM which contains the proper equates and jump table.
Modify USER.ASM using ED to suit your own purposes. Be sure to
change the equate for MSIZE in the file to the desired memory size of
your CP/M. Next, assemble your modified file with ASM.COM to produce
USER.HEX which will be used to overlay and modify your existing
system.
The existing system is then brought into memory at location 900H
(hex) using the SYSGEN program. This is the standard and most
convenient memory location for making changes in the CP/M system.
The copy of CP/M starting at 900H is usually called the SYSGEN IMAGE.
Save the SYSGEN IMAGE as a disk file by the command "SAVE 40
CPM.COM<CR>".
CPM.COM is then brought back into memory under the control of
DDT with the command "DDT CPM.COM<CR>". The SYSGEN IMAGE at 900H is
then overlaid with USER.HEX using the "I" insert and "R" read
commands of DDT.
Note that USER.HEX is read in with an offset so that the hex
file will be placed at 2700H. The offset changes for each different
CP/M system size and is equal to 2700 hex minus wherever your USER
AREA begins in memory (5A00 hex in standard 24K system). The proper
offset is automatically calculated for each MSIZE by the OFFSET
equate in USER.ASM or the DDT "H" hex calculator may be used as in
the example.
At this point, the new system has been patched with your custom
I/O and may be directly put on the system tracks with SYSGEN or saved
again as a disk file for later use.
