A MEMORY RESIDENT
                       FLOPPY DISK PROGRAM

                               by

                        Michail J. Karas




     This   is   from   the  Lifelines/The   Software   Magazine, 
September1982 issue,  about ADD-ON MODULE Construction. An add-on 
module  is designed for use under single user  CP/M-80  2.2.  The 
next several paragraphs, from the previous articles, January, and 
February  1982,  show  the  procedure  to get a  version  up  and 
running.

The source file,  RAMDISK.ASM,  being very similar to a BIOS  for 
CP/M-80 2.2, requires the extension disk definitions to have disk 
parameter  tables,   check  vector  and  allocation  vectors.  In 
addition,  a directory buffer must be allocated. An ADD-ON MODULE 
generates  the  appropriate tables through the use of  the  macro 
library  DISKDEF.LIB provided by Digital Research on the  CP/M-80 
2.2 distribution diskette. This macro capability requires a macro 
assembler to properly process the DISKDEF macros.

An  additional implementation requirement is the availability  of 
the  assembler  output  as  a .REL  file.  The  Digital  Research 
Relocating  Macro Assembler RMAC is just the tichet  to  generate 
the  appropriate  .REL  file and process  the  DISKDEF.LIB  macro 
includes.  The  command structure below shows how to assemble  an 
ADD-ON  MODULE program like the RAM disk demo listing on the disk 
as RAMDISK.ASM. The following is an example on the assembly.

     A>RMAC RAMDISK $+S PZ<cr>

The resulting .REL file has to be converted to a page relocatable 
format.  After agonizing over the problem of easily making a "BIT 
MAP",  I found that the Digital Research Link program distributed 
with  RMAC and PL/I-80 can generate page relocatable files  (.PRL 
type) which are compatible with MP/M-80. After reviewing the LINK 
output format of a .PRL file,  I designed the PRLMOVE program  to 
relocate the LINK output files. The command structure below shows 
how to produce the required .HEX file from PRLMOVE.

     A>MAC PRLMOVE $+S PZ<cr>

The  command  structure below shows how to produce the .PRL  file 
from the RMAC output.

     A>LINK RAMDISK[OP]<cr>

where  [OP] directs LINK to generate the desired .PRL  file.  The 
program PRLMOVE.HEX and the page relocatable RAMDISK.PRL must  be 
combined into a single executable command file as follows:
A>DDT<cr>
DDT Vers. 2.2
-IRAMDISK.PRL<cr>
-R<cr>                   <-Load  of .PRL file to RAM to  address 
                           0100H  with  code image  starting  at 
                           address 0200H.
MEXT PC
nnmm  0000               <-Convert nn to decimal  and  remember 
                           value.
-IPRLMOVE.HEX<cr>
-R<cr>                   <-Read  PRLMOVE program in  over  the 
                           .PRL file at load address of 0100H.
-G0<cr>                  <-Exit DDT to CP/M-80.

A>SAVE dd RAMDISK.COM    <-Save dd pages of memory to get command 
                           file. dd = converted nn from above!

This  results  in  the  .COM command file necessary  to  make  an 
executable module out of the ADD-ON MODULE.

The present design of the ADD-ON MODULE with the negative  offset 
pointer calculation reentry to CCP requires that the BDOS and CCP 
combination  be genuine (REAL Digital Research CP/M-80 Ver 2.2 of 
total length 01600H bytes. If your system uses a modified BDOS or 
CP/M-80 system look-alike,  then the negative offset to the stack 
pointer will require a different value. See NOT$PRES: in listing. 
Also,  for non-genuine CP/M-80 systems, if the CCP is a different 
size  (not  0800H)  bytes,  the program PRLMOVE will have  to  be 
modified  to set down the relocation load address calculation  by 
an amount equal to the difference in the alien CCP size.