C70:info-cpm (08/03/82)
>From keller@BRL Tue Aug 3 01:25:31 1982
I want to add one tiny routine to my North Star CP/M warm boot.
At 0000H I find C3 03 D3. An easy way to add the routine would
be to make the jump go to the user bios area, and then jump to
D303H. However, I can't find on the DISK the bytes that live at
0000H in the MEMORY once I have booted. Does anyone know how
these three bytes are generated and placed in memory?
GeorgeC70:info-cpm (08/04/82)
>From rconn@BRL Tue Aug 3 23:27:16 1982
Most CP/M systems initialize the low-memory bytes/instructions in
the cold boot routines within the BIOS. Some systems have been known
to also do this in the warm boot routines.
RickC70:info-cpm (08/04/82)
>From FJW@Mit-Mc Wed Aug 4 12:35:01 1982
Whatever you do, don't fool with the contents of the jump at 0, unless
you know exactly what you are doing. Fortunately, you haven't found
those bytes in an obvious place because the BIOS inserts those bytes
"by hand".
What you should do, instead, is change the JMP or CALL address at D303
(or to the address where THAT jump/call points to), and insert your
address there. Make sure your routine, in turn, jumps to the next
address after the patched address, and includes whatever instruction
you may have overlaid if you did not patch over a jump. Do not end
with a RET, of course...
--Frank