Hi Kurt,
Oh, I okay, I see. Yeah, this took me a minute to figure out back in my
days of disassembling lots of M100 ROM code.
Good old Mr. Gates found a way to really squeeze instructions into the
ROM. This is an example of a multi-entry point function. If you write
code to JMP 6F52H, then you get
MVI A,01H
LXI B,023EH
LXI B,033EH
PUSH H
This results in A being loaded with 1
But entering at 6F55H looks like:
MVI A,02H
LXI B,033EH
PUSH H
(i.e. A is loaded with 2)
And entering at 6F58H looks like:
MVI A,03H
PUSH H
So this routine can be entered from 3 different locations to perform 3
different functions (loaded into A as 01H, 02H or 03H) and it only costs
a couple of extra bytes. It uses dummy LXI B,#### opcodes (which only
cost 1 byte for the 01H) to "hide" the "MVI A,02" (3EH,02H) and "MVI
A,03" (3EH,03H) opcodes and make them look like the LXI B opcodes,
loading B with dummy values of 023EH and 033Eh).
Ken
On 4/10/26 1:35 PM, Kurt McCullum wrote:
Thanks Ken,
I do have an example.
1. Put a breakpoint at 6F58
2. Go to TELCOM and type TERM
3. Look at the Instruction Trace. It will say 6F58s MVI A, 03H
4. Go to the ROM source listing.
You will notice there is no command at 6F58H. Instead it says
; ======================================================
; Initialize RS232 or modem
; ======================================================
6F52H (3EH) MVI A,01H
6F54H (01H) LXI B,023EH
6F57H (01H) LXI B,033EH
6F5AH (E5H) PUSH H
To me it looks like the debugger got off track at the bottom of the ROM.
Kurt
On Fri, Apr 10, 2026, at 3:06 PM, Kenneth Pettit wrote:
Hi Kurt,
Umm, I believe the disassembler *should* be looking at the ROM if I
remember correctly. Could just be a latent bug. Do you have examples
of discrepancies?
Ken
On 4/10/26 12:35 PM, Kurt McCullum wrote:
I am running into a problem with the disassembler for the main ROM
in Virtual-T for the NEC 8201a.
In particular, the RS232 initialize routines don't match what the
ROM and debugger show as far as code.
For the main ROM, is the disassembler actually looking at the ROM,
or something else?
Kurt
