VIC-1001 in I/O magazine (JAP)

Mike · Post by **Mike** » Tue Oct 12, 2021 11:09 pm

The VIC-1001 once got quite some coverage in the Japanese I/O magazine.

There are several (good!) PDF scans of the magazine to be found on archive.org, and I set out to type in EX.MON, published in the 09/1982 issue, on pages 356, 357 and 358:

The tool features a memory hex dump editor and a direct assembler and disassembler, loads into the unexpanded VIC-20 (use LOAD "...",8,1) and it is 1.5 KB in size (download).

Start with SYS 4352
Enter 4 digit hex number (without $) to either disassemble a line or dump 8 bytes in hex
Changes are applied by overtyping disassembly or hex dump and entering line with RETURN
Address prompt auto-increments
X command switches to assembly mode
Y command switches to hex dump mode
Z command quits EX.MON and returns to BASIC

While I tested the tool, I found several idiosyncrasies (for not to say, "misfeatures" or "bugs"):

(Address) operands generally omit "$"
Branch target addresses are prefixed with ":"
Address mode (ZP,X) uses non-standard syntax "(ZP),X" as operand
LDX ZP,Y and STX ZP,Y use non-standard syntax "ZP,X" as operand
LDX ABS,Y uses non-standard syntax "ABS,X" as operand (could be fixed by swapping $147E/$147F)
CLD is not accepted (use "CDL" instead - could be fixed by swapping $15E7/$15E8)

That means, EX.MON successfully assembles and disassembles all 151 documented opcodes of the NMOS 6502 - albeit some of them with non-standard syntax. I kept the last two points above as they are, for preservation purposes. Fixing "(ZP),X" and the missing ZP,Y address mode likely requires some more reversing.

Greetings,

Michael