V & T Assembler Modifications

by Alan Marshall

The title of this article is somewhat of a misnomer as most of the modifications are actually made to Nas-Dis, though they are made to facilitate its use with the V & T assembler.

One apparently miraculous feature of Nas-Dis is its ability to disassemble a program directly into a ZEAP source file. Other assemblers are catered for by dis-assembling to tape and then reading the tape into the assembler in its automatic mode. While this works very well, there is no doubt that the ability to dis-assemble directly to the source area is a great advantage and so, using the excellent listing of Nas-Dis, I made the following modifications. These enable the tape version of Nas-Dis to dis-assemble any program directly to the source area of a V & T assembler.

As the V & T works in hex, the ‘£’, or hash, sign is not required, so addresses C4FFH and CF60H are changed from 23H (£) to 30H (0).

The message delimiter used by ZEAP is a “/” while V & T uses “'” so the following addresses need to be changed from 2FH to 27 :–

C6C0
C6D3
C704
CEBF

As the V & T can only put one DEFB on one line then the DJNZ instruction at C6E2/3 needs to be replaced by two NOPs. If this course is followed then the program will assemble with no editing. If, however, the 3 at C6E3 is replaced by 2 then the V & T DEFW can be used, though you must remember to reverse the order of the two bytes while editing. Depending on which course you follow, the 3 at CD6B needs to be changed to either 1 or 2.

The V & T is a little unusual in that it uses EX AF,AFG instead of EX AF,AF' to change to the alternate accumulator so 27H at CA43 needs to be changed to 47H, Another difference is that ZEAP uses a null as the end of line marker while V & T uses 1FH. CEDEH is the place to change.