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BC’ must be saved somewhere in RAM, I leave its actual position up to you, call it BCSAVE. Do not get this variable corrupted!

If you remember, when an IX of IY are used with a displacement, the displacement is always the third byte of an instruction. BC’ will get altered but the print routine will still need to know this third byte. Get a copy of the first byte of the op-code into the A register. Do not alter BC’ – its useful life is not over yet.

This part will test your knowledge of Z80 shifts and other bit-wise operations. Split A into 3 parts F, G and H (not to be confused with the actual register H). These variables must be stored in memory.

If the byte was £4F → 01 001 111 then F would be 1 (Not 64), G would be 1 (Not 4) and H would be 7. Got it? Easy wasn’t it? Now to do something similar to G. G goes into two parts called J and k (held in RAM). J consists of bite 2 and 1 of G, while K is just Bit 0. For the example above … J=0 and K=1. All these variables must be put somewhere safe. I keep emphasising this because I kept corrupting my variables and caused all sorts of things to happen!!

The next part is fundamental to the program. The print-out routine. Call it PRDIS, print the disassembled string. I will define it, but you can write it.

1. No registers or variables may be altered (corrupted). You must save them first (stack?) and restore them later
2. The routine is a subroutine, and as such must end with a £09 RET instruction.
3. Every INDIS in STRDIS is to be output as HL, IX+d or IY+d. Where dis the displacement (BCSAVE is used here). INDEX will hold 0, 1 or 2 respectively.
4. For every IND, print HL, IX or IY, depending on the contents of INDEX, as in (3)
5. If you wish you can print the contents of BCSAVE, this will tell you which location in memory the instruction came from. Nas-Sys is helpful here.

Check your subroutine with dummy data to test it.

Well, all I can say now is best of luck and be careful with those variables. The second part of this series will follow in the next issue.