Disassembler Part II

by M. Reed

Welcome to Part II – I hope that you managed to struggle through Part I alright, but don’t worry, it can only get worse. By now you should have your print routine working to your satisfaction – called PRDIS.

Now there are 9 subroutines to write, yes 9! and also a lot of data. One of these, is a sort of controller, and calls one of the remainder depending upon the data it is fed. A great deal of the disassembler is a great deal of data, organised into tables.

I will define the control routines;

i)

On entry HL’ contains a pointer to some data. This holds the routine number to which control will pass. This routine number is held in the least 3 significant bits (0, 1, 2) so ANDing the contents of HL’ [ie (HL&rsquo)] with 7 will give this number. Don’t forget to increment HL’ to point to the next piece of valuable data.

ii)

Bits 3, 4 and 5 may contain data that the called subroutine will need, though not the only data it may need. Consequently the data byte [ie (HL’) before incrementing!!] must be put somewhere – just in case.

iii)

Bit 7 of this data byte is used as a sort of ‘stop’ flag. If it is not set the Control will loop back on itself in order to call the next routine. If it is set then Control will break out of the loop, call PRDIS and then move onto the next op-code to be disassembled.

Obviously HL” must contain the correct address. This is achieved by creating a table of twelve addresses, corresponding to the 12 combinations of TYPE and F. Let me explain, TYPE represents whether the instruction to be disassembled began with £ED, £CB or neither. It can only have 3 possible values – use 0, 1 and 2 because life is so much easier. F will hold bits 6 and 7 of the instruction being disassembled. Therefore, it has 4 possible values. Hence 12 combinations. There should be an address in your table for each possible case. Get it into HL’.

This should cause no problems if you order your table correctly. For example: (and may I recommend this method)

Now the first byte of each address is given by TABST + (2 x