Re: Getting back into z80 programming (code included)

[Andrew Collier]

On 24 Oct 2009, at 23:46, Thomas Harte wrote:

By the way:

      ld d, (NUMVERTS)

I don't think you can do this?

No, you're right, you can't. It silently substituted ld a, (NUMVERTS),
so that loop was running quite a bit longer than it needed to and the
result not being visibly different unless the polygon hits the first
scanline. So easy to miss.


Which version of pyz80 are you using? For me, this instruction is  
caught by a testcase:

$ cat > test.z80s
NUMVERTS: db 0
ld d,(NUMVERTS)
$ pyz80 test.z80s
pass  1 ...
Error: Illegal combination of operands
test.z80s:1 "ld d,(NUMVERTS)"
Error: OpCode not recognised
test.z80s:1 "ld d,(NUMVERTS)"

Presumably your longer code sequence is catching it out somehow...

As an aside, it's rather unfortunate that zilog chose to use  
parentheses to denote memory dereferences, as they're ambiguous with  
mathematical ordering operators. It's not immediately obvious that the  
following examples generate entirely different instructions, but that  
is a consequence of the only useful way I could parse them!

        ld hl,(NUMVERTS + 1)
        ld hl,(NUMVERTS) + 1

Andrew

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