6502 ML programming

[Jim Choate <ravage@ssz.com>]

Forwarded message:

> Date: Wed, 8 Oct 1997 23:10:26 -0400
> From: ghio@temp0132.myriad.ml.org (Matthew Ghio)
> Subject: Re: [LONG, off-topic]] Interactive Programming

> Reminds me of a time, years ago, where I was trying to modify a program
> written in 6502 assembler.

> Consider the following line of C code:
> 
>   result = function(x,y,z);
> 
> One could write this in 6502 assembler as:
> 
>   JSR function
>   DATA x
>   DATA y
>   DATA z
>   STA result
> 
> where x,y,z,result are pointers to storage locations.

There in no 'DATA' construct in 6502 assembly.

This is some sort of macro that either the assembler or programmer
defined. All official Rockwell/Commodore [1] 6502 assembly mnemonics are
three letter. There are NO official macro's from MOS Technologies under
either Rockwell or Commodore other than the ORG. ORG simply defined the
starting address for the target code.

The most efficient way to store data in 6502 is to put it in the first 256
bytes of ram, then it could be called in no more than 2 clock cycles. Also,
in 6502 good programmers use the branch instructions (BCC, BCS, BEO, BMI,
BNE, BPL, BVC, BVS) because the JSR required three bytes versus 2 bytes for
a Bxx. The only constraint was that the branch target must reside in the same
256 byte page as the branch itself. The biggest boo-boo committed here was not
realizing that branch offsets that took you across the upper page boundary
would leave you at the lower page boundary plus some remaining offset.

Idealy a jump table was created that had the various target addresses for your
routines. Then a Bxx followed by a indirect mode JMP (NOT a JSR). If that table
was in the first 256 bytes it was possible to save a whole clock cycle AND a
byte in the process. At 64k and 1Mhz clock these sorts of short-cuts were
significant.

Because of the architecture of the 6502 the 0 page of RAM was a very busy
place indeed because any operation there automaticaly saved a byte and a
clock cycle.



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[1] Rockwell designed and sold the chip for several years and then in the
    late 70's Commodore bought MOS Technologies.