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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. ____________________________________________________________________ | | | The financial policy of the welfare state requires that there | | be no way for the owners of wealth to protect themselves. | | | | -Alan Greenspan- | | | | _____ The Armadillo Group | | ,::////;::-. Austin, Tx. USA | | /:'///// ``::>/|/ http:// www.ssz.com/ | | .', |||| `/( e\ | | -====~~mm-'`-```-mm --'- Jim Choate | | ravage@ssz.com | | 512-451-7087 | |____________________________________________________________________| [1] Rockwell designed and sold the chip for several years and then in the late 70's Commodore bought MOS Technologies.