At 11:39 PM 10/8/97 -0500, Jim Choate wrote:
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.
I concur.
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
This is not quite correct. I can't seem to find my Commodore 128 Programmer's Reference Guide, (of all the things I lost going to the PC, I miss my PRG the most. It contains complete BASIC & assembler language reference guides, lists EVERY memory address used by the OS and what it does, register-level programming info on every chip, and even schematics for my 128! Unfortunately, nobody seems to publish such a book for PC's, but I digress.) The branching instructions used 1 byte for the in struction, followed by 1 data byte which was a signed integer. This allowed one to branch up to 127 bytes ahead, or 128 bytes back from the current instruction address register value. The coolest thing about this was that if you were able write your code to branch and not JMP or JSR, your code could be located anywhere in memory and it would work perfectly, which was very useful when writing interrupt wedges that were supposed to work on the C-64 and the C-128, or that could be loaded in various places in memory. 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.
This is where self-modifying code came in handy. During initialization, a self-mod routine would increment the addresses in the jump table so they pointed to the proper addresses of the respective routines.
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.
If I remember correctly, there were all of 5 or 10 bytes available for "user programs", depending on which OS functions you were willing to break. My first computer was a Commodore VIC-20. 5 points and Geek of the Week to the first email correctly specifying the "Basic Bytes Free" on a vanilla VIC-20. Real Programmers don't use symbolic addresses. Jonathan Wienke What part of "the right of the people to keep and bear Arms, shall not be infringed" is too hard to understand? (From 2nd Amendment, U.S. Constitution) PGP 2.6.2 RSA Key Fingerprint: 7484 2FB7 7588 ACD1 3A8F 778A 7407 2928 DSS/D-H Key Fingerprint: 3312 6597 8258 9A9E D9FA 4878 C245 D245 EAA7 0DCC Public keys available at pgpkeys.mit.edu. PGP encrypted e-mail preferred. Get your assault crypto before they ban it! US/Canadian Windows 95/NT or Mac users: Get Eudora Light + PGP 5.0 for free at http://www.eudora.com/eudoralight/ Get PGP 5.0 for free at http://bs.mit.edu:8001/pgp-form.html Non-US PGP 5.0 sources: http://www.ifi.uio.no/pgp/ http://www.heise.de/ct/pgpCA/download.shtml ftp://ftp.pca.dfn.de/pub/pgp/V5.0/ ftp://ftp.fu-berlin.de/pub/pc/win95/pgp ftp://ftp.fu-berlin.de/pub/mac/pgp http://www.shopmiami.com/utopia.hacktic.nl/pub/replay/pub/pgp/pgp50/win/ RSA export-o-matic: print pack"C*",split/\D+/,`echo "16iII*o\U@{$/=$z;[(pop,pop,unpack"H*",<> )]}\EsMsKsN0[lN*1lK[d2%Sa2/d0<X+d*lMLa^*lN%0]dsXx++lMlN/dsM0<J]dsJxp"|dc`