From October 1980 through June 1988, Bob Sander-Cederlof published a newsletter called Apple Assembly Line. This newsletter focussed on assembly language for the Apple ][, //e, //c, and //gs computers, all of them based on the 6502 processor and its derivatives. Bob was the author of the S-C Macro Assembler, which lived as long as the newsletter, i.e., until 1988.
Since a few months ago, there's an Apple Assembly Line [online] Archive that includes all the AAL newsletters. It’s very fun, and instructive, to have a look at them, even if you don’t have an Apple ][. You can still pick some Steven Wozniak trick for your assembly programming, or learning how to add or substract one. If you know a bit of assembly, you’ll understand the code quickly (in fact, it looks a lot like Don Knuth’s MIXAL in TAOCP, and i’ve been able to run some code on my MIX simulator with minimal modifications).
If you’re thinking that you will learn little from a twenty years old newsletter, please try your hand at discovering what the DO.SOME.MAGIC subroutine below computes:
1010 *-------------------------------- 1020 BYTE .EQ 0 1030 *-------------------------------- 1040 T LDA #0 1050 STA BYTE 1060 .2 LDX #14 1070 .1 CPX #7 1080 BNE .4 1090 LDA #$A0 1100 JSR $FDED 1110 .4 JSR DO.SOME.MAGIC 1120 JSR $FDDA 1130 INC BYTE 1140 BEQ .3 1150 DEX 1160 BNE .1 1170 JSR $FD8E 1180 JMP .2 1190 .3 RTS 1200 *--------------------------- 1210 DO.SOME.MAGIC 1220 LDA BYTE 1230 LSR 1240 LSR 1250 LSR 1260 ADC BYTE 1270 ROR 1280 LSR 1290 LSR 1300 ADC BYTE 1310 ROR 1320 LSR 1330 LSR 1340 RTS 1350 *--------------------------------
If there’s some mnemonic that you don’t understand, here you have a complete reference of the 66 instructions understood by the 6502. The solution is somewhere in the AAL archive, but you will have a far greater time if you try to solve this little riddle by yourself. And if you get all worked up and want to try more algorithms for real, this page points to some 6502 emulators and assemblers. Have fun!
