[Project Log] Python on the 6502/C64, 8080, 6800, 6809 and AVR

[Bill]

@dave

The 6502 instruction set is kind of spare when you compare it with, for example, the 6809 (or even the lesser 6800, Z80, and 8080). However, even when you have to use several instructions in place of a single 6809 instruction, the number of cycles ends up being comparable. The code density is what suffers, and that’s because the 6502 is intentionally RISC. For many small embedded applications, the code density is comparable and the 6502 uses many fewer cycles.

Actually, the 6800 and 6809 are very similar to the 6502 in that they take one machine cycle to do a memory read or write or ALU operation, unlike the 8080 or Z80 which takes around 3 T cycles. If you can get it into the X register, the 680x can do a 16-bit increment or decrement in 4 machine cycles. The 680x has two capable accumulators. The advantage the 6502 has is that it pipelines the fetch of the next instruction.

@dave

A good macro assembler can make the 6502 much less tedious and can improve readability.

My assembler does macros. But there are many flavors of operations: zeropage or absolute addressing versus indirect using register Y, simple increment or decrement versus adding or subtracting a constant or another variable. Then there are standalone operations versus ones to be done in a sequence where a little of gain can be had by keeping a byte in the X register. If I create macros to cover every case, it will begin looking like the ARM with its conditional execution and optional updating of condition codes.