Sunday, November 28, 2010
The Original RISC Chip
I can see why the 6502 was once called "the original RISC processor." It's not, strictly, a RISC design, more of a parallel invention, but the BBC Micro used a 6502, and Acorn Research went on to produce the ARM architecture on similar principals. Looking at the instruction set, and how the registers work, it looks like you could even get it pretty close to the one-clock-per-instruction ideal, if you ignore the extra data fetches. It explains why it outperformed the z80.
I've only got experience with the 6502 and ARM instruction sets, and they're quite similar, the ARM being a heavily upgraded (and incompatible) version of the former. You've got arithmetic operations, memory operations, and program control, and that's it. ARM added multiplication which never gets used. Oh, and a bazillion registers.
Looking at the 8086 instruction set, it's got stuff that I don't even know what to do with. String operations, right there in machine language. An operation to compare two values in memory (the extra microcode to take care of that is probably slower than loading the values to registers, then comparing). Arithmetic shifts? Probably all useful stuff if C didn't exist and you don't have a macro assembler, though. Which was pretty much true with the original IBM PC.
VAX has a million different instructions, in fact it almost looks like it was created to run UNIX. Probably not too far from the truth. It has instructions to deal with polynomials.
Rambling now. I should probably go do something else.
¶ 1:22 PM
