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> ARM borrowed most of its design from 6502 Not really (perhaps leaving mnemonics aside). The 6502 has little in common with the first ARM. The ARM's designers
by klelatti 2y ago
> ARM borrowed most of its design from 6502
Not really (perhaps leaving mnemonics aside). The 6502 has little in common with the first ARM. The ARM's designers liked the simplicity and speed of the 6502 and it was their favourite 8-bit ISA. And a visit to the Western Design Center convinced them to design their own CPU.
However, the early RISC papers were the biggest influence on the design of the ARM. There is even a clue in the name 'Acorn RISC Machine'.
- rbanffy 2y agoI believe both are true. Early ARM feels like a refreshed 6502.
- klelatti 2y agoCould you say why? It doesn’t feel much like it to me at all!
- rbanffy 2y agoARM designers liked and had extensive experience with 6502 - it WAS their bread and butter for a long time - and this might be why the mnemonics are so similar (and carry in subtraction - that might have been for making porting easy, but I won't risk assuming that without a primary source). They obviously also studied foundational papers on RISC and understood the possibility of having a simple design (such as the 6502, which was very powerful considering its small transistor count) applied to a simple, more regular, instruction set. Assigning weight to these factors might a futile exercise, as the designers themselves might not agree. They were not blind to RISC and, therefore, it made sense to put it in the name of the architecture.
- klelatti 2y agoCompletely agree with all these points and that without the 6502 the ARM1 would not have looked like it did. One might say ARM1 was inspired by the 6502 and it prevented them going down the CISCy 68k route. But I don't think that ARM 'ARM borrowed most of its design from 6502' or (to my mind at least) looks like a refreshed 6502. There are just too many fundamental differences: - ARM1 was a load/store architecture / 6502 wasn't. - 6502 had a few special purpose registers / ARM1 had loads of general purpose registers. Plus there are lots of key innovations in ARM1 that weren't in either 6502 or RISC 1 such as conditional execution. Furber and Wilson were really quite innovative and didn't just borrrow ideas from other ISAs.
- rbanffy 2y ago> Furber and Wilson were really quite innovative and didn't just borrrow ideas from other ISAs. Nobody can ever question that.
- Dwedit 2y agoThings like conditional execution being always available, and the barrel shifter being always available feel a lot more like ideas from VLIW architectures. And when you come from 8-bit instructions and suddenly have 32 bits available, your instructions are a very long word.