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ARM does have microcode (though early ones didn't). Basically if you don't have microcode, you kinda tie your instruction set to your physical architecture, wh
by CHY872 12y ago
ARM does have microcode (though early ones didn't).
Basically if you don't have microcode, you kinda tie your instruction set to your physical architecture, which makes it hard to progress.
For example, at the time of the Pentium 3, all instructions were 64 bit - execution units, buses, register renaming tech. SSE had 128 bit vector instructions, which would screw with this. For this reason, Intel split them into 2 64 bit micro-instructions.
ARM generally doesn't have patchable microcode, which might be the source of your confusion?
- zurn 12y agoYou are confusing the concept of uops (or micro-instructions) with microcode. Microcode is when the CPU runs microcode from a microcode ROM to implement an instruction. In CISC days this was often all instructions (these were termed microcoded CPUs). The same architecture might have cheap microcoded and high end "hardwired" implementations. There's a conceptual similarity between uops and microcode, to quote the WP microcode article: "Modern CISC/RISC implementations, e.g. x86 designs, decode instructions into dynamically buffered micro-operations with instruction encodings similar to traditional fixed microcode. Ordinary static microcode is used as hardware assistance for complex multistep operations such as auto-repeating instructions and for transcendental functions in the floating point unit; it is also used for special purpose instructions (such as CPUID) and internal control and configuration purposes."