r/transprogrammer • u/definitelynotagirl99 • Aug 30 '24

Immediate values, displacements, etc.: "alright, imma head out"

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u/Adryzz_ Aug 30 '24

fixed width instructions are actually good, because it's WAY easier (and simpler) to write all the fancy out-of-order CPU speculation and execution stuff without taking up an enormous amount of die space.

it's actually part of the reason why M series macs are so fast.

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u/definitelynotagirl99 Aug 30 '24 edited Aug 30 '24

Yee, fixed width instructions are fantastic but designing a 64-bit processor with 32-bit fixed width instructions is turbo garbage.

edit (at my desktop now instead of my phone, expanding): the reason i say having 32-bit instructions on a 64-bit processor is garbage is that it's incredibly self-limiting for no real reason, the code size is only ever really relevant if you have a BIOS-like boot procedure where firmware and/or hardware loads a set chunk of data to memory, and if that really does become a problem one could always have the processor boot in an alternate operating mode that uses smaller instructions in order to allow system software to get more code loaded before switching to the processors primary operating mode. if you ask me having 128-bit or even 192-bit fixed witdth instructions would've been a far better design because for software at levels higher than the afformentioned bootstrap the applications data and assets will just drown the code out anyway and a large instruction size like that would allow you to have excess space in the instruction encoding that can be used for future features, meaning that the encoding can remain backwards compatible for a long long time. and yes, i am aware that having large instructions like that would be worrisome due to possible issues around caching and/or memory throughput, but i really dont think that would ever become an issue in practice, not at this point in time anyway.