• frezik@midwest.social
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    6 months ago

    http://www.quadibloc.com/arch/sriscint.htm

    The RISC architecture contains several common elements. Some of them are no longer present in most chips that still call themselves RISC:

    • All instructions execute in a single cycle.
    • Floating-point operations, specifically, are therefore excluded.

    But most of the defining characteristics of RISC do remain in force:

    • All instructions occupy the same amount of space in memory.
    • Only load, store, and jump instructions directly address memory. Calculations are performed only between operands in registers.

    https://groups.google.com/g/comp.arch/c/IZP5KUJprHw?pli=1

    MOST RISCs:
    3a) Have 1 size of instruction in an instruction stream
    3b) And that size is 4 bytes
    3c) Have a handful (1-4) addressing modes) (* it is VERY hard to count these things; will discuss later).
    3d) Have NO indirect addressing in any form (i.e., where you need one memory access to get the address of another operand in memory)
    4a) Have NO operations that combine load/store with arithmetic, i.e., like add from memory, or add to memory. (note: this means especially avoiding operations that use the value of a load as input to an ALU operation, especially when that operation can cause an exception. Loads/stores with address modification can often be OK as they don’t have some of the bad effects)
    4b) Have no more than 1 memory-addressed operand per instruction
    5a) Do NOT support arbitrary alignment of data for loads/stores
    5b) Use an MMU for a data address no more than once per instruction
    6a) Have >=5 bits per integer register specifier
    6b) Have >= 4 bits per FP register specifier

    Note that none of this has to do with reducing the number of instructions, which is what people tend to think of when they hear the name.

    • barsoap@lemm.ee
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      6 months ago

      All instructions occupy the same amount of space in memory.

      Both ARM and RISC-V have compressed instructions. Dunno how ARM works but with RISC-V the 16-bit instruction set is freely interspersable with the 32 bit one, which also get their alignment reduced to 16 bits. Gets like 95% of the space reduction possible with full variable-width instructions without overcomplicating the insn decoder.

      As to addressing and loads and arithmetic: No such instructions, but every CPU but the tiniest ones are expected to do macro-op fusion for things like indexed loads. Here’s an overview.

      The MMU thing… well the vector extension can do gather/scatter, I guess it could stay within the letter of “use the MMU once” but definitely not the spirit.