5 ms·

It's somewhat analogous to how we went from 16 bit registers to 32 bit registers with the extended opcode variants, so if you understand x86, x86_64 isn't reall

It's somewhat analogous to how we went from 16 bit registers to 32 bit registers with the extended opcode variants, so if you understand x86, x86_64 isn't really much of a stretch.

e12e 10y ago
I only have a vague understanding of assembly in general, and x86_64 in general - but I thought that with so many more registers, the recommended style of programming can change quite a bit (hence aggressive use of registers for passing arguments to function calls in the x86_64 C ABIs)?
- dkopi 10y ago
  That's less of an assembly issue and more of a compiler/calling convention issue. Assembly allows you to pass parameters on the stack or in registers. Calling conventions simply define a protocol for doing this consistently.
  - Annatar 10y ago
    Actually Id'd say that it's mostly the crappy processor design issue: for example, UltraSPARC has 32 physical registers in 32-bit mode, and 256 virtual registers (through register windows, specifically designed for compilers). Even Motorola 68000 with eight general purpose address registers and eight general purpose data registers is far more elegant than a 32-bit four register intel CPU. intel CPU is just crap from a design standpoint, and since they had to remain backward compatible, it's gotten a lot faster with lots and lots of tricks, but it still sucks in 32-bit mode. No amount of tricks will change that. It has to be run in 64-bit mode to gain a performance boost and simplify the code, whereas processors with fixed 32-bit instruction encoding run faster in 32-bit mode and code simplicity is a constant.
    - vardump 10y ago
      While x86 is arguably pretty ugly, I think you're being unfair. > ... far more elegant than a 32-bit four register intel CPU. I count eight: EAX, EBX, ECX, EDX, ESI, EDI, EBP, ESP. x86 has also advantage of supporting arbitrary immediate values, so you don't need to allocate registers just for constants.
      - Annatar 10y ago
        eax is the accumulator register, ebx the base register, ecx the counter register, edx the data register, esi is the source index, edi is the destination index, ebp is the base pointer, and esp is the stack pointer. When I originally wrote four general purpose registers, I had eax, ebx, ecx and edx in mind, but after fully listing them above, I revise my earlier statement: the x86 assembler has two general purpose registers. Crappy processor architecture with lots of specialized registers, but too few general purpose ones. Compared to MOS 6502, Motorola MC680##, or SPARC, only the eax and edx are really general purpose registers -- even mentioning ecx, the counter register, would be iffy.
        vardump 10y ago
        6502 has only accumulator anyways. X and Y are not general purpose. 68k is pretty nice, d0-d7 registers are indeed interchangeable. Of course a0-a7 are just for addressing, I think a7 was usually stack pointer. SPARC I've never programmed, so no comments about it. I've written x86 code in the past (20 years ago) using all 8 registers for general purpose task -- yes, even ESP. It was faster that way to implement a texture mapper. Ugly but fast. Those 8 x86 registers are mostly general purpose, apart from some exceptions. Multiplication was the only annoying one, getting result in EAX:EDX. I always succeeded making x86 do whatever I wanted, despite some limitations with register use.
        qb45 10y ago
        Indeed. While repurposing ESP might be rightfully considered ugly, repurposing EBP is quite common and EBX, ECX, ESI, EDI pretty much are general purpose because nobody has been using them for their fixed functions for two decades.

It's somewhat analogous to how we went from 16 bit registers to 32 bit registers with the extended opcode variants, so if you understand x86, x86_64 isn't really much of a stretch.

e12e 10y ago

I only have a vague understanding of assembly in general, and x86_64 in general - but I thought that with so many more registers, the recommended style of programming can change quite a bit (hence aggressive use of registers for passing arguments to function calls in the x86_64 C ABIs)?

dkopi 10y ago
That's less of an assembly issue and more of a compiler/calling convention issue. Assembly allows you to pass parameters on the stack or in registers. Calling conventions simply define a protocol for doing this consistently.
- Annatar 10y ago
  Actually Id'd say that it's mostly the crappy processor design issue: for example, UltraSPARC has 32 physical registers in 32-bit mode, and 256 virtual registers (through register windows, specifically designed for compilers). Even Motorola 68000 with eight general purpose address registers and eight general purpose data registers is far more elegant than a 32-bit four register intel CPU. intel CPU is just crap from a design standpoint, and since they had to remain backward compatible, it's gotten a lot faster with lots and lots of tricks, but it still sucks in 32-bit mode. No amount of tricks will change that. It has to be run in 64-bit mode to gain a performance boost and simplify the code, whereas processors with fixed 32-bit instruction encoding run faster in 32-bit mode and code simplicity is a constant.
  - vardump 10y ago
    While x86 is arguably pretty ugly, I think you're being unfair. > ... far more elegant than a 32-bit four register intel CPU. I count eight: EAX, EBX, ECX, EDX, ESI, EDI, EBP, ESP. x86 has also advantage of supporting arbitrary immediate values, so you don't need to allocate registers just for constants.
    - Annatar 10y ago
      eax is the accumulator register, ebx the base register, ecx the counter register, edx the data register, esi is the source index, edi is the destination index, ebp is the base pointer, and esp is the stack pointer. When I originally wrote four general purpose registers, I had eax, ebx, ecx and edx in mind, but after fully listing them above, I revise my earlier statement: the x86 assembler has two general purpose registers. Crappy processor architecture with lots of specialized registers, but too few general purpose ones. Compared to MOS 6502, Motorola MC680##, or SPARC, only the eax and edx are really general purpose registers -- even mentioning ecx, the counter register, would be iffy.
      - vardump 10y ago
        6502 has only accumulator anyways. X and Y are not general purpose. 68k is pretty nice, d0-d7 registers are indeed interchangeable. Of course a0-a7 are just for addressing, I think a7 was usually stack pointer. SPARC I've never programmed, so no comments about it. I've written x86 code in the past (20 years ago) using all 8 registers for general purpose task -- yes, even ESP. It was faster that way to implement a texture mapper. Ugly but fast. Those 8 x86 registers are mostly general purpose, apart from some exceptions. Multiplication was the only annoying one, getting result in EAX:EDX. I always succeeded making x86 do whatever I wanted, despite some limitations with register use.
        qb45 10y ago
        Indeed. While repurposing ESP might be rightfully considered ugly, repurposing EBP is quite common and EBX, ECX, ESI, EDI pretty much are general purpose because nobody has been using them for their fixed functions for two decades.