Register operands are always prefixed with %. The 80386 registers consist of
- the 8 32-bit registers %eax (the accumulator), %ebx, %ecx, %edx, %edi, %esi, %ebp (the frame pointer), and %esp (the stack pointer).
- the 8 16-bit low-ends of these: %ax, %bx, %cx, %dx, %di, %si, %bp, and %sp.
- the 8 8-bit registers: %ah, %al, %bh, %bl, %ch, %cl, %dh, and %dl (These are the high-bytes and low-bytes of %ax, %bx, %cx, and %dx)
- the 6 section registers %cs (code section), %ds (data section), %ss (stack section), %es, %fs, and %gs.
- the 5 processor control registers %cr0, %cr2, %cr3, %cr4, and %cr8.
- the 6 debug registers %db0, %db1, %db2, %db3, %db6, and %db7.
- the 2 test registers %tr6 and %tr7.
- the 8 floating point register stack %st or equivalently %st(0), %st(1), %st(2), %st(3), %st(4), %st(5), %st(6), and %st(7). These registers are overloaded by 8 MMX registers %mm0, %mm1, %mm2, %mm3, %mm4, %mm5, %mm6 and %mm7.
- the 8 128-bit SSE registers registers %xmm0, %xmm1, %xmm2, %xmm3, %xmm4, %xmm5, %xmm6 and %xmm7.
The AMD x86-64 architecture extends the register set by:
- enhancing the 8 32-bit registers to 64-bit: %rax (the accumulator), %rbx, %rcx, %rdx, %rdi, %rsi, %rbp (the frame pointer), %rsp (the stack pointer)
- the 8 extended registers %r8%r15.
- the 8 32-bit low ends of the extended registers: %r8d%r15d.
- the 8 16-bit low ends of the extended registers: %r8w%r15w.
- the 8 8-bit low ends of the extended registers: %r8b%r15b.
- the 4 8-bit registers: %sil, %dil, %bpl, %spl.
- the 8 debug registers: %db8%db15.
- the 8 128-bit SSE registers: %xmm8%xmm15.
With the AVX extensions more registers were made available:
- the 16 256-bit SSE %ymm0%ymm15 (only the first 8 available in 32-bit mode). The bottom 128 bits are overlaid with the xmm0xmm15 registers.
The AVX512 extensions added the following registers:
- the 32 512-bit registers %zmm0%zmm31 (only the first 8 available in 32-bit mode). The bottom 128 bits are overlaid with the %xmm0%xmm31 registers and the first 256 bits are overlaid with the %ymm0%ymm31 registers.
- the 8 mask registers %k0%k7.