9.16.5 Register Naming

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 ‘xmm0’–‘xmm15’ registers.

The AVX2 extensions made in 64-bit mode more registers available:

• the 16 128-bit registers ‘%xmm16’–‘%xmm31’ and the 16 256-bit registers ‘%ymm16’–‘%ymm31’.

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’.