5.27.5 AMD64 (x86_64) Assembler

The AMD64 assembler is a slightly modified version of the 386 assembler, and as such shares most of the syntax. Two new prefixes, .q and .qa, are provided to select 64-bit operand and address sizes respectively. 64-bit sizes are the default, so normally you only have to use the other prefixes. Also there are additional register operands R8-R15.

The registers lack the ’e’ or ’r’ prefix; even in 64 bit mode, rax is called ax. Additional register operands are available to refer to the lowest-significant byte of all registers: R8L-R15L, SPL, BPL, SIL, DIL.

The Linux-AMD64 calling convention is to pass the first 6 integer parameters in rdi, rsi, rdx, rcx, r8 and r9 and to return the result in rax and rdx; to pass the first 8 FP parameters in xmm0–xmm7 and to return FP results in xmm0–xmm1. So abi-code words get the data stack pointer in di and the address of the FP stack pointer in si, and return the data stack pointer in ax. The other caller-saved registers are: r10, r11, xmm8-xmm15. This calling convention reportedly is also used in other non-Microsoft OSs.

Windows x64 passes the first four integer parameters in rcx, rdx, r8 and r9 and return the integer result in rax. The other caller-saved registers are r10 and r11.

Here is an example of an AMD64 abi-code word:

abi-code my+  ( n1 n2 -- n3 )
\ SP passed in di, returned in ax,  address of FP passed in si
8 di d) ax lea        \ compute new sp in result reg
di )    dx mov        \ get old tos
dx    ax ) add        \ add to new tos
ret
end-code

Here’s a AMD64 example that deals with FP values:

abi-code my-f+  ( r1 r2 -- r )
\ SP passed in di, returned in ax,  address of FP passed in si
si )       dx mov         \ load fp
8 dx d)  xmm0 movsd       \ r2
dx )     xmm0 addsd       \ r1+r2
xmm0  8 dx d) movsd       \ store r
8 #      si ) add         \ update fp
di         ax mov         \ sp into return reg
ret
end-code