dest = dest - source
The subsd instruction subtracts the source value (second operand) from the destination (an XMM register). The source can be an XMM register or a 64 bit memory location. There is also vsubsd on CPUs with AVX instructions which allows using 3 XMM registers or 2 XMM registers and a memory location which can simplify coding.
subsd xmm0, xmm1 ; subtract xmm1 from xmm0
; leave the rest of xmm0 as is
subsd xmm0, [x] ; subtract 64 bit variable x from xmm0
; leave the rest of xmm0 as is
subsd xmm0, [rsi] ; subtract 64 bit value [rsi] from xmm0
; rsi holds the address of a double
; leave the rest of xmm0 as is
vsubsd xmm3, xmm0, xmm15 ; xmm3 = xmm0 - xmm15
; Note: operates by copying 128 bits from xmm0 to xmm3
; before subtracting. This might occasionally be useful.
; The rest of ymm0 is left as is
; using subsd would change either xmm0 or xmm15
; possibly requiring an additional instruction
vsubsd xmm3, xmm0, [x] ; xmm3 = xmm0 - x