dest = dest - source
The subss instruction subtracts the source value (second operand) from the destination (an XMM register). The source can be an XMM register or a 32 bit memory location. There is also vsubss on CPUs with AVX instructions which allows using 3 XMM registers or 2 XMM registers and a memory location which can simplify coding.
subss xmm0, xmm1 ; subtract xmm1 from xmm0 ; leave the rest of xmm0 as is subss xmm0, [x] ; subtract 32 bit variable x from xmm0 ; leave the rest of xmm0 as is subss xmm0, [x+4*rcx] ; subtract element of array x from xmm0 ; rcx contains the array index vsubss 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 subss would change either xmm0 or xmm15 ; possibly requiring an additional instruction vsubss xmm3, xmm0, [x] ; xmm3 = xmm0 - x