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