dest = max(source)
The maxss, maxsd, maxps, and maxpd instructions computes the maximum of the source and the destination and store the result(s) in the destination operand. There are also vmaxss, vmaxsd, vmaxps, and vmaxpd on CPUs with AVX instructions which can compute multiple values at once. Behaviorally these instructions are similar to the various add instructions should you wish to see more details.
maxss xmm0, xmm1 ; xmm0 gets 32 bit max of xmm1 maxsd xmm0, [x] ; xmm0 gets 64 bit max of x maxps xmm0, [rdi] ; xmm0 gets 4 32 bit maxs of [rdi] maxpd xmm0, [x+4*rcx] ; xmm0 gets 2 64 bit maxs of array x vmaxps xmm3, xmm4, xmm15 ; xmm3 gets 4 32 bit maxs of xmm4 & xmm15 vmaxps ymm3, ymm4, ymm15 ; ymm3 gets 8 32 bit maxs of ymm4 & ymm15 vmaxpd ymm3, ymm5, [x] ; ymm3 gets 4 64 bit maxs of ymm5 & array x