Comment by kika

Yeah, like very quick lowercasing of blocks of ASCII text, 8 characters at a time:

  char *strlwrmmx(char *buffer,int len=-1)
  {
        if (len==-1) len=strlen(buffer);

        unsigned __int64 a=0x4040404040404040; // 0x40='A'-1
        unsigned __int64 z=0x5B5B5B5B5B5B5B5B; // 0x5B='Z'+1
        unsigned __int64 c=0x2020202020202020; // conversion offset for lowercase->uppercase
        asm {
                mov edi,buffer
                mov ecx,len

        convblock:
                cmp ecx,8
                jl rest

                movq mm4,c              // Eight copies of conversion value
                movq mm2,a              // Put eight "A" characters in mm2
                movq mm3,z              // Put eight "Z" characters in mm3

                movq mm0,[edi]  // Get next eight characters of our string
                movq mm1,mm0    // We need two copies
                pcmpgtb mm1,mm2 // Generate 1's in MM1 everywhere chars >= 'A'
                pcmpgtb mm3,mm0 // Generate 1's in MM3 everywhere chars <= 'Z'
                pand mm1,mm3    // Generate 1's in MM1 when 'A'<=chars<='Z'
                pand mm1,mm4    // Generates $20 in each spot we have a l.c. char
                paddb mm0,mm1   // Convert uppercase chars to lowercase by adding $20
                movq [edi],mm0  // Store back in buffer

                sub ecx,8
                add edi,8
                jmp convblock
        rest:                           // convert any left characters one by one
                cmp ecx,0
                je done
        convchar:
                mov al,[edi]
                cmp al,'A'
                jb skip
                cmp al,'Z'
                ja skip
                add al,0x20
                mov [edi],al
        skip:
                inc edi
                loop convchar
        done:
                }
        return buffer;
  }