Understanding the source code of memcpy()

Question

00018 void *memcpy(void *dst, const void *src, size_t len)
00019 {
00020         size_t i;
00021 
00022         /*
00023          * memcpy does not support overlappi         


        

Answer 1

As if you see assembly code of memcpy it show that in 32 bit system each register is 32 bit it can store 4 byte at a time, if you will copy only one byte in 32 bit register, CPU need extra Instruction cycle.

If len/count is aliged in the multiple of 4 , we can copy 4 byte in one cycle

    MOV FROM, R2
    MOV TO,   R3
    MOV R2,   R4
    ADD LEN,  R4
CP: MOV (R2+), (R3+) ; "(Rx+)" means "*Rx++" in C
    CMP R2, R4
    BNE CP