How does the internal implementation of memcpy work?

Question

How does the standard C function \'memcpy\' work? It has to copy a (large) chunk of RAM to another area in the RAM. Since I know you cannot move straight from RAM to RAM in

Answer 1

The implementation of memcpy is highly specific to the system in which it is implemented. Implementations are often hardware-assisted.

Memory-to-memory mov instructions are not that uncommon - they have been around since at least PDP-11 times, when you could write something like this:

    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

The commented line is roughly equivalent to C's

*to++ = *from++;

Contemporary CPUs have instructions that implement memcpy directly: you load special registers with the source and destination addresses, invoke a memory copy command, and let CPU do the rest.