Note however that this is less efficient than executing MOV twice, but it does execute the copy in a single instruction. T_cur db 0x9a DB tells NASM that we want to declare a byte Mov di, t_last Load DI with address of 't_last' mov si, t_cur Load SI with address of 't_cur' You want to transfer byte(s), word(s), etc. Try using MOVS (move string), and setting SI and DI, depending on whether It is technically possible to move from memory to memory.
See and other x86 performance links in the x86 tag wiki Unless somewhere on the stack is the desired destination and you can't optimize that local variable into a register, in which case push is just fine to copy it there and allocate stack space for it.
It introduces an extra store/reload store-forwarding latency before the data can be read from the final destination, and takes more uops. For a word, dword, or qword (depending on CPU mode), you could push / pop, but that's a lot slower than copying via a register. How exactly do partial registers on Haswell/Skylake perform? Writing AL seems to have a false dependency on RAX, and AH is inconsistentįirst of all, you can't push a single byte at all, so there's no way you could do a byte load / byte store from the stack.Generally avoid writing AH, BH, CH, or DH for performance reasons, unless you've read and understood the following links and any false dependencies or partial-register merging stalls aren't going to be a problem or happen at all in your code.
CX PROGRAMMER MEMORY MOV INSTRUCTION CODE
In 64-bit mode, SIL, DIL, r8b, r9b and so on are also fine choices, but require a REX prefix in the machine code for the store so there's a minor code-size reason to avoid them. movzx ecx, byte load CL, zero-extending into RCX x86's movzx is the analogue of instructions like ARM ldrb. Only use a narrow 16 or 8-bit mov load if you actively want to merge into the low bits of another value. This avoids a false dependency on the old value of the register. For a byte or word copy, you typically want a movzx load into a 32-bit register, then an 8-bit or 16-bit store. In all the mainstream 32-bit and 64-bit calling conventions, EAX, ECX, and EDX are call-clobbered, so AL, CL, and DL are good choices.
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