copy WORD to BYTE array

[Agilaz]

Hi, i need the fastest way to copy the lowbytes of large WORD arrays into BYTE arrays for an exported function in a dll.

this is what i have so far...

void  __stdcall CopyIntArray(unsigned char * ucDest, unsigned short * usSource, unsigned int * uiLen)
{

	__asm
	{

		mov esi, usSource
		mov edi, ucDest
		mov eax, uiLen
		mov eax, [eax]
		mov ebx, eax
		shr eax, 2
		jz SHORT lendDW
lloopDW:	
		mov ecx, [esi+4]
		mov dl, cl
		shr ecx, 8
		mov dh, ch
		shl edx, 16
		mov ecx, [esi]
		mov dl, cl
		shr ecx, 8
		mov dh, ch
		mov [edi], edx
		add esi, 8
		add edi, 4
		dec eax
		jnz SHORT lloopDW
lendDW:
		and ebx, 3
		jz SHORT lend
lloopB:
		mov cx, [esi]
		mov [edi+eax], cl
		inc eax
		add esi, 2
		cmp ebx, eax
		jnz Short lloopB
lend:

	}

}

the code works fine, but is there a faster way of doing this?

[Al42]

Hmmm - pseudo-code, but:

point at the start of the word array
point at the start of the byte array
load a byte from the word array
store the byte to the byte array
inc the byte pointer
inc the word pointer by 2
loop

(If the word array is in low-byte-at-high-address order, initially point at the word array + 1)

[Agilaz]

that's that's the first thing i tried and what the second loop does. but it's slower. the reason is most likely that you have more memory read/write operations and more loop iterations that way. i already optimized the code to read 2 dwords from the word array and write one dword to the byte array inside the first loop. the only way to make it faster seems to be to reduce the slow memory read/write operations, but i have no idea how to reduce them even more.

[Al42]

The only thing faster than memory r/w is register r/w. Get a processor with a few gigs of register space.

Seriously, other than a register push being faster than a store, I can't think of anything.

[Agilaz]

a few gigs of register space where can i oder that processor?

i guess i can live with this code if this is already the fastet way.

thanks Al42

[Al42]

you could try to implement an old tric - I'll leave the coding to you, but you could save the SP, point it to the top of the destination, point another register to the top of the source, pop the register, transfer the low byte to the SP, pop the register, transfer the low byte to the high byte of the SP, push the SP, etc. Saves having to inc one register.