Day 21 - LDRH And STRH

Say What?

	Today, we're going to take a look at 2 instructions that for some reason
I had forgot about. Infact, I had been writting some fairly inefficient code because
of that. These instructions are ldrh and strh.

	What these instructions do is load/store a Half word (notice the capital 'H'),
which is two bytes (assuming 32bit 'words'). They work just like the normal STR and
LDR instructions except they only work with the lower 16bits of the register(s) you
use with them. Appearantly this should help with those mem. registers that are only
16bits long...

	I was on the GBADev Forum recently asking if anyone could help with a macro
that I was having trouble with. It was supposed to set the tile number of a sprite
into OAM. But for some reason it kept overwritting the X value of the sprite instead.

	So this very clever and intelligent person (not me, believe it or not) gave
me a piece of code using LDRH and STRH that was short and worked beautifully. I had
totally forgotten that these 2 instructions existed. So, in order to hopefully remind
you about these magical instruments of data moving power, I am going to explain that
macro to you. This macro "SetNumber" has been added to sprites.h. I suggest you either
download it now, or just add the macro to the file when we
get to it.
	
Us Getting To It
	Guess what? We got to it. Yep. We're here. Short one today. Anyway, here's
the macro:

	@macro SetNumber sprite, number
		ldr r3,=(OAM+(sprite*8))+6  ; load r3 with the address of the sprite's tile number.
		ldr r4,=number  ; Then load the number into the lower bits (just a regular load).
		strh r4,[r3]  ; And store the lower 16bits into place.
	@endm

I realize that only demonstrates the STRH instruction, to load something with LDRH
looks like:

	ldrh r3,[r1]  ; note that we take the 16bits at address in r1 and put that into r3.

Just like old times except 16bits.

Day In Review

	Many thanks to the person on the GBADev Forum who reminded me of these 
instructions.

	These instructions are extremely useful in creating more efficient code.
Use them well.

	Happy Coding,
		- GbaGuy

Intro - Day 22