Day 3

Video Interface Registers

	The VI (Video Interface) Memory Mapped Registers start at 0xA4400000 in N64 main memory
and allow you to initialize the bitmapped screen in many different ways (See Anarko's N64OPS on Dextrose.
Anarko's document lists them starting at 0x04400000, I don't know why. The Video Registers are:
(As listed by Nemu's debugger):

	VI_STATUS_REG (0xA4400000)     - Chooses color-depth, and some gamma/antialias/interlacing options
	VI_DRAM_ADDR_REG (0xA4400004)  - The address of the currect framebuffer (set it to the memory you're going to
					use for the N64's bitmapped screen.)
	VI_H_WIDTH_REG (0xA4400008)    - The Horizontal Resolution of the framebuffer.
	VI_V_INTR_REG (0xA440000C)     - Not sure. Check out N64OPS.
	VI_V_CURRENT_LINE_REG (0xA4400010)  - I think it gives the current line that is being drawn. (use it to check
						for VBlank??)
	VI_TIMING_REG (0xA4400014)     - Not sure what it means that N64OPS says..
	VI_V_SYNC_REG (0xA4400018)     - "number of half-lines per field", what the **** does that mean?
	VI_H_SYNC_REG (0xA440001C)     - Don't know. See N64OPS.
	VI_H_SYNC_LEAP_REG (0xA4400020)  - Don't know. See N64OPS.
	VI_H_VIDEO_REG (0xA4400024)      - Ditto.
	VI_V_VIDEO_REG (0xA4400028)      - Ditto.
	VI_V_BURST_REG (0xA440002C)      - Ditto.
	VI_X_SCALE_REG (0xA4400030)      - Seems obvious, but description is a little wierd, commonly seen at 0x200.
	VI_Y_SCALE_REG (0xA4400034)      - Seems obvious, but description is a little wierd, commonly seen at 0x400.

I'll fill out this chart as I figure out exactly what each one does. See Anarko's N64OPS for his
description (which come almost straight from the comments from the official dev headers).

If anyone has some more info on these, please email me (I'm going to be saying that alot, aren't I? :) )

Before We Get Started
	You're probably wondering how to run your ROM. Well, Nemu (which you can get from Zophar)
has a pretty good debugger. To run the ROMs that we'll eventually get to making, you need this
Plugin for Nemu to support the bitmap-mode
drawing that we'll be doing (Thanks again to HCS, for pointing me to it).

Initialization of Video
	But first, there's 2 instructions we still need to learn: LW and SW. They work like so:
	
	LW Loads a 32-bit Word from memory like this:
		lw t1,0(t3) ; the 32-bit word at the address created by the memory location 
			    ; in t3 + zero in this case, will be put into t1. 
	Here's another:
		lw t0,4(t1) ; the 32bit word at the address created by the memory location in t1 +4 will be put into
			    ; t0. So if the address 0xA0200004 has 32 in it,
			    ; and t1 has 0xA0200000 in it plus the 4 will be 0xA0200004 and then t0 will end up with
			    ; the 32.

	I hope you understood that.

	SW works in precisely the same way except that it will store the register on the left into the memory address
created by the register plus offset on the right. (The 4 and the zero in the examples are the offsets.)

Here's our video initialization code:
	;;---CODE START---;;
	; I should mention that while I use 0x in this tutorial's text for hexadecimal numbers,
	; ASMN6432.exe uses $.
        la   t0,$A4400000 ; VI status register (start of VI reg. mem.)
        li   t1, $103002 ; basically 16bit color and some other things
        sw   t1,0(t0)
        la   t1,0xa0200000 ; address of screen bitmap (framebuffer)
        sw   t1,4(t0)
        li   t1,320        ; width of screen (horizontal resolution?)
        sw   t1,8(t0)
        li   t1,$2         ; Don't know.
        sw   t1,12(t0)
        li   t1,$0         ; Ditto.
        sw   t1,16(t0)
        li   t1,$3e52239   ; Ditto until next comment
        sw   t1,20(t0)
        li   t1,$0000020d
        sw   t1,24(t0)
        li   t1,$00000c15
        sw   t1,28(t0)
        li   t1,$0c150c15
        sw   t1,32(t0)
        li   t1,$006c02ec
        sw   t1,36(t0)
        li   t1,$002501ff
        sw   t1,40(t0)
        li   t1,$000e0204
        sw   t1,44(t0)
        li   t1,$200      ; something about horizontal scaling?
        sw   t1,48(t0)
        li   t1,$400      ; something about vertical scaling?
        sw   t1,52(t0)
	;;---CODE END---;;

It basically selects a 320x240 16 bit mode, and does some other stuff I don't know yet.
(That code is essentailly the same as code that HCS was again, nice enough to show me, so
now I use 320x240 16 bit mode.)
It probably wouldn't be too hard to change that to 32 bit color, but I just don't feel like
it (I think you'd have to change $103002 to $103003) and 16 bit color is plenty for most things.
The offsets are not permanently added to the register in ()s, so in the code, the offsets just keep
gaining by 4, for each VI register set.

This Day In Review

	Tomorrow, we'll learn some math instructions and then the next day, we'll do some loops.
Then, we'll put together a code file that clears the screen (with code that, yep, HCS showed me.)

	Hold on! We're getting there!,
		- GBAGuy

Intro - Day 4