Day 22 - Mapper 2

Mapper 2

NOTE: This code doesn't work :(, if someone could figure out why
I'll be extremely gratefull.


	I'd imagine that you'd know what a mapper is by now, maybe you've even already
figured out how to use a mapper better than me. Either way, today I'll do some simple
code showing how to use Mapper 2. Mapper 2 is probably one of the simplest mappers,
it's basically a ROM switch. When a Mapper 2 ROM is loaded, the last ROM bank is put
at $C000 and the first at $8000. The one at $C000 can't be switched, so naturally 
Mapper 2 switches the ROM Bank at $8000. To do this we just write the bank number
to ROM anywhere from $8000-$FFFF (doesn't matter).

The Code
	This code is simple enough, it starts at $C000, jumps to $8000 which then
switches the bank and the newly loaded code bank sets a BG color of pink (I just
picked an arbitrary number and it turned out to be pink, oh well :) ). It's quite
likely I'm not doing something right, so if you find what it is or if you're just
reading this to find my silly errors and laugh at them, please let me know!

The code:

;-- CODE START --;
	.inesmir 0 ;- don't care about mirroring
	.inesmap 2 ;- Mapper 2!!!
	.inesprg 4 ;- This might be 2 (I think NESASM.EXE goes by 8kb PRGs, so therefore the 4)
	.ineschr 0

	.bank 3  ; should be the last bank
	.org $FFFA
	.dw 0
	.dw START
	.dw 0

	.bank 0    ; first bank loaded at $8000
	.org $8000 
	lda #2     ; will switch the bank to .bank 2 down there... 
	sta $8000
infin:
	jmp infin

	.bank 1
	.org $C000 ; the second 16kb PRG at $C000
START:              ; the startup location (see vector table in bank 3)
	jmp $8000

	.bank 2
	.org $8000 ; another bank that'll get swapped in
	lda #0
	sta $2000
	lda #%00011000
	sta $2001      ; this code will enable PPU and put a nice color for
	lda #$3F       ; the background
	sta $2006
	lda #0
	sta $2006
	lda #$33
	sta $2007
label:
	jmp label
;-- CODE END --;

One Last Thing
	
	In all seriousness NESASM.EXE is a pain in the left, little pinky toe to use...
I had written a fairly good assembly a while back, but it had some limitations that would
make it useless for serious coders (labels didn't work in indirect addressing modes, etc...).

	If I ever recode that assembler (i.e. get it working fully, hopefully with macros), then
I'll switch to it. I must say that I really hate linking and link scripts and all that stuff; I
just want to be able to write code and run it.

This Day In Review

	The next installment WILL be attributes.

	Happy (no, nothing inparticular, just HAPPY!),
		-Mike H a.k.a GbaGuy

Intro - Day 23