NES Audio Programming (Page 1 of 3)

It's fairly straightforward - you need to change your ROM configuration to use MMC1 mapper (I think that's the minimum you need mapper-wise) and you'll need to change the ROM header. After that, NVRAM (battery-backed RAM) is mapped at $6000-$7FFF and you can just read and write to it like normal RAM.

I'll send you some code.

Actually, turns out you don't even need MMC1 (in Nestopia at least). Here's some bare-bones code that will write 00-FF in the first 256 bytes of a .sav file. The key part is setting bit 1 of the 6th byte of the iNES header (http://wiki.nesdev.com/w/index.php/INES)

                ;NES Header
                DB "NES",$1A
                DB 2            ;number of PRG ROMs
                DB 0            ;number of CHR ROMs
                DB %00000010    ; BIT 1 = enable(1)/disable(0) battery RAM
                DB %00000000
                DB 0,0,0,0,0,0,0,0

WRAM            EQU $6000
                
                .ORG $8000
RESET           sei
                ldx #$FF
                txs

                ldx #$00
@a              txa
                sta WRAM,x
                inx
                bne @a

LOOP            jmp LOOP

NMI             RTI

IRQ             RTI

                ORG $FFFA
                DW NMI,RESET,IRQ
                

Last edited by neilbaldwin (Jan 8, 2011 11:26 pm)

I realised afterwards that some of that might be confusing (or that your code for the Chords program is set up a little differently) - I wanted to give you something that would compile and work in ASM6 (I guessed that was what you used?)

Really, the only parts you need that can be transplanted in your own code is setting bit 1 of the 6th header byte and then knowing you just read/write addresses $6000-$7FFF to access the battery RAM. The rest of my example is just reset code and the necessary layout to make a working ROM.

LOL just spotted a schoolboy error - I'd wrote 'sta WRAM,y' instead of 'sta WRAM,x'

Fixed now

It's because the NMI never gets triggered. You need to enable NMIs through the PPU Control Register 1 ($2000). To enable the NMI you just set bit 7. Put this somewhere before the LOOP;

lda #$80
sta $2000

The PPU control registers do a lot more so you should be familiar with the various functions they provide;

http://nesdev.parodius.com/NinTech.txt

(near the bottom of the page is a table listing what each bit of PPU 1/2 Control and PPU Status does)

It's also common practice to 'acknowledge' the NMI in your NMI code by 'reading' the PPU Status Register ($2002)

bit $2002

at the start of the NMI.

Something else you should do in the NMI is save the contexts of the A, X and Y registers - this is because the NMI can occur at any point in your background loop and once the NMI has finished, the register contents could have been changed. You use the stack instructions PHA and PLA.

NMI:
    pha                ;push A onto stack
    txa                ;transfer X to A
    pha                ;push A onto stack
    tya                ;transfer Y to A
    pha                ;push A onto stack

    bit $2000                ;acknowledge interrupt
   
    ;do other stuff

    pla                ;pull top of stack to A
    tay                ;transfer A to Y
    pla                ;pull next by off stack to A
    tax                ;transfer A to X
    pla                ;pull A off stack
    rti

The stack is a LIFO buffer (last in, first out) so you restore the registers in the reverse order to what you saved them. Because you can only push (PHA) and pull (PLA) a value to/from the A register, in order to save the content of X and Y you transfer them to the accumulaor (A) first when saving, and then reverse the procedure for restoring.

Edited to add:

In fact, most people clear the two PPU Control registers at the start of the reset code too:

PPU0 EQU $2000
PPU1 EQU $2001

RESET:
    sei
    lda #$00
    sta PPU0
    sta PPU1
    ldx #$FF
    txs
    
    etc.

Last edited by neilbaldwin (Jan 10, 2011 9:11 am)

Sorry, that happens a lot to stuff I get involved with. It's a constant burden.

I think its more than on topic, and interesting to read, even if  barely understand anything.