mirror of
https://github.com/sehugg/8bitworkshop.git
synced 2024-11-25 18:33:11 +00:00
98 lines
2.4 KiB
C
98 lines
2.4 KiB
C
|
|
/*
|
|
Multiple screen splits with the MMC3 mapper using IRQs.
|
|
The main loop updates the scroll counters continuously.
|
|
When a IRQ fires, we change the X scroll register, setting
|
|
it immediately via the PPU struct.
|
|
The NMI and IRQ use the same callback function.
|
|
*/
|
|
|
|
// bank-switching configuration
|
|
#define NES_MAPPER 4 // Mapper 4 (MMC3)
|
|
#define NES_PRG_BANKS 4 // # of 16KB PRG banks
|
|
#define NES_CHR_BANKS 1 // # of 8KB CHR banks
|
|
|
|
#include <peekpoke.h>
|
|
#include <string.h>
|
|
#include <nes.h>
|
|
#include "neslib.h"
|
|
|
|
// link the pattern table into CHR ROM
|
|
//#link "chr_generic.s"
|
|
|
|
// "strobe" means "write any value"
|
|
#define STROBE(addr) __asm__ ("sta %w", addr)
|
|
|
|
#define MMC3_IRQ_SET_VALUE(n) POKE(0xc000, (n));
|
|
#define MMC3_IRQ_RELOAD() STROBE(0xc001)
|
|
#define MMC3_IRQ_DISABLE() STROBE(0xe000)
|
|
#define MMC3_IRQ_ENABLE() STROBE(0xe001)
|
|
|
|
void draw_text(word addr, const char* text) {
|
|
vram_adr(addr);
|
|
vram_write(text, strlen(text));
|
|
}
|
|
|
|
word counters[128];
|
|
byte irqcount = 0;
|
|
|
|
void __fastcall__ irq_nmi_callback(void) {
|
|
// check high bit of A to see if this is an IRQ
|
|
if (__A__ & 0x80) {
|
|
// it's an IRQ from the MMC3 mapper
|
|
// change PPU scroll registers
|
|
PPU.scroll = counters[irqcount & 0x7f] >> 8;
|
|
PPU.scroll = 0;
|
|
// advance to next scroll value
|
|
++irqcount;
|
|
// acknowledge interrupt
|
|
MMC3_IRQ_DISABLE();
|
|
MMC3_IRQ_ENABLE();
|
|
} else {
|
|
// this is a NMI
|
|
// reload IRQ counter
|
|
MMC3_IRQ_RELOAD();
|
|
// reset scroll counter
|
|
irqcount = 0;
|
|
}
|
|
}
|
|
|
|
void main(void)
|
|
{
|
|
// More accurate NES emulators simulate the mapper's
|
|
// monitoring of the A12 line, so the background and
|
|
// sprite pattern tables must be different.
|
|
// https://forums.nesdev.com/viewtopic.php?f=2&t=19686#p257380
|
|
set_ppu_ctrl_var(get_ppu_ctrl_var() | 0x08);
|
|
// Enable Work RAM
|
|
POKE(0xA001, 0x80);
|
|
// Mirroring - horizontal
|
|
POKE(0xA000, 0x01);
|
|
|
|
// set up MMC3 IRQs every 8 scanlines
|
|
MMC3_IRQ_SET_VALUE(7);
|
|
MMC3_IRQ_RELOAD();
|
|
MMC3_IRQ_ENABLE();
|
|
// enable CPU IRQ
|
|
__asm__ ("cli");
|
|
// set IRQ callback
|
|
nmi_set_callback(irq_nmi_callback);
|
|
// set palette colors
|
|
pal_col(1,0x04);
|
|
pal_col(2,0x20);
|
|
pal_col(3,0x30);
|
|
// fill vram
|
|
vram_adr(NTADR_A(0,0));
|
|
vram_fill('A', 32*28);
|
|
// turn on PPU/interrupts
|
|
ppu_on_all();
|
|
// loop forever, updating each counter at a different rate
|
|
while(1) {
|
|
byte i;
|
|
for (i=0; i<128; i++) {
|
|
counters[i] += i*16;
|
|
}
|
|
ppu_wait_frame();
|
|
}
|
|
}
|