nese/src/ppu.c

#include <stdio.h>

#include "ppu.h"


// TODO: Retain open bus bits?


#define ppu_reg_ctrl    (0x2000U)
#define ppu_reg_mask    (0x2001U)
#define ppu_reg_status  (0x2002U)
#define oam_reg_addr    (0x2003U)
#define oam_reg_data    (0x2004U)
#define ppu_reg_scroll  (0x2005U)
#define ppu_reg_addr    (0x2006U)
#define ppu_reg_data    (0x2007U)
#define oam_reg_dma     (0x4014U)


uint8_t nes_ppu_read(nes_ppu* ppu, uint16_t addr) {
    uint8_t val = 0;

    if (ppu_reg_status == addr) {
        val = ppu->status;
        ppu->latch = 0;

    } else if (oam_reg_data == addr) {
        val = ppu->oam[ppu->oam_addr];

    } else if (ppu_reg_data == addr) {
        val = ppu->data;

        if (ppu->addr < nes_ppu_mem_vram_start) {
            ppu->data = ppu->chr_mem[ppu->addr];
        } else if (ppu->addr < nes_ppu_mem_vram_start +
                               nes_ppu_mem_vram_size) {
            ppu->data = ppu->vram[ppu->addr -
                                  nes_ppu_mem_vram_start];
        } else if (ppu->addr < nes_ppu_mem_pal_start) {
            ppu->data = ppu->chr_mem[ppu->addr];
        } else if (ppu->addr < nes_ppu_mem_size) {
            uint8_t pal_addr =
                    (ppu->addr - nes_ppu_mem_pal_start) &
                    (nes_ppu_mem_pal_size - 1);
            ppu->data = ppu->palette[pal_addr];
        }

        ppu->addr += (ppu->status & ppu_Control_VRAM_Inc) ?
                     32 : 1;
    }

//    fprintf(stdout, "PPU: <-R $%04x %02x\n", addr, val);

    return val;
}

void nes_ppu_write(nes_ppu* ppu, uint16_t addr, uint8_t val) {
//    fprintf(stdout, "PPU: W-> $%04x %02x\n", addr, val);

    if (ppu_reg_ctrl == addr) {
        ppu->control = val;
        // TODO: Trigger NMI if it's enabled during VBlank?

    } else if (oam_reg_addr == addr) {
        ppu->oam_addr = val;

    } else if (oam_reg_data == addr) {
        ppu->oam[ppu->oam_addr++] = val;

    } else if (ppu_reg_mask == addr) {
        ppu->mask = val;

    } else if (ppu_reg_scroll == addr) {
        if (ppu->latch) {
            ppu->scroll &= 0xFF00U;
            ppu->scroll |= val;
        } else {
            ppu->scroll &= 0x00FFU;
            ppu->scroll |= (uint16_t)val << 8;
        }
        ppu->latch = !ppu->latch;

    } else if (ppu_reg_addr == addr) {
        if (ppu->latch) {
            ppu->addr &= 0x3F00U;
            ppu->addr |= val;
//            printf("PPU: VRAM ADDR %04x\n", ppu->addr);

        } else {
            ppu->addr &= 0x00FFU;
            ppu->addr |= (uint16_t)val << 8;
        }
        ppu->latch = !ppu->latch;

    } else if (ppu_reg_data == addr) {
        if (ppu->addr >= nes_ppu_mem_size) {
            printf("!!! PPU: MEM OOB: %04x", ppu->addr);

        } else if (ppu->addr >= nes_ppu_mem_pal_start) {
            uint8_t pal_addr =
                    (ppu->addr - nes_ppu_mem_pal_start) &
                    (nes_ppu_mem_pal_size - 1);
//            fprintf(stderr, "PPU PAL %02x < %02x\n", pal_addr, val);
            ppu->palette[pal_addr] = val;

        } else if (ppu->addr >= nes_ppu_mem_vram_start) {
            uint16_t vram_addr = ppu->addr - nes_ppu_mem_vram_start;
            if (vram_addr >= nes_ppu_mem_vram_size) {
                printf("!!! PPU: VRAM OOB: %04x", vram_addr);
                vram_addr &= (nes_ppu_mem_vram_size - 1);
            }
            ppu->vram[vram_addr] = val;
        }

        ppu->addr += (ppu->status & ppu_Control_VRAM_Inc) ?
                     32 : 1;
    }
}

void nes_ppu_reset(nes_ppu* ppu) {
    ppu->control  = 0;
    ppu->mask     = 0;
    ppu->latch    = 0;
    ppu->scroll   = 0;
    ppu->data     = 0;
    ppu->frame    = 0;
    ppu->scanline = 0;
    ppu->cycle    = 0;
}

void nes_ppu_init(nes_ppu* ppu, uint8_t* chr_mem) {
    ppu->chr_mem  = chr_mem;
    ppu->status   = 0;
    ppu->oam_addr = 0;
    ppu->addr     = 0;
    nes_ppu_reset(ppu);
}

int nes_ppu_run(nes_ppu* ppu, int cycles) {
    int vblank = 0;

    ppu->cycle += cycles;

    if (    0 != ppu->hit_line &&
            ppu->scanline > ppu->hit_line &&
            ppu->cycle > ppu->hit_dot) {
        ppu->status |= ppu_Status_Hit;
    }

    while (ppu->cycle >= nes_ppu_dots) {
        ppu->cycle -= nes_ppu_dots;
        if (    ppu->scanline <= nes_ppu_prerender &&
                (ppu->frame & 1)) {
            // Prerender line is one dot shorter in odd frames
            // Fake it by incrementing the cycle in this case
            ppu->cycle++;
        }
        ppu->scanline++;
        if (ppu->scanline >= nes_ppu_prerender +
                             nes_ppu_height +
                             nes_ppu_postrender +
                             nes_ppu_vblank) {
            ppu->status &= ~(ppu_Status_VBlank | ppu_Status_Hit);
            ppu->hit_line = 0;
            ppu->hit_dot = 0;
            ppu->scanline = 0;
            ppu->frame++;
            // TODO: Render callback if vblank was previously set
        } else if (ppu->scanline >= nes_ppu_prerender +
                                    nes_ppu_height +
                                    nes_ppu_postrender) {
            ppu->status |= ppu_Status_VBlank;
            if (ppu->control & ppu_Control_VBlank) {
                vblank = 1;
            }
        }
    }

    return vblank;
}


int nes_ppu_cycles_til_vblank(nes_ppu* ppu) {
    int cycles_til_vblank = nes_ppu_active_cycles - (
            ppu->cycle + (ppu->scanline * nes_ppu_dots));
    return (cycles_til_vblank > 0 ? cycles_til_vblank :
            nes_ppu_vblank_cycles + cycles_til_vblank);
}