nesem/ppu.c

#include "ppu.h"
#include "common.h"

#include "cartridge.h" //needs acces to the cartridge to load CHR maps, since r/w functions are in-house instead of bus-wide like it is for busRead/Write it needs to be imported here too, quite like there are physical wires connecting the cartridge CHR bank pins to the PPU

#define PPU_BUS_SIZE (0x3FFF)
byte ppuBus[PPU_BUS_SIZE];

PPU ppu;

inline word resolveNameTableAddress(word regData){
    return (regData & 0b0000111111111111) | (1 << (14 - 1));
}

/*
10NNYYYYYXXXXX
10100010100011  ==> $28A3
--~~-----~~~~~
^ ^ ^    ^
| | |    |
| | y=5  x=3
| |
| 3rd name table
|
fixed
*/

word resolveAttributeTableAddress(word regData){
    locationRegister n;
    n.data = regData;
    byte trimmedCoarseX = n.field.coarseX >> 2; //trim the lowest 2 bits of coarse X
    byte trimmedCoarseY = n.field.coarseY >> 2; //trim the lowest 2 bits of coarse Y

    word ret = trimmedCoarseX | (trimmedCoarseY << 3); //YYYXXX
    ret = 0b1111 >> 6 | ret; //1111YYYXXX
    ret = n.field.nameTableID << 10 | ret; //NN1111YYYXXX
    ret = 0b10 << 12 | ret; //10NN1111YYYXXX

    return ret;
}

/*
10NN1111YYYXXX
10101111001000
--~~----~~~---
^ ^ ^   ^  ^
| | |   |  |
| | |   |  x=3>>2
| | |   |
| | |   y=5>>2
| | |
| | offset of the attribute table from a name table
| | each name table has 960 bytes, i.e. $3C0 == 1111000000
| | that's why the address has 1111 in the middle
| |
| 3rd name table
|
fixed
*/


// ----
//
//BUS ENGINE FUNCTIONS
//
// ----

byte ppuRead(word address){

    word cartResponse;

    if((cartResponse = mapper000_Read(address, true)) == 0x0100){

        if(address >= 0x3000 && 0x3EFF <= address) //mirrored region
            address -= 0x1000;

        if(address >= 0x3F20 && 0x3FFF <= address) //mirrored region
            address = (address - 0x3F00) % 0x20 + 0x3F00;

        return ppuBus[address];
    }

    else return cartResponse;

}

void ppuWrite(word address, byte data){

    if(!mapper000_Write(address, data, true)){

        if(address >= 0x3000 && 0x3EFF <= address) //mirrored region
            address -= 0x1000;

        if(address >= 0x3F20 && 0x3FFF <= address) //mirrored region
            address = (address - 0x3F00) % 0x20 + 0x3F00;

        ppuBus[address] = data;
    }

}


void ppuRegWrite(word address, byte data){
    address -= 0x2000;
    switch(address){
        case 0: //ppuctrl

            ppu.control.full = data;
            ppu.tReg.field.nameTableID = ppu.control.nameTableID;

        break;

        case 1: //ppumask

            ppu.mask.full = data;

        break;

        case 2: //ppustatus

            ppu.status.full = data;

        break;

        case 3: //oamAddress

        break;

        case 4: //oamData

        break;

        case 5: //ppuScroll

            if(ppu.expectingLsb){
                //second write
                ppu.tReg.field.fineY = data & 0b00000111; //set fineY
                ppu.tReg.field.coarseY = data >> 3; //set coarse Y
            }else{
                //first write
                ppu.xReg = data & 0b00000111; //set fineX
                ppu.tReg.field.coarseX = data >> 3; //set coarse X
            }

            ppu.expectingLsb = !ppu.expectingLsb;

        break;

        case 6: //ppuAddr

            if(ppu.expectingLsb){
                //second write
                ppu.tReg.data = (ppu.tReg.data & 0xFF00) | (data & 0x00FF);

                ppu.vReg.data = ppu.tReg.data; //IMPORTANT V SYNC

            }else{
                //first write
                word buf;
                buf = data & 0b00111111; //eliminate highest 2 bits
                buf = buf << 8;
                buf = buf | (ppu.tReg.data & 0xFF);
                ppu.tReg.data = buf;

                ppu.tReg.field.fineY = ppu.tReg.field.fineY & 0b011; //clear highest bit of fine Y
            }

            ppu.expectingLsb = !ppu.expectingLsb;

        break;

        case 7: //ppuData

            ppuWrite(ppu.vReg.data, data);

            if(ppu.control.vramIncrement) ppu.vReg.data += 32;
            else ppu.vReg.data++;

        break;

        default:
            fprintf(stderr, "ERR: Invalid write to PPU register of address %X\n!", address + 0x2000);
            abort();
        break;
    }
}

byte ppuRegRead(word address){ //send the registers to the bus so the components can read them
    
    address -= 0x2000;

    byte returnData;

    switch(address){
        case 0: //ppuctrl

            return ppu.control.full;

        break;

        case 1: //ppumask

            return ppu.mask.full;
            
        break;

        case 2: //ppustatus

            return ppu.status.full;
            ppu.status.vblank = 0;
            ppu.expectingLsb = false;

        break;

        case 3: //oamAddress


        break;

        case 4: //oamData


        break;

        case 5: //ppuScroll


        break;

        case 6: //ppuAddr

            //makes no sense

        break;

        case 7: //ppuData

            returnData = ppu.dataByteBuffer; //reads are lagged back by one cycle, the data you read is the data of the last query

            ppu.dataByteBuffer = ppuRead(ppu.vReg.data);

            if(ppu.vReg.data >= 0x3F00 && 0x3FFF <= ppu.vReg.data) //except when reading palette info
                returnData = ppuRead(ppu.vReg.data); //then the query is immediate


            return returnData;

        break;

        default:
            fprintf(stderr, "ERR: Invalid read to PPU register of address %X\n!", address + 0x2000);
            abort();
        break;
    }

}

void dumpPpuBus(){
    
    for(int i = 0; i < 0x3FFF; i++){
        printf("%02X ", ppuRead(i));
    }

}

void initPpu(){
    ppu.dataByteBuffer = 0;
    ppu.expectingLsb = 0;
    ppu.control.full = 0;
    ppu.dataByteBuffer = 0;
    ppu.mask.full = 0;
    ppu.status.full = 0;
}

void ppuClock(){
    //one tick of the PPU clock

    

}