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MemoryAccess.c
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MemoryAccess.c
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#include <string.h>
#include <assert.h>
#include "Nes.h"
#define NES ((Nes*)sys) // some syntax de-clutter to compensate for the unfortunate void *sys
// -------------------------------------------------------------------------------
// $0..$7FF unmirrored RAM
byte read_ram( void *sys, word address )
{
return NES->ram[address];
}
void write_ram( void *sys, word address, byte value )
{
NES->ram[address] = value;
}
// -------------------------------------------------------------------------------
// $800..$1FFF mirrored RAM
byte read_ram_mirror( void *sys, word address )
{
return NES->ram[address & 0x7FF]; // Convert mirrors to actual address
}
void write_ram_mirror( void *sys, word address, byte value )
{
NES->ram[address & 0x7FF] = value; // Convert mirrors to actual address
}
// -------------------------------------------------------------------------------
byte read_save_ram( void *sys, word address )
{
return NES->save_ram[ address - 0x6000 ];
}
void write_save_ram( void *sys, word address, byte value )
{
NES->save_ram[ address - 0x6000 ] = value;
}
// -------------------------------------------------------------------------------
// $8000..$FFFF PRG-ROM
byte read_prg_rom( void *sys, word address )
{
address -= 0x8000; // make address zero-based ( $0..$7FFF )
if( NES->prg_rom_count == 1 ) // WIP bankswitching hell here in the future
{
address &= 0x3FFF; // Convert mirror in actual ROM address ( $0..$3FFF )
}
return NES->prg_rom[address];
}
// -------------------------------------------------------------------------------
// $2000
void write_ppu_control1( void *sys, word address, byte value )
{
NES->ppu.nmi_enabled = ( value & (1<<7) ) ? 1 : 0;
NES->ppu.sprite_height = ( value & (1<<5) ) ? 16 : 8;
NES->ppu.back_pattern = ( value & (1<<4) ) ? 0x1000 : 0;
NES->ppu.sprite_pattern = ( value & (1<<3) ) ? 0x1000 : 0;
NES->ppu.increment_vram = ( value & (1<<2) ) ? 32 : 1;
NES->ppu.scroll_high_bits = value & 3; // & %11
}
// -------------------------------------------------------------------------------
// $2001
void write_ppu_control2( void *sys, word address, byte value )
{
NES->ppu.color_emphasis = ( value & 0xE0 ) >>5; // & %11100000
NES->ppu.sprites_visible = ( value & (1<<4) ) ? 1 : 0;
NES->ppu.background_visible = ( value & (1<<3) ) ? 1 : 0;
NES->ppu.sprite_clip = ( value & (1<<2) ) ? 0 : 1;
NES->ppu.background_clip = ( value & (1<<1) ) ? 0 : 1;
NES->ppu.monochrome = ( value & (1<<1) ) ? 1 : 0;
}
// -------------------------------------------------------------------------------
// $2002
byte read_ppu_status( void *sys, word address )
{
// Unused bits should actually return the open bus
byte value =
( NES->ppu.vblank_flag <<7 ) |
( NES->ppu.sprite0_hit <<6 ) |
( NES->ppu.sprites_lost <<5 );
NES->ppu.vblank_flag = 0; // reset flag once read
NES->ppu.write_count = 0; // writes count is reset
#ifdef _Cpu6502_Disassembler
NES->cpu->disasm.value = value;
#endif
return value;
}
// -------------------------------------------------------------------------------
// $2003
void write_spr_ram_address( void *sys, word address, byte value )
{
// assert( 0 && "sprite RAM address register not yet implemented" );
}
// -------------------------------------------------------------------------------
// $2004
byte read_spr_ram_io( void *sys, word address )
{
// assert( 0 && "Read from sprite RAM not yet implemented" );
return 0;
}
// -------------------------------------------------------------------------------
void write_spr_ram_io( void *sys, word address, byte value )
{
// assert( 0 && "Write to sprite RAM not yet implemented" );
}
// -------------------------------------------------------------------------------
// $2005
void write_scroll( void *sys, word address, byte value )
{
if( NES->ppu.write_count == 0 ) {
NES->ppu.horz_scroll = value;
NES->ppu.write_count = 1;
}
else {
NES->ppu.vert_scroll = value;
NES->ppu.write_count = 0;
}
}
// -------------------------------------------------------------------------------
// $2006
void write_vram_address( void *sys, word register_address, byte value )
{
if( NES->ppu.write_count == 0 ) {
NES->ppu.vram_address = ((word) value & 0x3F ) <<8; // put 6 bits of value in vram_address msb
NES->ppu.write_count = 1;
}
else {
NES->ppu.vram_address |= value;
NES->ppu.write_count = 0;
}
}
// -------------------------------------------------------------------------------
// $2007
byte read_vram_io( void *sys, word register_address )
{
byte old_latch = NES->ppu.vram_latch;
if( NES->ppu.write_count > 0 ) {
assert( 0 && "Trying to write to VRAM after only setting half of VRAM address, what to do here?" );
}
word vram_address = NES->ppu.vram_address;
NES->ppu.vram_address += NES->ppu.increment_vram;
NES->ppu.vram_address &= 0x3FFF;
// Palettes
if( vram_address >= 0x3F00 )
{
vram_address &= 0x1F; // Make VRAM address zero based and Unmirror
if( vram_address == 0x10 || vram_address == 0x14 || vram_address == 0x18 || vram_address == 0x1C ) {
vram_address -= 0x10; // Sprite colors 0 mirror background colors 0
}
NES->ppu.vram_latch = NES->ppu.palettes[ vram_address ];
return NES->ppu.vram_latch;
}
// Name tables and attributes
else if( vram_address < 0x3F00 ) {
vram_address &= 0x7FF; // Make VRAM address zero based and Unmirror
NES->ppu.vram_latch = NES->ppu.name_attr[ vram_address ];
}
else {
assert(0 && "Reading Pattern tables.");
}
return old_latch;
}
// -------------------------------------------------------------------------------
void write_vram_io( void *sys, word register_address, byte value )
{
if( NES->ppu.write_count > 0 ) {
assert( 0 && "Trying to write to VRAM after only setting half of VRAM address, what to do here?" );
}
word vram_address = NES->ppu.vram_address;
// Palettes
if( vram_address >= 0x3F00 )
{
assert( NES->ppu.vram_address < 0x4000 ); // WIP remove this once checked
vram_address &= 0x1F; // Make VRAM address zero based and Unmirror
if( vram_address == 0x10 || vram_address == 0x14 || vram_address == 0x18 || vram_address == 0x1C ) {
vram_address -= 0x10; // Sprite colors 0 mirror background colors 0
}
NES->ppu.palettes[ vram_address ] = value & 0x3F;
}
// Name tables and attributes
else if( NES->ppu.vram_address < 0x3F00 ) {
vram_address &= 0x7FF; // Make VRAM address zero based and Unmirror
NES->ppu.name_attr[ vram_address ] = value;
}
// else tries to write to pattern tables, do nothing for now
NES->ppu.vram_address += NES->ppu.increment_vram;
NES->ppu.vram_address &= 0x3FFF; // Wrap around $4000
}
// -------------------------------------------------------------------------------
// $4014
// WIP: OAM DMA starts on RAM address written to $2003
void write_sprite_dma( void *sys, word address, byte value )
{
if( value > 0x1F ) {
assert( 0 && "Copying sprite DMA from outside RAM, weird." );
}
address = ( value & 0x7 ) <<8; // shamelessly reusing unused `address`
memcpy( NES->ppu.sprites, &NES->ram[address], 0x100 );
int cpu_cycles = ( NES->cpu_cycles % 2 == 1 ) ? 514 : 513; // +1 cycle on odd CPU cycles
NES->cpu_cycles += cpu_cycles;
NES->ppu_cycles += 3 * cpu_cycles;
}
// -------------------------------------------------------------------------------
// $4016
byte read_gamepad( void *sys, word address )
{
byte value = 0;
if( NES->input.strobe_state == Nes_Strobe_clear )
{
NES->input.strobe_state = Nes_Strobe_reading;
NES->input.read_count[0] = NES->input.read_count[1] = 0;
}
if( NES->input.strobe_state == Nes_Strobe_reading )
{
if(( address == 0x4016 ) && ( NES->input.read_count[0] <= 7 )) {
value = NES->input.gamepad[0][ NES->input.read_count[0] ];
NES->input.read_count[0]++;
}
else if(( address == 0x4017 ) && ( NES->input.read_count[1] <= 7 )) {
value = NES->input.gamepad[1][ NES->input.read_count[1] ];
NES->input.read_count[1]++;
}
}
return value;
}
void write_gamepad( void *sys, word address, byte value )
{
// Ignore writes to 0x4017 for now
if( address == 0x4016 )
{
if( value == 1 ) {
NES->input.strobe_state = Nes_Strobe_reset;
}
else if( value == 0 ) {
if( NES->input.strobe_state == Nes_Strobe_reset ) {
NES->input.strobe_state = Nes_Strobe_clear;
}
// else: what should happen if 0 is written when not in reset state?
}
// else: Some games write $C0 here for APU thingies. Ignore for now
}
}
// -------------------------------------------------------------------------------
byte read_unimplemented( void *sys, word address )
{
assert( 0 && "Memory read not implemented" );
return 0;
}
// -------------------------------------------------------------------------------
void write_unimplemented( void *sys, word address, byte value )
{
assert( 0 && "Memory write not implemented" );
}