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package ppu
import "crecher.net/gones/v2/cart"
// maps a $2000-$3EFF address onto an offset into the 2KB of nametable RAM,
// according to the cartridge's mirroring mode. four-screen carts will be
// expected to use their own extra 2KB of RAM via the mapper, but since no
// implemented mapper does that yet, four-screen just falls back to the vertical
// mirroring as a placeholder
func (p *PPU) nametableOffset(addr uint16) int {
addr &= 0x0FFF
table := addr / 0x400
offset := int(addr % 0x400)
var page uint16
switch p.mirroring {
case cart.Horizontal:
page = table / 2
case cart.SingleScreenLower:
page = 0
case cart.SingleScreenUpper:
page = 1
default: // TODO: add correct four-screen behavior once four-screen carts are added
page = table % 2
}
return int(page)*0x400 + offset
}
// paletteIndex applies the $3F10/$3F14/$3F18/$3F1C -> $3F00/$3F04/$3F08/$3F0C
// mirroring
func paletteIndex(addr uint16) uint16 {
idx := addr & 0x1F
if idx >= 0x10 && idx%4 == 0 {
idx -= 0x10
}
return idx
}
func (p *PPU) readPalette(addr uint16) uint8 {
return p.palette[paletteIndex(addr)]
}
func (p *PPU) writePalette(addr uint16, v uint8) {
p.palette[paletteIndex(addr)] = v
}
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