ppuLoopyReg.go

package nes

// Loopy registers are 15 bit internal PPU registers used for implementing
// scrolling.
// Loopy register layout:
//   yyy NN YYYYY XXXXX
//
//   yyy   - fine Y scroll
//   NN    - nametable select
//   YYYYY - coarse Y scroll
//   XXXXX - coarse X scroll
type PpuLoopyReg uint16

const (
	loopyCoarseX   PpuLoopyReg = 0b11111
	loopyCoarseY               = 0b11111 << 5
	loopyNametable             = 0b11 << 10
	loopyFineY                 = 0b111 << 12
)

// Returns the value fo the loopy register as a unsigned 16-bit integer.
func (r *PpuLoopyReg) value() uint16 {
	return uint16(*r)
}

// Sets coarse X (bits 0-4) of the loopy register with the low 5 bits of the
// given value.
func (r *PpuLoopyReg) setCoarseX(val byte) {
	// Get relevant 5 bits
	setBits := PpuLoopyReg(val) & 0b11111

	// Clear bits about to be set
	*r &^= loopyCoarseX

	// Set new bits
	*r |= (setBits)
}

// Sets coarse Y (bits 5-9) of the loopy register with the low 5 bits of the
// given value.
func (r *PpuLoopyReg) setCoarseY(val byte) {
	// Get relevant 5 bits
	setBits := PpuLoopyReg(val) & 0b11111

	// Clear bits about to be set
	*r &^= loopyCoarseY

	// Set new bits
	*r |= (setBits << 5)
}

// Sets nametable (bits 10-11) of the loopy register with the low 2 bits of the
// given value.
func (r *PpuLoopyReg) setNametable(val byte) {
	// Get relevant 2 bits
	setBits := PpuLoopyReg(val) & 0b11

	// Clear bits about to be set
	*r &^= loopyNametable

	// Set new bits
	*r |= (setBits << 10)
}

// Sets fine Y (bits 12-14) of the loopy register with the low 3 bits of the
// given value.
func (r *PpuLoopyReg) setFineY(val byte) {
	// Get relevant 3 bits
	setBits := PpuLoopyReg(val) & 0b111

	// Clear bits about to be set
	*r &^= loopyFineY

	// Set new bits
	*r |= (setBits << 12)
}

func (r *PpuLoopyReg) getCoarseX() byte {
	return byte(*r & loopyCoarseX)
}

func (r *PpuLoopyReg) getCoarseY() byte {
	return byte((*r & loopyCoarseY) >> 5)
}

func (r *PpuLoopyReg) getNametable() byte {
	return byte((*r & loopyNametable) >> 10)
}

func (r *PpuLoopyReg) getFineY() byte {
	return byte((*r & loopyFineY) >> 12)
}

// Swap to the other horizontal nametable.
// See: https://wiki.nesdev.com/w/index.php/PPU_scrolling#Wrapping_around
func (r *PpuLoopyReg) toggleNametableH() {
	*r ^= 0x0400
}

// Swap to the other vertical nametable.
func (r *PpuLoopyReg) toggleNametableV() {
	*r ^= 0x0800
}