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emulate.go
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emulate.go
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package main
import (
"math/rand"
"time"
)
func (chip *chip8) EmulateNext() {
decodedInstruction := chip.DecodeInstruction(chip.pc)
chip.EmulateDecodedInstruction(decodedInstruction)
}
func (chip *chip8) EmulateDecodedInstruction(op uint16) {
// flag showing whether we should inc program counter
retflag := true
// mask to get first hex digit
switch op & 0xF000 {
case 0x0000:
switch op & 0xFF {
case 0xE0: // CLS
for i := 0; i < len(chip.disp); i++ {
chip.disp[i] = 0x00
}
chip.graphics.Render()
case 0xEE: // RET
chip.SetPC(chip.stack[chip.sp])
chip.sp--
}
case 0x1000: // SET PC
chip.SetPC(address(op & 0x0FFF))
retflag = false
case 0x2000: // CALL --> 2NNN
chip.sp++
chip.stack[chip.sp] = chip.pc // must return past the last instruction
chip.SetPC(address(op & 0x0FFF))
retflag = false
case 0x3000: // SE
targetReg := uint8((op >> 8) & 0x000F)
compareVal := uint8(op & 0x00FF)
if chip.ReadRegister(targetReg) == compareVal {
chip.IncrementPC()
}
case 0x4000: // SNE
targetReg := uint8((op >> 8) & 0x000F)
compareVal := uint8(op & 0x00FF)
if chip.ReadRegister(targetReg) != compareVal {
chip.IncrementPC()
}
case 0x5000: // SE Vx Vy -> 5XY0
rxval := chip.ReadRegister(uint8((op >> 8) & 0x000F))
ryval := chip.ReadRegister(uint8((op >> 4) & 0x000F))
if rxval == ryval {
chip.IncrementPC()
}
case 0x6000: // LD Vx, byte --> 6xKK
targetReg := uint8((op >> 8) & 0x000F)
ldVal := uint8(op & 0x00FF)
chip.SetRegister(targetReg, ldVal)
case 0x7000: // ADD Vx, byte --> 7xKK
targetReg := uint8((op >> 8) & 0x000F)
kk := uint8(op & 0x00FF)
sum := chip.ReadRegister(targetReg) + kk
chip.SetRegister(targetReg, sum)
case 0x8000: // these take form 8xyZ
rx := uint8((op >> 8) & 0x000F)
ry := uint8((op >> 4) & 0x000F)
rxval := chip.ReadRegister(rx)
ryval := chip.ReadRegister(ry)
switch op & 0x0F {
case 0x0:
chip.SetRegister(rx, ryval)
case 0x1: // OR Vx, Vy
chip.SetRegister(rx, rxval|ryval)
case 0x2: // AND Vx, Vy
chip.SetRegister(rx, rxval&ryval)
case 0x3: // XOR Vx, Vy
chip.SetRegister(rx, rxval^ryval)
case 0x4: // ADD Vx, Vy
sum := uint16(rxval) + uint16(ryval)
trimmedSum := byte(sum)
chip.SetRegister(rx, trimmedSum)
chip.setRegisterOnCondition(0xf, sum > 255)
case 0x5: // SUB Vx, Vy
chip.setRegisterOnCondition(0xf, rxval > ryval)
chip.SetRegister(rx, rxval-ryval)
case 0x6: // SHR Vx {, Vy}
chip.setRegisterOnCondition(0xf, rxval&0x1 == 0x1)
chip.SetRegister(rx, rxval/2)
case 0x7: // SUBN Vx, Vy
chip.setRegisterOnCondition(0xf, ryval > rxval)
chip.SetRegister(rx, ryval-rxval)
case 0xE: // SHL Vx {, Vy}
chip.setRegisterOnCondition(0xf, (rxval>>7)&0x1 == 0x1)
chip.SetRegister(rx, rxval*2)
}
case 0x9000: // SNE Vx, Vy --> 9xy0
rxval := chip.ReadRegister(uint8((op >> 8) & 0x000F))
ryval := chip.ReadRegister(uint8((op >> 4) & 0x000F))
if rxval != ryval {
chip.IncrementPC()
}
case 0xA000: // LD I, addr --> ANNN
chip.I = 0x0FFF & op
case 0xB000: // JP V0, addr --> BNNN
baseAddr := address(op & 0x0FFF)
valV0 := chip.ReadRegister(0x0)
chip.SetPC(baseAddr + address(valV0)) // can we make this conversion?
retflag = false // noinc
case 0xC000: // RND Vx, byte --> CxKK
targetReg := uint8((op >> 8) & 0x000F)
val := uint8(op & 0x00FF)
rand.Seed(time.Now().UnixNano())
result := uint8(rand.Intn(256)) & val
chip.SetRegister(targetReg, byte(result))
case 0xD000: // DRW Vx, Vy --> 0xDXYN
rxval := uint16(chip.ReadRegister(uint8((op >> 8) & 0x000F)))
ryval := uint16(chip.ReadRegister(uint8((op >> 4) & 0x000F)))
n := uint16(op & 0x000F)
var b byte
chip.SetRegister(0xf, 0x0)
for yline := uint16(0); yline < n; yline++ {
b = chip.mem[chip.I+uint16(yline)]
for xline := uint16(0); xline < 8; xline++ {
xWrapped := (xline + rxval) % displayColumns
yWrapped := (yline + ryval) % displayRows
if b&(0x80>>xline) != 0 {
if chip.isPixelSet(xWrapped, yWrapped) {
chip.SetRegister(0xf, 0x1)
}
chip.setPixel(xWrapped, yWrapped)
}
}
}
chip.graphics.Render()
case 0xE000: // SKP Vx --> Ex9E
targetReg := uint8((op >> 8) & 0x000F)
switch op & 0xFF {
case 0x9E:
if chip.keys[chip.ReadRegister(targetReg)] {
chip.IncrementPC()
}
case 0xA1:
if !chip.keys[chip.ReadRegister(targetReg)] {
chip.IncrementPC()
}
}
case 0xF000:
targetReg := uint8((op >> 8) & 0x000F)
// swith over last two digits of opcode for 0xF___
switch op & 0xFF {
case 0x07: // LD Vx, DT --> FX07
chip.SetRegister(targetReg, chip.dt)
case 0x0A: // LD Vx, K --> FX0A
// there should be input channel that some keypress handler writes to,
// and this should wait on that channel, for now scanf
event := <-chip.input
value := SdlKeyToValue(event.Keysym.Sym)
chip.SetRegister(targetReg, value)
case 0x15: // LD DT, Vx --> FX15
chip.dt = chip.ReadRegister(targetReg)
case 0x18:
chip.st = chip.ReadRegister(targetReg)
case 0x1E:
chip.I = uint16(chip.ReadRegister(targetReg)) + chip.I
case 0x29:
hexDigToPointTo := chip.ReadRegister(targetReg)
chip.I = uint16(hexDigToPointTo) * 0x5
case 0x33: // LD B, Vx --> FX33 (BCD into I, I+1, I+2)
val := uint8(chip.ReadRegister(targetReg))
chip.mem[chip.I] = byte(val / 100)
chip.mem[chip.I+1] = byte((val / 10) % 10)
chip.mem[chip.I+2] = byte(val % 10)
case 0x55: // LD [I], Vx
for i := 0; i <= int(targetReg); i++ {
chip.mem[int(chip.I)+i] = chip.ReadRegister(byte(i))
}
case 0x65: // LD Vx, [I]
for i := 0; i <= int(targetReg); i++ {
chip.SetRegister(uint8(i), chip.mem[int(chip.I)+i])
}
}
}
// increment program counter if needed, and decrement
if retflag {
chip.IncrementPC()
}
// update timers
if chip.st > 0 {
chip.sound <- true
chip.st--
} else {
chip.sound <- false
}
if chip.dt > 0 {
chip.dt = chip.dt - 1
}
}
// DecodeInstruction takes an index and decodes the contained bytes as opcode
func (chip *chip8) DecodeInstruction(ind address) uint16 {
decodedInstruction := (uint16(chip.mem[ind]) << 8) | uint16(chip.mem[ind+1])
return decodedInstruction
}
func (chip *chip8) setRegisterOnCondition(reg uint8, predicate bool) {
if predicate {
chip.SetRegister(reg, 1)
} else {
chip.SetRegister(reg, 0)
}
}
func (chip *chip8) setPixel(x, y uint16) {
ind := x + y*displayColumns
chip.disp[ind] = chip.disp[ind] ^ 1
return
}
func (chip *chip8) isPixelSet(x, y uint16) bool {
ind := x + y*displayColumns
return chip.disp[ind] == 0x1
}
// Max provides basic byte min
func Max(a, b byte) byte {
if a > b {
return a
}
return b
}