Get nibbles before-hand for clarity
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174259f1ea
commit
821c327ed3
106
src/cpu.ts
106
src/cpu.ts
@ -11,7 +11,7 @@ const REGISTERS = 16;
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class CPU {
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pc: number; // 16b
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indexReg: number; // 16b
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registers: Uint8Array;
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regs: Uint8Array;
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delayTimer: number; // 8b
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soundTimer: number; // 8b
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@ -33,7 +33,7 @@ class CPU {
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this.stack = new Array<number>(STACK_SIZE);
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this.delayTimer = 0;
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this.soundTimer = 0;
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this.registers = new Uint8Array(REGISTERS);
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this.regs = new Uint8Array(REGISTERS);
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this.rng = new RNG(0);
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@ -48,7 +48,7 @@ class CPU {
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}
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clearRegisters() {
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this.registers.fill(0);
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this.regs.fill(0);
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this.pc = 0x200; // Start of the program on normal programs
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this.indexReg = 0;
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this.stackPointer = 0;
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@ -105,10 +105,12 @@ class CPU {
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let opcode: number = firstOpcode << 8 | secondOpcode;
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// Decode opcode
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// X000
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let firstNibble = firstOpcode >> 4;
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let nib1 = firstOpcode >> 4; // X000
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let nib2 = firstOpcode & 0xF; // 0X00
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let nib3 = secondOpcode >> 4; // 00X0
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let nib4 = secondOpcode & 0xF; // 000X
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switch(firstNibble) {
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switch(nib1) {
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case 0x0:
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if(opcode == 0x00E0) {
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// Clear the screen
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@ -134,9 +136,8 @@ class CPU {
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case 0x3: {
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// 3XNN
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// Skips the next instruction if VX equals NN
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let secondNibble = firstOpcode & 0xF; //0X00
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let val = opcode & 0x00FF; //0x00NN
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if(this.registers[secondNibble] == val){
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if(this.regs[nib2] == val){
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this.incrementPC();
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}
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this.incrementPC();
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@ -145,9 +146,8 @@ class CPU {
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case 0x4: {
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// 4XNN
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// Skips the next instruction if VX NOT equals NN
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let secondNibble = firstOpcode & 0xF; //0X00
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let val = opcode & 0x00FF; //0x00NN
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if(this.registers[secondNibble] != val){
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if(this.regs[nib2] != val){
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this.incrementPC();
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}
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this.incrementPC();
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@ -156,9 +156,7 @@ class CPU {
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case 0x5: {
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// 5XY0
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// Skips the next instruction if VX == VY
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let secondNibble = firstOpcode & 0xF; //0X00
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let thirdNibble = secondOpcode >> 4; //00Y0
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if(this.registers[secondNibble] == this.registers[thirdNibble]) {
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if(this.regs[nib2] == this.regs[nib3]) {
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this.incrementPC();
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}
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this.incrementPC();
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@ -167,76 +165,71 @@ class CPU {
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case 0x6: {
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// 6XNN
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// Sets VX to NN
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let secondNibble = firstOpcode & 0xF; //0X00
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this.registers[secondNibble] = secondOpcode;
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this.regs[nib2] = secondOpcode;
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this.incrementPC();
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}
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break;
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case 0x7: {
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// 7XNN
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// Adds NN to VX
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let secondNibble = firstOpcode & 0xF; //0X00
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this.registers[secondNibble] = secondOpcode;
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this.regs[nib2] = secondOpcode;
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this.incrementPC();
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}
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break;
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case 0x8: {
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let secondNibble = firstOpcode & 0xF; //0X00
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let thirdNibble = secondOpcode >> 4; //00Y0
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let lastNibble = secondOpcode & 0xF; //000N
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if(lastNibble == 0x0) {
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if(nib4 == 0x0) {
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// 8XY0
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// Vx |= Vy
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this.registers[secondNibble] = this.registers[thirdNibble];
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this.regs[nib2] = this.regs[nib3];
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this.incrementPC();
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}else if(lastNibble == 0x1) {
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}else if(nib4 == 0x1) {
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// 8XY1
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// Vx |= Vy
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this.registers[secondNibble] |= this.registers[thirdNibble];
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this.regs[nib2] |= this.regs[nib3];
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this.incrementPC();
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}else if(lastNibble == 0x2) {
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}else if(nib4 == 0x2) {
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// 8XY2
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// Vx &= Vy
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this.registers[secondNibble] &= this.registers[thirdNibble];
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this.regs[nib2] &= this.regs[nib3];
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this.incrementPC();
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}else if(lastNibble == 0x3) {
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}else if(nib4 == 0x3) {
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// 8XY3
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// Vx ^= Vy
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this.registers[secondNibble] ^= this.registers[thirdNibble];
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this.regs[nib2] ^= this.regs[nib3];
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this.incrementPC();
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}else if(lastNibble == 0x4) {
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}else if(nib4 == 0x4) {
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// 8XY4
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// Vx += Vy
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let sum = this.registers[secondNibble] + this.registers[thirdNibble];
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this.registers[0xF] = sum > 255 ? 1 : 0; // Set carry flag
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this.registers[secondNibble] = sum & 0xFF; // Get only the first byte
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let sum = this.regs[nib2] + this.regs[nib3];
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this.regs[0xF] = sum > 255 ? 1 : 0; // Set carry flag
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this.regs[nib2] = sum & 0xFF; // Get only the first byte
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this.incrementPC();
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}else if(lastNibble == 0x5) {
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}else if(nib4 == 0x5) {
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// 8XY5
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// Vx -= Vy
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this.registers[0xF] = this.registers[secondNibble] > this.registers[thirdNibble] ? 1 : 0;
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let subs = this.registers[secondNibble] - this.registers[thirdNibble];
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this.registers[secondNibble] = subs;
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this.regs[0xF] = this.regs[nib2] > this.regs[nib3] ? 1 : 0;
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let subs = this.regs[nib2] - this.regs[nib3];
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this.regs[nib2] = subs;
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this.incrementPC();
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}else if(lastNibble == 0x6) {
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}else if(nib4 == 0x6) {
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// 8XY6
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// Vx >>= 1
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this.registers[0xF] = this.registers[0xF] & 1; // ???? // Store the least significant bit of VX prior
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this.registers[secondNibble] >>= 1;
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this.regs[0xF] = this.regs[0xF] & 1; // ???? // Store the least significant bit of VX prior
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this.regs[nib2] >>= 1;
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this.incrementPC();
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}else if(lastNibble == 0x7) {
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}else if(nib4 == 0x7) {
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// 8XY7
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// Vx = Vy - Vx
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this.registers[0xF] = this.registers[thirdNibble] > this.registers[secondNibble] ? 1 : 0;
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let subs = this.registers[thirdNibble] - this.registers[secondNibble];
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this.registers[secondNibble] = subs;
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this.regs[0xF] = this.regs[nib3] > this.regs[nib2] ? 1 : 0;
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let subs = this.regs[nib3] - this.regs[nib2];
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this.regs[nib2] = subs;
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this.incrementPC();
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}else if(lastNibble == 0xE) {
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}else if(nib4 == 0xE) {
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// 8XYE
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// Vx <<= 1
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// Set VF to 1 if the most significant bit of VX prior to that shift was set
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this.registers[0xF] = this.registers[0xF] >> 3;
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this.registers[secondNibble] <<= 1;
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this.regs[0xF] = this.regs[0xF] >> 3;
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this.regs[nib2] <<= 1;
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this.incrementPC();
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}
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@ -245,9 +238,7 @@ class CPU {
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case 0x9: {
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// 9XY0
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// Skips the next instruction if VX != VY
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let secondNibble = firstOpcode & 0xF; //0X00
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let thirdNibble = secondOpcode >> 4; //00Y0
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if(this.registers[secondNibble] != this.registers[thirdNibble]) {
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if(this.regs[nib2] != this.regs[nib3]) {
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this.incrementPC();
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}
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this.incrementPC();
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@ -264,31 +255,26 @@ class CPU {
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case 0xB:{
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// BNNN
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// Jumps to the address NNN plus V0
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this.pc = this.registers[0] + opcode & 0x0FFF;
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this.pc = this.regs[0] + opcode & 0x0FFF;
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}
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break;
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case 0xC: {
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// CXNN
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// Vx = rand() & NN
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let secondNibble = firstOpcode & 0xF; //0X00
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this.registers[secondNibble] = this.rng.next255() & secondOpcode;
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this.regs[nib2] = this.rng.next255() & secondOpcode;
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this.incrementPC();
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}
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break;
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case 0xD: {
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// DXYN
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// draw(Vx, Vy, N)
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let secondNibble = firstOpcode & 0xF; //0X00
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let thirdNibble = secondOpcode >> 4; //00Y0
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let fourthNibble = secondOpcode & 0xF;
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this.regs[0xF] = 0;
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this.registers[0xF] = 0;
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let regX = this.registers[secondNibble];
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let regY = this.registers[thirdNibble];
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let regX = this.regs[nib2];
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let regY = this.regs[nib3];
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let y = 0;
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while(y < fourthNibble) {
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while(y < nib4) {
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// A sprite is always 8 bits horizontal
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let pixel = this.memory[this.indexReg + y];
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let x = 0;
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