cpu - Fix overflow flag!
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n loewen
parent
7b143ab000
commit
c84c86c160
136
src/cpu.js
136
src/cpu.js
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@ -152,55 +152,21 @@ module.exports = class CPU {
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add_lit: (lit) => {
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this.dbg.currentMnemonic = 'ADD lit';
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// Calculate sum
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let sum = this.acc + lit;
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if (sum > 255) {
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this.flags.C = true;
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sum = (sum % 255) - 1;
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} else {
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this.flags.C = false;
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}
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// Calculate overflow flag status
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let bitSixCarry = 0;
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if ((this.acc & 64) && (lit & 64)) { bitSixCarry = 1; }
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// let overflow = bitSixCarry ^ (this.flags & 8);
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// FIXME - re-implement overflow
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// I'm on a plane and can't remember how this works
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let overflow = 0;
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if (overflow) {
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this.flags.O = true;
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} else {
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this.flags.O = false;
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}
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const [sum, carry, overflow] = sumCarryOverflow(this.acc, lit);
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this.acc = sum;
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this.flags.C = carry;
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this.flags.O = overflow;
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this._updateFlagNegative();
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this._updateFlagZero();
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this._incrementIP(2);
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},
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add_addr: (addr) => {
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this.dbg.currentMnemonic = 'ADD addr';
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// Calculate sum
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let sum = this.acc + this.memory[addr];
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if (sum > 255) {
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this.flags.C = true;
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sum = (sum % 255) - 1;
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} else {
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this.flags.C = false;
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}
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// Calculate overflow flag status
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let bitSixCarry = 0;
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if ((this.acc & 64) && (addr & 64)) { bitSixCarry = 1; }
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// let overflow = bitSixCarry ^ (this.flags & 8);
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// FIXME - re-implement overflow
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// I'm on a plane and can't remember how this works
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let overflow = 0;
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if (overflow) {
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this.flags.O = true;
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} else {
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this.flags.O = false;
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}
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this.dbg.currentMnemonic = `ADD addr; @ addr: $${num2hex(this.memory[addr])}`;
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const [sum, carry, overflow] = sumCarryOverflow(this.acc, this.memory[addr]);
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this.acc = sum;
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this.flags.C = carry;
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this.flags.O = overflow;
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this._updateFlagNegative();
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this._updateFlagZero();
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this._incrementIP(2);
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@ -208,55 +174,21 @@ module.exports = class CPU {
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sub_lit: (lit) => {
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this.dbg.currentMnemonic = 'SUB lit';
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// Calculate sum
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let sum = this.acc - lit;
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if (sum < 0) {
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this.flags.C = true;
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sum = sum + 256;
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} else {
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this.flags.C = false;
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}
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// Calculate overflow flag status
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let bitSixCarry = 0;
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if ((this.acc & 64) && (lit & 64)) { bitSixCarry = 1; }
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// let overflow = bitSixCarry ^ (this.flags & 8);
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// FIXME - re-implement overflow
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// I'm on a plane and can't remember how this works
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let overflow = 0;
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if (overflow) {
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this.flags.O = true;
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} else {
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this.flags.O = false;
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}
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this.acc = sum;
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const [difference, carry, overflow] = differenceCarryOverflow(this.acc, lit);
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this.acc = difference;
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this.flags.C = carry;
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this.flags.O = overflow;
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this._updateFlagNegative();
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this._updateFlagZero();
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this._incrementIP(2);
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},
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sub_addr: (addr) => {
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this.dbg.currentMnemonic = 'SUB addr';
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// Calculate sum
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let sum = this.acc - this.memory[addr];
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if (sum < 0) {
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this.flags.C = true;
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sum = sum + 256;
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} else {
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this.flags.C = false;
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}
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// Calculate overflow flag status
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let bitSixCarry = 0;
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if ((this.acc & 64) && (addr & 64)) { bitSixCarry = 1; }
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// let overflow = bitSixCarry ^ (this.flags & 8);
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// FIXME - re-implement overflow
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// I'm on a plane and can't remember how this works
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let overflow = 0;
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if (overflow) {
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this.flags.O = true;
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} else {
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this.flags.O = false;
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}
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this.acc = sum;
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this.dbg.currentMnemonic = `SUB addr; @ addr: $${num2hex(this.memory[addr])}`;
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const [difference, carry, overflow] = differenceCarryOverflow(this.acc, this.memory[addr]);
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this.acc = difference;
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this.flags.C = carry;
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this.flags.O = overflow;
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this._updateFlagNegative();
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this._updateFlagZero();
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this._incrementIP(2);
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@ -350,3 +282,39 @@ module.exports = class CPU {
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process.exit();
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}
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}
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/**
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* @arg {number} n1
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* @arg {number} n2
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* @returns {[number, boolean, boolean]} [sum, carry, overflow]
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**/
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function sumCarryOverflow(n1, n2) {
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let sum = n1 + n2;
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let carry = false;
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if (sum > 255) {
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carry = true;
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sum = (sum % 255) - 1;
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}
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let n1_bit6 = (n1 & 64) === 64; // Bit 6 is the 64s place
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let n2_bit6 = (n2 & 64) === 64; // 64 & n == 64 where n >= 64
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let carryIntoLastBit = n1_bit6 && n2_bit6;
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console.log('c_in', carryIntoLastBit, 'c_out', carry);
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let overflow = carryIntoLastBit != carry;
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return [sum, carry, overflow];
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}
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/**
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* @arg {number} n1
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* @arg {number} n2
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* @returns {[number, boolean, boolean]} [sum, carry, overflow]
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**/
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function differenceCarryOverflow(n1, n2) {
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// https://www.righto.com/2012/12/the-6502-overflow-flag-explained.html
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// > SBC simply takes the ones complement of the second value and then performs an ADC.
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//
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// https://stackoverflow.com/a/8966863
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// > The signed overflow flag value, however, must be the same for both A-B and A+(-B) because it depends on whether or not the result has the correct sign bit and in both cases the sign bit will be the same.
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return sumCarryOverflow(n1, -n2);
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}
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@ -0,0 +1,63 @@
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;; Test behaviour of flags during addition and subtraction
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;; with a focus on the Overflow flag
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2023-08-29
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; http://teaching.idallen.com/dat2343/11w/notes/040_overflow.txt:
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;
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; > 1. If the sum of two numbers with the sign bits off yields a result number
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; with the sign bit on, the "overflow" flag is turned on.
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; >
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; > 0100 + 0100 = 1000 (overflow flag is turned on)
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; >
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; > 2. If the sum of two numbers with the sign bits on yields a result number
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; > with the sign bit off, the "overflow" flag is turned on.
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; >
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; > 1000 + 1000 = 0000 (overflow flag is turned on)
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; >
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; > Otherwise, the overflow flag is turned off.
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; > * 0100 + 0001 = 0101 (overflow flag is turned off)
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; > * 0110 + 1001 = 1111 (overflow flag is turned off)
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; > * 1000 + 0001 = 1001 (overflow flag is turned off)
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; > * 1100 + 1100 = 1000 (overflow flag is turned off)
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;; Check simple addition and subtraction
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; LDA 1
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; STO 0
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; LDA 1
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; ADD 1
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; LDA 1
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; ADD (0)
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; LDA 3
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; SUB 1
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; LDA 3
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; SUB (0)
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;; Check zero flag, negative flag
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; LDA 0
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; LDA 255
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;; Check overflow flag
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LDA 0b01000000
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ADD 0b01000000 ; 10000000 ; Overflow flag is on
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LDA 0b10000000
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ADD 0b10000000 ; 00000000 ; Overflow flag is on
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; > * 0100 + 0001 = 0101 (overflow flag is turned off)
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; > * 0110 + 1001 = 1111 (overflow flag is turned off)
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; > * 1000 + 0001 = 1001 (overflow flag is turned off)
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; > * 1100 + 1100 = 1000 (overflow flag is turned off)
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LDA 0b01000000
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ADD 0b00010000 ; 01010000 ; overflow off
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LDA 0b01100000
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ADD 0b10010000 ; 11110000 ; overflow off
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LDA 0b10000000
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ADD 0b00010000 ; 10010000 ; overflow off
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LDA 0b11000000
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ADD 0b11000000 ; 10000000 ; overflow off
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