Feature (simulator): Implement Negative, Zero, and Overflow flags

simulator.js

@@ -2,12 +2,6 @@ const { INITIAL_IP_ADDRESS, CYCLE_LIMIT } = require('./machine.config');
 const { num2hex, num2bin_4bit } = require('./logging.js');
 const display = require('./display.js');
 
-// TODO:
-// implement setting and clearing of:
-// - [ ] overflow flag
-// - [ ] zero flag
-// - [ ] negative flag
-
 // STATE
 const CPU = {
   running: false,
@@ -36,6 +30,17 @@ const CPU = {
   unsetFlagZero:     () => { CPU.FLAGS &= ~4 },
   unsetFlagOverflow: () => { CPU.FLAGS &= ~2 },
   unsetFlagCarry:    () => { CPU.FLAGS &= ~1 },
+
+  updateFlagZero: () => {
+    if (CPU.Acc === 0) {
+      CPU.setFlagZero();
+    } else {
+      CPU.unsetFlagZero();
+    }
+  },
+
+  updateFlagNegative: () => {
+    CPU.Acc & 128 ? CPU.setFlagNegative : CPU.unsetFlagNegative },
 }
 
 
@@ -62,68 +67,116 @@ const Instructions = {
   load_lit: (lit) => {
     CPU.currentInstruction.mnemonic = 'LDA lit';
     CPU.Acc = lit;
+    CPU.updateFlagNegative();
+    CPU.updateFlagZero();
     CPU.IP = CPU.IP += 2;
   },
 
   load_addr: (addr) => {
     CPU.currentInstruction.mnemonic = `LDA addr; @ addr: ${num2hex(CPU.memory[addr])}`;
     CPU.Acc = CPU.memory[addr];
+    CPU.updateFlagNegative();
+    CPU.updateFlagZero();
     CPU.IP = CPU.IP += 2;
   },
 
   add_lit: (lit) => {
     CPU.currentInstruction.mnemonic = 'ADD lit';
+    // Calculate sum
     let sum = CPU.Acc + lit;
-    // TODO: implement NZO flags
     if (sum > 255) {
       CPU.setFlagCarry();
-      CPU.Acc = (sum % 255) - 1;
+      sum = (sum % 255) - 1;
     } else {
       CPU.unsetFlagCarry();
-      CPU.Acc = sum;
     }
+    // Calculate overflow flag status
+    let bitSixCarry = 0;
+    if ((CPU.Acc & 64) && (lit & 64)) { bitSixCarry = 1; }
+    let overflow = bitSixCarry ^ (CPU.FLAGS & 1)
+    if (overflow) {
+      CPU.setFlagOverflow();
+    } else {
+      CPU.unsetFlagOverflow();
+    }
+    CPU.Acc = sum;
+    CPU.updateFlagNegative();
+    CPU.updateFlagZero();
     CPU.IP = CPU.IP += 2;
   },
 
   add_addr: (addr) => {
     CPU.currentInstruction.mnemonic = 'ADD addr';
+    // Calculate sum
     let sum = CPU.Acc + CPU.memory[addr];
-    // TODO: implement NZO flags
     if (sum > 255) {
       CPU.setFlagCarry();
-      CPU.Acc = (sum % 255) - 1;
+      sum = (sum % 255) - 1;
     } else {
       CPU.unsetFlagCarry();
-      CPU.Acc = sum;
     }
+    // Calculate overflow flag status
+    let bitSixCarry = 0;
+    if ((CPU.Acc & 64) && (addr & 64)) { bitSixCarry = 1; }
+    let overflow = bitSixCarry ^ (CPU.FLAGS & 1)
+    if (overflow) {
+      CPU.setFlagOverflow();
+    } else {
+      CPU.unsetFlagOverflow();
+    }
+    CPU.Acc = sum;
+    CPU.updateFlagNegative();
+    CPU.updateFlagZero();
     CPU.IP = CPU.IP += 2;
   },
 
   sub_lit: (lit) => {
     CPU.currentInstruction.mnemonic = 'SUB lit';
+    // Calculate sum
     let sum = CPU.Acc - lit;
-    // TODO: implement NZO flags
     if (sum < 0) {
       CPU.setFlagCarry();
-      CPU.Acc = 255 + sum + 1;
+      sum = sum + 256;
     } else {
       CPU.unsetFlagCarry();
-      CPU.Acc = sum;
     }
+    // Calculate overflow flag status
+    let bitSixCarry = 0;
+    if ((CPU.Acc & 64) && (lit & 64)) { bitSixCarry = 1; }
+    let overflow = bitSixCarry ^ (CPU.FLAGS & 1)
+    if (overflow) {
+      CPU.setFlagOverflow();
+    } else {
+      CPU.unsetFlagOverflow();
+    }
+    CPU.Acc = sum;
+    CPU.updateFlagNegative();
+    CPU.updateFlagZero();
     CPU.IP = CPU.IP += 2;
   },
 
   sub_addr: (addr) => {
     CPU.currentInstruction.mnemonic = 'SUB addr';
+    // Calculate sum
     let sum = CPU.Acc - CPU.memory[addr];
-    // TODO: implement NZO flags
     if (sum < 0) {
       CPU.setFlagCarry();
-      CPU.Acc = 255 + sum + 1;
+      sum = sum + 256;
     } else {
       CPU.unsetFlagCarry();
-      CPU.Acc = sum;
     }
+    // Calculate overflow flag status
+    let bitSixCarry = 0;
+    if ((CPU.Acc & 64) && (addr & 64)) { bitSixCarry = 1; }
+    let overflow = bitSixCarry ^ (CPU.FLAGS & 1)
+    if (overflow) {
+      CPU.setFlagOverflow();
+    } else {
+      CPU.unsetFlagOverflow();
+    }
+    CPU.Acc = sum;
+    CPU.updateFlagNegative();
+    CPU.updateFlagZero();
     CPU.IP = CPU.IP += 2;
   },
 
@@ -163,21 +216,18 @@ const Instructions = {
     if (flagNum === 2) { mask = 4; }
     if (flagNum === 3) { mask = 8; }
     if (mask === null) { throw new Error('Invalid flag number'); }
-    console.log('flag mask:', mask);
     CPU.FLAGS = CPU.FLAGS ^= mask;
     CPU.IP += 2;
   },
 
   flag_hop: (flagNum) => {
     CPU.currentInstruction.mnemonic = `FHP; IP+2: ${CPU.memory[CPU.IP+2]}, IP+3: ${CPU.memory[CPU.IP+3]}`;
-    console.log('flag number:', flagNum);
     let mask = null;
     if (flagNum === 0) { mask = 1; }
     if (flagNum === 1) { mask = 2; }
     if (flagNum === 2) { mask = 4; }
     if (flagNum === 3) { mask = 8; }
     if (mask === null) { throw new Error('Invalid flag number'); }
-    console.log('flag mask:', mask);
     if (CPU.FLAGS & mask) {
       CPU.IP += 4;
     } else {
@@ -264,7 +314,7 @@ async function logCPUState(debug = false) {
     console.log('Mnemonic:', CPU.currentInstruction.mnemonic);
     console.log(`Machine:  $${num2hex(CPU.currentInstruction.opcode)} $${num2hex(CPU.currentInstruction.operand)}`);
     console.log();
-    console.log(`IP: $${num2hex(CPU.IP)}  Acc: $${num2hex(CPU.Acc)}  FLAGS: ${num2bin_4bit(CPU.FLAGS)}  ${CPU.running ? "running" : "halted" }`);
+    console.log(`IP: $${num2hex(CPU.IP)}  Acc: $${num2hex(CPU.Acc)}  NVOC: ${num2bin_4bit(CPU.FLAGS)}  ${CPU.running ? "running" : "halted" }`);
     console.log();
     // Pause to show animated display:
     if (!debug) await new Promise(resolve => setTimeout(resolve, 75));

test-programs/flag-overflow.asm

@@ -0,0 +1,36 @@
+;; test overflow flag
+; https://www.righto.com/2012/12/the-6502-overflow-flag-explained.html
+
+LDA $50
+ADD $10 ; CF, OF: 0 0
+LDA $50
+ADD $50 ; CF, OF: 0 1
+LDA $50
+ADD $90 ; CF, OF: 0 0
+LDA $50
+ADD $D0 ; CF, OF: 1 0
+LDA $D0
+ADD $10 ; CF, OF: 0 0
+LDA $D0
+ADD $50 ; CF, OF: 1 0
+LDA $D0
+ADD $90 ; CF, OF: 1 1
+LDA $D0
+ADD $D0 ; CF, OF: 1 0
+
+LDA $50
+SUB $F0   ; CF, OF: 1 0
+LDA $50
+SUB $B0 ; CF, OF: 1 1
+LDA $50
+SUB $70 ; CF, OF: 1 0
+LDA $50
+SUB $30 ; CF, OF: 0 0
+LDA $D0
+SUB $F0 ; CF, OF: 1 0
+LDA $D0
+SUB $B0 ; CF, OF: 0 0
+LDA $D0
+SUB $70 ; CF, OF: 0 1
+LDA $D0
+SUB $30 ; CF, OF: 0 0