fixed bcd (but the bcd test code still fails, strange)

sim65/src/Sim65Main.kt

@@ -70,19 +70,29 @@ private fun startSimulator(args: Array<String>) {
     //ram.dump(0x8000, 0x802f)
     //cpu.disassemble(ram, 0x8000, 0x802f)
 
-    while(true) {
+    try {
-        bus.clock()
+        while (true) {
-        if(cpu.totalCycles > 300)
+            bus.clock()
-            break
+        }
+    } catch(e: InstructionError) {
+
     }
 
     if(ram.read(0x0400)==0.toShort())
         println("BCD TEST: OK!")
     else {
+        val code = ram.read(0x0400)
         val v1 = ram.read(0x0401)
         val v2 = ram.read(0x0402)
-        println("BCD TEST: FAIL!! value1=${hexB(v1)} value2=${hexB(v2)}")
+        val predictedA = ram.read(0x00fc)
+        val actualA = ram.read(0x00fd)
+        val predictedF = ram.read(0x00fe)
+        val actualF = ram.read(0x00ff)
+        println("BCD TEST: FAIL!! code=${hexB(code)} value1=${hexB(v1)} value2=${hexB(v2)}")
+        println("  predictedA=${hexB(predictedA)}")
+        println("  actualA=${hexB(actualA)}")
+        println("  predictedF=${predictedF.toString(2).padStart(8,'0')}")
+        println("  actualF=${actualF.toString(2).padStart(8,'0')}")
     }
-    ram.dump(0x0400, 0x0402)
 
 }

sim65/src/components/Cpu6502.kt

@@ -1,6 +1,5 @@
 package sim65.components
 
-
 class InstructionError(msg: String) : RuntimeException(msg)
 
 interface ICpu {
@@ -17,7 +16,6 @@ interface ICpu {
 }
 
 // TODO: add the optional additional cycles to certain instructions and addressing modes
-// TODO: fix sbc and adc with BCD arithmetic.
 // TODO: add IRQ and NMI signaling.
 // TODO: make a 65c02 variant as well (and re-enable the unit tests for that).
 
@@ -699,23 +697,31 @@ class Cpu6502(private val illegalInstrsAllowed: Boolean) : BusComponent(), ICpu
         val operand = getFetched()
         if (Status.D) {
             // BCD add
-            var lo = (A and 0x0f) + (operand and 0x0f) + if (Status.C) 1 else 0
+            // see http://www.6502.org/tutorials/decimal_mode.html
-            if (lo and 0xff > 9) lo += 6
+            // and http://nesdev.com/6502.txt
-            var hi = (A shr 4) + (operand shr 4) + if (lo > 15) 1 else 0
+            // and https://sourceforge.net/p/vice-emu/code/HEAD/tree/trunk/vice/src/6510core.c#l598
-            Status.N = (hi and 8) != 0      // strange... other sources say "BCD is never negative on NMOS 6502 (bug)"
+            // (the implementation below is based on the code used by Vice)
-            Status.V = ((((hi shl 4) xor A) and 0x80) !=0) && ((A xor operand) and 0x80)==0   // strange... other sources say "V is never set in BCD mode"
+            var tmp = (A and 0xf) + (operand and 0xf) + (if (Status.C) 1 else 0)
-            if (hi and 0xff > 9) hi += 6
+            if (tmp > 0x9) tmp += 0x6
-            A = lo and 0x0f or (hi shl 4) and 0xff
+            tmp = if (tmp <= 0x0f) {
-            Status.C = hi > 15
+                (tmp and 0xf) + (A and 0xf0) + (operand and 0xf0)
-            Status.Z = A == 0
+            } else {
+                (tmp and 0xf) + (A and 0xf0) + (operand and 0xf0) + 0x10
+            }
+            Status.Z = ((A + operand + (if (Status.C) 1 else 0)) and 0xff) == 0
+            Status.N = (tmp and 0b10000000) != 0
+            Status.V = ((A xor tmp) and 0x80) != 0 && ((A xor operand) and 0x80) == 0
+            if (tmp > 0x90) tmp += 0x60
+            Status.C = tmp > 0xf0
+            A = tmp and 255
         } else {
             // normal add
-            val result = A + operand + if (Status.C) 1 else 0
+            val tmp = operand + A + if (Status.C) 1 else 0
-            Status.C = result > 255
+            Status.N = (tmp and 0b10000000) != 0
-            Status.V = (A xor operand).inv() and (A xor result) and 0x0080 != 0
+            Status.Z = (tmp and 255) == 0
-            A = result and 255
+            Status.V = ((A xor operand) and 0x80) == 0 && ((A xor tmp) and 0x80) != 0
-            Status.N = (A and 0b10000000) != 0
+            Status.C = tmp > 255
-            Status.Z = A == 0
+            A = tmp and 255
         }
     }
 
@@ -989,29 +995,30 @@ class Cpu6502(private val illegalInstrsAllowed: Boolean) : BusComponent(), ICpu
     }
 
     private fun iSbc() {
+        val operand = getFetched()
+        val tmp = (A - operand - if (Status.C) 0 else 1) and 65535
         if (Status.D) {
-            val operand = getFetched()
+            // BCD subtract
-            var lo: Int
+            // see http://www.6502.org/tutorials/decimal_mode.html
-            var hi: Int
+            // and http://nesdev.com/6502.txt
-            lo = (A and 0x0f) - (operand and 0x0f) - if (Status.C) 0 else 1
+            // and https://sourceforge.net/p/vice-emu/code/HEAD/tree/trunk/vice/src/6510core.c#l1396
-            if (lo and 0x10 != 0) lo -= 6
+            // (the implementation below is based on the code used by Vice)
-            hi = (A shr 4) - (operand shr 4) - if (lo and 0x10 != 0) 1 else 0
+            var tmpA = ((A and 0xf) - (operand and 0xf) - if (Status.C) 0 else 1) and 65535
-            if (hi and 0x10 != 0) hi -= 6
+            tmpA = if ((tmpA and 0x10) != 0) {
-            A = lo and 0x0f or (hi shl 4) and 0xff
+                ((tmpA - 6) and 0xf) or (A and 0xf0) - (operand and 0xf0) - 0x10
-            Status.C = hi and 0xff < 15
+            } else {
-            Status.V = false  // BCD never sets overflow flag
+                (tmpA and 0xf) or (A and 0xf0) - (operand and 0xf0)
-            Status.Z = A==0
+            }
-            Status.N = (A and 0b10000000) != 0
+            if ((tmpA and 0x100) != 0) tmpA -= 0x60
+            A = tmpA and 255
         } else {
-            // normal sub
+            // normal subtract
-            val invertedOperand = getFetched() xor 255
+            A = tmp and 255
-            val result = A + invertedOperand + if (Status.C) 1 else 0
-            Status.C = result > 255
-            Status.V = (A xor invertedOperand) and (A xor result) and 0x0080 != 0
-            A = result and 255
-            Status.N = (A and 0b10000000) != 0
-            Status.Z = A == 0
         }
+        Status.C = tmp < 0x100
+        Status.Z = (tmp and 255) == 0
+        Status.N = (tmp and 0b10000000) != 0
+        Status.V = ((A xor tmp) and 0x80) != 0 && ((A xor operand) and 0x80) != 0
     }
 
     private fun iSec() {

sim65/test/Test6502CpuBasics.kt

@@ -19,7 +19,8 @@ class Test6502CpuBasics {
         assertEquals(0, cpu.X)
         assertEquals(0, cpu.Y)
         assertEquals(0, cpu.currentOpcode)
-        assertEquals(Cpu6502.StatusRegister(C = false, Z = false, I = false, D = false, B = false, V = false, N = false), cpu.Status)
+        assertEquals(Cpu6502.StatusRegister(C = false, Z = false, I = true, D = false, B = false, V = false, N = false), cpu.Status)
+        assertEquals(0b00100100, cpu.Status.asByte())
     }
 
 }

sim65/test/TestCommon6502.kt

@@ -507,8 +507,8 @@ abstract class TestCommon6502 {
         assertEquals(0x80, mpu.A)
         assertFalse(mpu.Status.C)
         assertFalse(mpu.Status.Z)
-        assertTrue(mpu.Status.V)
         assertTrue(mpu.Status.N)
+        assertTrue(mpu.Status.V)
     }
 
     @Test
@@ -534,6 +534,7 @@ abstract class TestCommon6502 {
 
         mpu.Status.D = true
         mpu.Status.C = false
+        mpu.Status.N = true
         mpu.A = 0x9c
         // $0000 ADC #$9d
         // $0002 ADC #$9d
@@ -545,10 +546,10 @@ abstract class TestCommon6502 {
         mpu.step()
         assertEquals(0x0004, mpu.PC)
         assertEquals(0x93, mpu.A)
-        assertFalse(mpu.Status.N)
         assertFalse(mpu.Status.Z)
         assertTrue(mpu.Status.C)
         assertTrue(mpu.Status.V)
+        assertFalse(mpu.Status.N)
     }
 
     // ADC Absolute, X-Indexed
@@ -5696,8 +5697,8 @@ abstract class TestCommon6502 {
         assertEquals(0x9a, mpu.A)
         assertFalse(mpu.Status.Z)
         assertTrue(mpu.Status.C)
-        assertFalse(mpu.Status.V)
         assertTrue(mpu.Status.N)
+        assertFalse(mpu.Status.V)
     }
 
     @Test
@@ -5714,9 +5715,9 @@ abstract class TestCommon6502 {
         assertEquals(0x0002, mpu.PC)
         assertEquals(0x99, mpu.A)
         assertFalse(mpu.Status.Z)
-        assertFalse(mpu.Status.C)
-        assertFalse(mpu.Status.V)
         assertTrue(mpu.Status.N)
+        assertFalse(mpu.Status.V)
+        assertFalse(mpu.Status.C)
     }
 
     @Test
@@ -5729,10 +5730,10 @@ abstract class TestCommon6502 {
         mpu.step()
         assertEquals(0x0002, mpu.PC)
         assertEquals(0x1f, mpu.A)
-        assertFalse(mpu.Status.N)
-        assertFalse(mpu.Status.V)
         assertFalse(mpu.Status.Z)
         assertTrue(mpu.Status.C)
+        assertFalse(mpu.Status.N)
+        assertFalse(mpu.Status.V)
     }
 
     // SBC Absolute, X-Indexed