o cpu

- fixed decimal mode subtraction and addition

hardware/cpu/cpu_test.go

@@ -450,6 +450,19 @@ func testSubroutineInstructions(t *testing.T, mc *cpu.CPU, mem *MockMem) {
 	assert.CheckValueVCS(t, mc.SP, 255)
 }
 
+func testDecimalMode(t *testing.T, mc *cpu.CPU, mem *MockMem) {
+	var origin uint16
+	mem.Clear()
+	mc.Reset()
+
+	origin = mem.putInstructions(origin, 0xf8, 0xa9, 0x20, 0x38, 0xe9, 0x01)
+	step(t, mc) // SED
+	step(t, mc) // LDA #$20
+	step(t, mc) // SEC
+	step(t, mc) // SBC #$00
+	assert.CheckValueVCS(t, mc.A, 0x19)
+}
+
 func TestCPU(t *testing.T) {
 	mem := NewMockMem()
 	mc, err := cpu.New(mem)
@@ -467,4 +480,5 @@ func TestCPU(t *testing.T) {
 	testJumps(t, mc, mem)
 	testComparisonInstructions(t, mc, mem)
 	testSubroutineInstructions(t, mc, mem)
+	testDecimalMode(t, mc, mem)
 }

hardware/cpu/register/decimal_mode.go

@@ -26,11 +26,6 @@ func (r *Register) AddDecimal(v interface{}, carry bool) (rcarry bool) {
 		panic(fmt.Sprintf("decimal mode addition only supported for uint8 values with 8 bit registers"))
 	}
 
-	// no need to do anything if operand is zero
-	if val == 0 {
-		return carry
-	}
-
 	runits := uint8(r.value) & 0x0f
 	rtens := (uint8(r.value) & 0xf0) >> 4
 
@@ -69,11 +64,6 @@ func (r *Register) SubtractDecimal(v interface{}, carry bool) (rcarry bool) {
 		panic(fmt.Sprintf("decimal mode subtraction only supported for uint8 values with 8 bit registers"))
 	}
 
-	// no need to do anything if operand is zero
-	if val == 0 {
-		return carry
-	}
-
 	runits := int(r.value) & 0x0f
 	rtens := (int(r.value) & 0xf0) >> 4