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Author SHA1 Message Date
Seth Morabito cda9a218af
Merge pull request #6 from Screwtapello/irq-disabled-at-reset
Make the Processor Status register match a real 6502 at power-on.
2023-02-03 16:38:06 -08:00
Tim Allen b5a470d3ba Make the Processor Status register match a real 6502 at power-on.
When describing the CPU's reset pin, the W65C02S data sheet says:

> All Registers are initialized by software except the Decimal and Interrupt
> disable mode select bits of the Processor Status Register (P) are initialized
> by hardware.

It then has a diagram of the power-on state of the processor status register:

>     7 6 5 4 3 2 1 0
>     * * 1 1 0 1 * *
>     N V - B D I Z C
>
> * = software initialized

Confusingly the text indicates that only the D and I flags are initialised by
hardware, while the diagram indicates that the B flag is initialised too.

Meanwhile, https://www.nesdev.org/wiki/CPU_power_up_state says that
the power-on state of the NES CPU is $34 (exactly matching the diagram above)
but https://www.nesdev.org/wiki/Status_flags#The_B_flag says that the B flag
does not physically exist within P register, it's only relevant in the copy
of P that gets pushed to the stack by BRK (set), PHP (set), or an interrupt
signal (cleared).

As a result, the most sensible power-on state for the processor status register
is with the "interrupt disable" flag set and everything else cleared.
2023-02-03 18:16:57 +11:00
16 changed files with 40 additions and 38 deletions

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@ -126,7 +126,7 @@ public class Cpu implements InstructionTable {
// Clear status register bits.
state.carryFlag = false;
state.zeroFlag = false;
state.irqDisableFlag = false;
state.irqDisableFlag = true;
state.decimalModeFlag = false;
state.breakFlag = false;
state.overflowFlag = false;

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@ -29,7 +29,7 @@ public class CpuAbsoluteModeTest extends TestCase {
assertEquals(0, cpu.getYRegister());
assertEquals(0x200, cpu.getProgramCounter());
assertEquals(0xff, cpu.getStackPointer());
assertEquals(0x20, cpu.getProcessorStatus());
assertEquals(0x24, cpu.getProcessorStatus());
}
/*
@ -169,7 +169,7 @@ public class CpuAbsoluteModeTest extends TestCase {
assertEquals(0x04, bus.read(0x1fe, true));
// No flags should have changed.
assertEquals(0x20, cpu.getProcessorStatus());
assertEquals(0x24, cpu.getProcessorStatus());
}
/* BIT - Bit Test - $2c */
@ -362,7 +362,7 @@ public class CpuAbsoluteModeTest extends TestCase {
cpu.step();
assertEquals(0x3400, cpu.getProgramCounter());
// No change to status flags.
assertEquals(0x20, cpu.getProcessorStatus());
assertEquals(0x24, cpu.getProcessorStatus());
}
/* EOR - Exclusive OR - $4d */

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@ -29,7 +29,7 @@ public class CpuAbsoluteXModeTest extends TestCase {
assertEquals(0, cpu.getYRegister());
assertEquals(0x200, cpu.getProgramCounter());
assertEquals(0xff, cpu.getStackPointer());
assertEquals(0x20, cpu.getProcessorStatus());
assertEquals(0x24, cpu.getProcessorStatus());
}
/*

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@ -29,7 +29,7 @@ public class CpuAbsoluteYModeTest extends TestCase {
assertEquals(0, cpu.getYRegister());
assertEquals(0x200, cpu.getProgramCounter());
assertEquals(0xff, cpu.getStackPointer());
assertEquals(0x20, cpu.getProcessorStatus());
assertEquals(0x24, cpu.getProcessorStatus());
}
/*

View File

@ -30,7 +30,7 @@ public class CpuAccumulatorModeTest extends TestCase {
assertEquals(0, cpu.getYRegister());
assertEquals(0x200, cpu.getProgramCounter());
assertEquals(0xff, cpu.getStackPointer());
assertEquals(0x20, cpu.getProcessorStatus());
assertEquals(0x24, cpu.getProcessorStatus());
}
/*
@ -306,4 +306,4 @@ public class CpuAccumulatorModeTest extends TestCase {
assertFalse(cpu.getCarryFlag());
}
}
}

View File

@ -29,7 +29,7 @@ public class CpuImmediateModeTest extends TestCase {
assertEquals(0, cpu.getYRegister());
assertEquals(0x200, cpu.getProgramCounter());
assertEquals(0xff, cpu.getStackPointer());
assertEquals(0x20, cpu.getProcessorStatus());
assertEquals(0x24, cpu.getProcessorStatus());
}
/*

View File

@ -33,7 +33,7 @@ public class CpuImpliedModeTest {
assertEquals(0, cpu.getYRegister());
assertEquals(0x200, cpu.getProgramCounter());
assertEquals(0xff, cpu.getStackPointer());
assertEquals(0x20, cpu.getProcessorStatus());
assertEquals(0x24, cpu.getProcessorStatus());
}
/*
@ -76,6 +76,7 @@ public class CpuImpliedModeTest {
public void test_BRK() throws MemoryAccessException {
cpu.setCarryFlag();
cpu.setOverflowFlag();
cpu.clearIrqDisableFlag();
assertEquals(0x20 | Cpu.P_CARRY | Cpu.P_OVERFLOW,
cpu.getProcessorStatus());
assertEquals(0x00, cpu.stackPeek());
@ -343,7 +344,7 @@ public class CpuImpliedModeTest {
assertEquals(0, cpu.getYRegister());
assertEquals(0x201, cpu.getProgramCounter());
assertEquals(0xff, cpu.getStackPointer());
assertEquals(0x20, cpu.getProcessorStatus());
assertEquals(0x24, cpu.getProcessorStatus());
}
/* PHA - Push Accumulator - $48 */
@ -432,7 +433,7 @@ public class CpuImpliedModeTest {
cpu.step();
assertEquals(0x0f12, cpu.getProgramCounter());
assertEquals(0x20, cpu.getProcessorStatus());
assertEquals(0x24, cpu.getProcessorStatus());
}
/* SEC - Set Carry Flag - $38 */
@ -641,4 +642,4 @@ public class CpuImpliedModeTest {
assertFalse(cpu.getZeroFlag());
assertTrue(cpu.getNegativeFlag());
}
}
}

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@ -31,7 +31,7 @@ public class CpuIndexedIndirectModeTest {
assertEquals(0, cpu.getYRegister());
assertEquals(0x200, cpu.getProgramCounter());
assertEquals(0xff, cpu.getStackPointer());
assertEquals(0x20, cpu.getProcessorStatus());
assertEquals(0x24, cpu.getProcessorStatus());
}
@Test
@ -105,4 +105,4 @@ public class CpuIndexedIndirectModeTest {
assertEquals(0x11, cpu.getAccumulator());
assertEquals(0x31, bus.read(0xc51f, true));
}
}
}

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@ -31,7 +31,7 @@ public class CpuIndirectIndexedModeTest {
assertEquals(0, cpu.getYRegister());
assertEquals(0x200, cpu.getProgramCounter());
assertEquals(0xff, cpu.getStackPointer());
assertEquals(0x20, cpu.getProcessorStatus());
assertEquals(0x24, cpu.getProcessorStatus());
}
@Test
@ -81,4 +81,4 @@ public class CpuIndirectIndexedModeTest {
assertEquals(0xe3, bus.read(0xd828, true));
}
}
}

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@ -29,7 +29,7 @@ public class CpuIndirectModeTest extends TestCase {
assertEquals(0, cpu.getYRegister());
assertEquals(0x200, cpu.getProgramCounter());
assertEquals(0xff, cpu.getStackPointer());
assertEquals(0x20, cpu.getProcessorStatus());
assertEquals(0x24, cpu.getProcessorStatus());
}
/*
@ -48,7 +48,7 @@ public class CpuIndirectModeTest extends TestCase {
cpu.step();
assertEquals(0x5400, cpu.getProgramCounter());
// No change to status flags.
assertEquals(0x20, cpu.getProcessorStatus());
assertEquals(0x24, cpu.getProcessorStatus());
}
public void test_JMP_with_ROR_Bug() throws MemoryAccessException {
@ -60,7 +60,7 @@ public class CpuIndirectModeTest extends TestCase {
cpu.step();
assertEquals(0x2200, cpu.getProgramCounter());
// No change to status flags.
assertEquals(0x20, cpu.getProcessorStatus());
assertEquals(0x24, cpu.getProcessorStatus());
}
public void test_JMP_withIndirectBug() throws MemoryAccessException {
@ -72,7 +72,7 @@ public class CpuIndirectModeTest extends TestCase {
cpu.step();
assertEquals(0x2200, cpu.getProgramCounter());
// No change to status flags.
assertEquals(0x20, cpu.getProcessorStatus());
assertEquals(0x24, cpu.getProcessorStatus());
}
public void test_JMP_withOutIndirectBug() throws MemoryAccessException {
@ -84,7 +84,7 @@ public class CpuIndirectModeTest extends TestCase {
cpu.step();
assertEquals(0x5400, cpu.getProgramCounter());
// No change to status flags.
assertEquals(0x20, cpu.getProcessorStatus());
assertEquals(0x24, cpu.getProcessorStatus());
}
public void test_JMP_cmos() throws MemoryAccessException {
@ -96,7 +96,7 @@ public class CpuIndirectModeTest extends TestCase {
cpu.step();
assertEquals(0x5400, cpu.getProgramCounter());
// No change to status flags.
assertEquals(0x20, cpu.getProcessorStatus());
assertEquals(0x24, cpu.getProcessorStatus());
}
}
}

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@ -28,7 +28,7 @@ public class CpuIndirectXModeTest extends TestCase {
assertEquals(0, cpu.getYRegister());
assertEquals(0x200, cpu.getProgramCounter());
assertEquals(0xff, cpu.getStackPointer());
assertEquals(0x20, cpu.getProcessorStatus());
assertEquals(0x24, cpu.getProcessorStatus());
}
/*

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@ -29,7 +29,7 @@ public class CpuRelativeModeTest extends TestCase {
assertEquals(0, cpu.getYRegister());
assertEquals(0x200, cpu.getProgramCounter());
assertEquals(0xff, cpu.getStackPointer());
assertEquals(0x20, cpu.getProcessorStatus());
assertEquals(0x24, cpu.getProcessorStatus());
}
/*
@ -278,4 +278,4 @@ public class CpuRelativeModeTest extends TestCase {
assertEquals(0x202, cpu.getProgramCounter());
}
}
}

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@ -44,7 +44,7 @@ public class CpuTest extends TestCase {
assertEquals(0x0200, cpu.getProgramCounter());
assertFalse(cpu.getCarryFlag());
assertFalse(cpu.getZeroFlag());
assertFalse(cpu.getIrqDisableFlag());
assertTrue(cpu.getIrqDisableFlag());
assertFalse(cpu.getDecimalModeFlag());
assertFalse(cpu.getBreakFlag());
assertFalse(cpu.getOverflowFlag());
@ -205,14 +205,12 @@ public class CpuTest extends TestCase {
}
public void testGetProcessorStatus() {
// By default, no flags are set. Remember, bit 5
// By default, only "interrupt disable" is set. Remember, bit 5
// is always '1'.
assertEquals(0x20, cpu.getProcessorStatus());
assertEquals(0x24, cpu.getProcessorStatus());
cpu.setCarryFlag();
assertEquals(0x21, cpu.getProcessorStatus());
assertEquals(0x25, cpu.getProcessorStatus());
cpu.setZeroFlag();
assertEquals(0x23, cpu.getProcessorStatus());
cpu.setIrqDisableFlag();
assertEquals(0x27, cpu.getProcessorStatus());
cpu.setDecimalModeFlag();
assertEquals(0x2f, cpu.getProcessorStatus());
@ -237,13 +235,16 @@ public class CpuTest extends TestCase {
assertEquals(0xa0, cpu.getProcessorStatus());
cpu.clearNegativeFlag();
assertEquals(0x20, cpu.getProcessorStatus());
cpu.setIrqDisableFlag();
assertEquals(0x24, cpu.getProcessorStatus());
}
public void testSetProcessorStatus() {
// Default
assertFalse(cpu.getZeroFlag());
assertFalse(cpu.getZeroFlag());
assertFalse(cpu.getIrqDisableFlag());
assertTrue(cpu.getIrqDisableFlag());
assertFalse(cpu.getDecimalModeFlag());
assertFalse(cpu.getBreakFlag());
assertFalse(cpu.getOverflowFlag());
@ -669,4 +670,4 @@ public class CpuTest extends TestCase {
cpu.step();
assertEquals(0x3E, cpu.getAccumulator());
}
}
}

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@ -29,7 +29,7 @@ public class CpuZeroPageModeTest extends TestCase {
assertEquals(0, cpu.getYRegister());
assertEquals(0x200, cpu.getProgramCounter());
assertEquals(0xff, cpu.getStackPointer());
assertEquals(0x20, cpu.getProcessorStatus());
assertEquals(0x24, cpu.getProcessorStatus());
}
/*

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@ -29,7 +29,7 @@ public class CpuZeroPageXModeTest extends TestCase {
assertEquals(0, cpu.getYRegister());
assertEquals(0x200, cpu.getProgramCounter());
assertEquals(0xff, cpu.getStackPointer());
assertEquals(0x20, cpu.getProcessorStatus());
assertEquals(0x24, cpu.getProcessorStatus());
}
/*

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@ -29,7 +29,7 @@ public class CpuZeroPageYModeTest extends TestCase {
assertEquals(0, cpu.getYRegister());
assertEquals(0x200, cpu.getProgramCounter());
assertEquals(0xff, cpu.getStackPointer());
assertEquals(0x20, cpu.getProcessorStatus());
assertEquals(0x24, cpu.getProcessorStatus());
}
/*