breadloaf/BreadedSNES
1
0
Fork 0
Code Issues Pull Requests Actions Packages Projects Releases Wiki Activity

Implement BIT Instructions

Browse Source
This commit is contained in:
Palindromic Bread Loaf
2025-07-23 11:45:40 -04:00
parent 259b27c823
commit 34d30ed47c
2 changed files with 167 additions and 0 deletions
Download Patch File Download Diff File Expand all files Collapse all files

155
src/cpu.cpp
View File

@@ -220,6 +220,69 @@ void CPU::UpdateASLFlags16(const uint16_t original_value, const uint16_t result)
UpdateNZ16(result);
}
// Helper function to update flags after BIT operation (non-immediate modes)
void CPU::UpdateBITFlags8(uint8_t memory_value, uint8_t acc_value) {
// Z flag: set if (A & memory) == 0
if (const uint8_t result = acc_value & memory_value; result == 0) {
P |= FLAG_Z;
} else {
P &= ~FLAG_Z;
}
// N flag: copy bit 7 of memory
if (memory_value & 0x80) {
P |= FLAG_N;
} else {
P &= ~FLAG_N;
}
// V flag: copy bit 6 of memory
if (memory_value & 0x40) {
P |= FLAG_V;
} else {
P &= ~FLAG_V;
}
}
void CPU::UpdateBITFlags16(const uint16_t memory_value, const uint16_t acc_value) {
if (const uint16_t result = acc_value & memory_value; result == 0) {
P |= FLAG_Z;
} else {
P &= ~FLAG_Z;
}
// N flag: copy bit 15 of memory
if (memory_value & 0x8000) {
P |= FLAG_N;
} else {
P &= ~FLAG_N;
}
// V flag: copy bit 14 of memory
if (memory_value & 0x4000) {
P |= FLAG_V;
} else {
P &= ~FLAG_V;
}
}
// Helper function for immediate mode BIT
void CPU::UpdateBITImmediateFlags8(const uint8_t memory_value, const uint8_t acc_value) {
if (const uint8_t result = acc_value & memory_value; result == 0) {
P |= FLAG_Z;
} else {
P &= ~FLAG_Z;
}
}
void CPU::UpdateBITImmediateFlags16(const uint16_t memory_value, const uint16_t acc_value) {
if (const uint16_t result = acc_value & memory_value; result == 0) {
P |= FLAG_Z;
} else {
P &= ~FLAG_Z;
}
}
void CPU::ExecuteInstruction() {
// TODO: Actual Opcode decoding
switch (const uint8_t opcode = bus->Read(PC++)) {
@@ -272,6 +335,13 @@ void CPU::ExecuteInstruction() {
case 0x30: BMI_Relative(); break; // BMI $nn
case 0x10: BPL_Relative(); break; // BPL $nn
// BIT - Test Bits Instructions
case 0x89: BIT_Immediate(); break; // BIT #$nn or #$nnnn
case 0x2C: BIT_Absolute(); break; // BIT $nnnn
case 0x3C: BIT_AbsoluteX(); break; // BIT $nnnn,X
case 0x24: BIT_DirectPage(); break; // BIT $nn
case 0x34: BIT_DirectPageX(); break; // BIT $nn,X
// CMP - Compare Accumulator
case 0xC9: CMP_Immediate(); break; // CMP #$nn or #$nnnn
case 0xCD: CMP_Absolute(); break; // CMP $nnnn
@@ -2571,3 +2641,88 @@ void CPU::ASL_DirectPageX() {
if (D & 0xFF) cycles++;
}
}
void CPU::BIT_Immediate() {
if (P & FLAG_M) { // 8-bit mode
const uint8_t operand = ReadByte(PC++);
UpdateBITImmediateFlags8(operand, A & 0xFF);
cycles += 2;
} else { // 16-bit mode
const uint16_t operand = ReadWord(PC);
PC += 2;
UpdateBITImmediateFlags16(operand, A);
cycles += 3;
}
}
void CPU::BIT_Absolute() {
const uint16_t address = ReadWord(PC);
PC += 2;
const uint32_t full_address = (DB << 16) | address;
if (P & FLAG_M) { // 8-bit mode
const uint8_t operand = ReadByte(full_address);
UpdateBITFlags8(operand, A & 0xFF);
cycles += 4;
} else { // 16-bit mode
const uint16_t operand = ReadWord(full_address);
UpdateBITFlags16(operand, A);
cycles += 5;
}
}
void CPU::BIT_AbsoluteX() {
const uint16_t base_address = ReadWord(PC);
PC += 2;
const uint32_t full_address = (DB << 16) | (base_address + X);
if (P & FLAG_M) { // 8-bit mode
const uint8_t operand = ReadByte(full_address);
UpdateBITFlags8(operand, A & 0xFF);
cycles += 4;
if ((base_address & 0xFF00) != ((base_address + X) & 0xFF00)) {
cycles++;
}
} else { // 16-bit mode
const uint16_t operand = ReadWord(full_address);
UpdateBITFlags16(operand, A);
cycles += 5;
if ((base_address & 0xFF00) != ((base_address + X) & 0xFF00)) {
cycles++;
}
}
}
void CPU::BIT_DirectPage() {
const uint8_t offset = ReadByte(PC++);
const uint32_t address = D + offset;
if (P & FLAG_M) { // 8-bit mode
const uint8_t operand = ReadByte(address);
UpdateBITFlags8(operand, A & 0xFF);
cycles += 3;
if (D & 0xFF) cycles++;
} else { // 16-bit mode
const uint16_t operand = ReadWord(address);
UpdateBITFlags16(operand, A);
cycles += 4;
if (D & 0xFF) cycles++;
}
}
void CPU::BIT_DirectPageX() {
const uint8_t offset = ReadByte(PC++);
const uint32_t address = D + offset + X;
if (P & FLAG_M) { // 8-bit mode
const uint8_t operand = ReadByte(address);
UpdateBITFlags8(operand, A & 0xFF);
cycles += 4;
if (D & 0xFF) cycles++;
} else { // 16-bit mode
const uint16_t operand = ReadWord(address);
UpdateBITFlags16(operand, A);
cycles += 5;
if (D & 0xFF) cycles++;
}
}

12
src/cpu.h
View File

@@ -79,6 +79,12 @@ public:
void UpdateASLFlags8(uint8_t original_value, uint8_t result);
void UpdateASLFlags16(uint16_t original_value, uint16_t result);
// Helper methods for BIT
void UpdateBITFlags8(uint8_t memory_value, uint8_t acc_value);
void UpdateBITFlags16(uint16_t memory_value, uint16_t acc_value);
void UpdateBITImmediateFlags8(uint8_t memory_value, uint8_t acc_value);
void UpdateBITImmediateFlags16(uint16_t memory_value, uint16_t acc_value);
// Instruction implementations
// TODO: Implement remaining instructions
void CMP_Immediate();
@@ -232,6 +238,12 @@ public:
void ASL_AbsoluteX();
void ASL_DirectPage();
void ASL_DirectPageX();
void BIT_Immediate();
void BIT_Absolute();
void BIT_AbsoluteX();
void BIT_DirectPage();
void BIT_DirectPageX();
};
#endif //CPU_H