426 lines
14 KiB
C
426 lines
14 KiB
C
// cpu.c
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// SplitBit CPU Emulator Core
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// Written by Anachronaut
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// 10/16/2024
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#include "cpu.h"
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#include "io.h"
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uint16_t shiftRegister;
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void initializeCPU(CPURegisters *cpu, uint8_t *programMemory, uint8_t *dataMemory) {
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cpu->A = 0;
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cpu->B = 0;
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cpu->Q = 0;
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cpu->Status = 0;
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cpu->ProgramCounter = 0x0000;
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// Every Data Pointer starts at the bottom of Data Memory.
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for (int i = 0; i < DATA_POINTERS; i++) {
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cpu->DataPointer[i] = 0x0000;
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}
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cpu->StackPointer = 0xFFFF;
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cpu->Program = programMemory;
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cpu->Data = dataMemory;
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}
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void genericBranch(CPURegisters *cpu){
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// Load the next two bytes from program memory into the Program Counter.
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// Byte order is imporant. Most Significant first, then Least Significant.
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cpu->ProgramCounter++; // Move to the next byte. (MSB)
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uint16_t DestinationAddress;
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DestinationAddress = (uint16_t)cpu->Program[cpu->ProgramCounter] << 8; // Cast the 8 bit value to a 16 bit value and shifts it up to the high byte.
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cpu->ProgramCounter++; // Move to the next byte. (LSB)
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DestinationAddress = DestinationAddress | (uint16_t)cpu->Program[cpu->ProgramCounter]; // Cast the 8 bit value to a 16 bit value and or it to add it to the desination.
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cpu->ProgramCounter = DestinationAddress-1;
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}
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void genericCall(CPURegisters *cpu){
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// Order, low byte, high byte
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cpu->Data[cpu->StackPointer] = cpu->ProgramCounter & 0xFF;
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cpu->StackPointer--;
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cpu->Data[cpu->StackPointer] = (cpu->ProgramCounter >> 8) & 0xFF;
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cpu->StackPointer--;
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// Push the preserved Data Pointers to the Stack, lowest numbered first.
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// Order within each one, low byte, high byte.
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// The pointers above PRESERVED_DATA_POINTERS are deliberately left alone, so a
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// subroutine can use one to hand an address back to whoever called it.
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for (int i = 0; i < PRESERVED_DATA_POINTERS; i++) {
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cpu->Data[cpu->StackPointer] = cpu->DataPointer[i] & 0xFF;
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cpu->StackPointer--;
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cpu->Data[cpu->StackPointer] = (cpu->DataPointer[i] >> 8) & 0xFF;
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cpu->StackPointer--;
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}
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// Push B to the Stack.
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cpu->Data[cpu->StackPointer] = cpu->B;
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cpu->StackPointer--;
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// Push A to the Stack.
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cpu->Data[cpu->StackPointer] = cpu->A;
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cpu->StackPointer--;
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// Perform a Generic Branch to the Address.
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genericBranch(cpu);
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}
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uint16_t *selectDataPointer(CPURegisters *cpu) {
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// Every instruction that works through a Data Pointer names which one in the
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// byte immediately after the opcode. Out of range selectors are masked down
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// rather than rejected, the way a narrow field in hardware would be. It is the
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// assembler's job to refuse to emit one in the first place.
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cpu->ProgramCounter++;
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return &cpu->DataPointer[cpu->Program[cpu->ProgramCounter] & (DATA_POINTERS - 1)];
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}
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uint8_t executeOperation(uint8_t Instruction, CPURegisters *cpu) {
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switch(Instruction) {
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// 0x - Arithmetic and Logic Operations.
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case 0x00:
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// ADD - A + B + Carry -> Q
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uint16_t result = (uint16_t)cpu->A + (uint16_t)cpu->B + (cpu->Status & 0x01);
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if (result > 255) {
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cpu->Status |= 0x01;
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} else {
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cpu->Status &= ~0x01;
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}
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cpu->Q = result & 0xFF;
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break;
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case 0x01:
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// SUB - A - B - Carry -> Q
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result = (uint16_t)cpu->A - (uint16_t)cpu->B - (cpu->Status & 0x01);
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if (result > 255) {
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cpu->Status |= 0x01;
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} else {
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cpu->Status &= ~0x01;
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}
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cpu->Q = result & 0xFF;
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break;
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case 0x02:
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// AND - A and B -> Q
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cpu->Q = cpu->A&cpu->B;
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break;
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case 0x03:
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// OR - A or B -> Q
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cpu->Q = cpu->A|cpu->B;
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break;
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case 0x04:
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// XOR - A xor B -> Q
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cpu->Q = cpu->A^cpu->B;
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break;
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case 0x05:
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// NOTA - not A -> Q
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cpu->Q = ~cpu->A;
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break;
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case 0x06:
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// NOTB - not B -> Q
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cpu->Q = ~cpu->B;
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break;
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case 0x07:
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// SHL - Shift AB left.
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shiftRegister = ((uint16_t)cpu->A << 8) | cpu->B;
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shiftRegister = (shiftRegister << 1) | (shiftRegister >> 15);
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cpu->A = shiftRegister >> 8;
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cpu->B = shiftRegister & 0xFF;
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break;
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case 0x08:
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// SHR - Shift AB right.
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shiftRegister = ((uint16_t)cpu->A << 8) | cpu->B;
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shiftRegister = (shiftRegister >> 1) | (shiftRegister << 15);
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cpu->A = shiftRegister >> 8;
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cpu->B = shiftRegister & 0xFF;
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break;
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//
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// 1x - Branch Operations:
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//
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case 0x10:
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// BRI - Branch Immediately
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genericBranch(cpu);
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break;
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case 0x11:
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// BRQ - Branch if Q = 0
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if(cpu->Q == 0) {
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genericBranch(cpu);
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} else {
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cpu->ProgramCounter+=2;
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}
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break;
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case 0x12:
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// BRA - Branch if A = 0
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if(cpu->A == 0) {
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genericBranch(cpu);
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} else {
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cpu->ProgramCounter+=2;
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}
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break;
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case 0x13:
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// BRB - if B = 0
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if(cpu->B == 0) {
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genericBranch(cpu);
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} else {
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cpu->ProgramCounter+=2;
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}
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break;
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case 0x14:
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// BRC - Do an immediate branch if the Carry Flag is set.
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if (cpu->Status & 0x01) {
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genericBranch(cpu);
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} else {
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cpu->ProgramCounter+=2;
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}
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break;
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case 0x17:
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// CALL - Push the Program Counter to the Stack, and perform an immediate branch.
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genericCall(cpu);
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break;
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case 0x1F:
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// RET - Return from subroutine, restore the registers and set the Program Counter to the Return Address.
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// Pop A from the Stack.
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cpu->StackPointer++;
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cpu->A = cpu->Data[cpu->StackPointer];
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// Pop B from the Stack.
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cpu->StackPointer++;
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cpu->B = cpu->Data[cpu->StackPointer];
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// Pop the preserved Data Pointers from the Stack. This walks the pointers
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// in the opposite order to genericCall, and takes the high byte before the
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// low byte, so that it exactly mirrors the way they were pushed.
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for (int i = PRESERVED_DATA_POINTERS - 1; i >= 0; i--) {
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cpu->StackPointer++;
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cpu->DataPointer[i] = (uint16_t)cpu->Data[cpu->StackPointer] << 8;
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cpu->StackPointer++;
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cpu->DataPointer[i] |= (uint16_t)cpu->Data[cpu->StackPointer];
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}
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// Pop the Return Address from the Stack.
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cpu->StackPointer++;
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cpu->ProgramCounter = (uint16_t)cpu->Data[cpu->StackPointer] << 8;
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cpu->StackPointer++;
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cpu->ProgramCounter = cpu->ProgramCounter | (uint16_t)cpu->Data[cpu->StackPointer];
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// Add 2 to the Program Counter to skip over the address when it returns.
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cpu->ProgramCounter += 2;
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break;
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//
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// 2x - Register Operations:
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//
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case 0x20:
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// RSTA - Reset A to 0.
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cpu->A = 0;
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break;
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case 0x21:
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// RSTB - Reset B to 0.
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cpu->B = 0;
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break;
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case 0x22:
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// INCA - Add 1 to A.
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// Set the Carry Flag if the register overflows.
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if (cpu->A == 0xFF) {
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cpu->Status |= 0x01;
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} else {
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cpu->Status &= ~0x01;
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}
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cpu->A++;
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break;
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case 0x23:
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// INCB - Add 1 to B.
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// Set the Carry Flag if the register overflows.
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if (cpu->B == 0xFF) {
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cpu->Status |= 0x01;
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} else {
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cpu->Status &= ~0x01;
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}
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cpu->B++;
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break;
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case 0x24:
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// DECA - Subtract 1 from A.
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// Set the Carry Flag if the register underflows.
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if (cpu->A == 0x00) {
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cpu->Status |= 0x01;
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} else {
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cpu->Status &= ~0x01;
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}
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cpu->A--;
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break;
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case 0x25:
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// DECB - Subtract 1 from B.
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// Set the Carry Flag if the register underflows.
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if (cpu->B == 0x00) {
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cpu->Status |= 0x01;
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} else {
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cpu->Status &= ~0x01;
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}
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cpu->B--;
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break;
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case 0x26:
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// INIA - Initialize A Immediately from Program Memory.
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cpu->ProgramCounter++;
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cpu->A = cpu->Program[cpu->ProgramCounter];
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break;
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case 0x27:
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// INIB - Initialize A Immediately from Program Memory.
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cpu->ProgramCounter++;
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cpu->B = cpu->Program[cpu->ProgramCounter];
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break;
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case 0x28:
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// CCF - Clear the Carry Flag.
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cpu->Status &= ~0x01;
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break;
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//
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// 3x - Stack Operations:
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//
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case 0x30:
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// PSHQ - Push Q to the Stack.
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cpu->Data[cpu->StackPointer] = cpu->Q;
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cpu->StackPointer--;
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break;
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case 0x31:
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// PSHA - Push A to the Stack.
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cpu->Data[cpu->StackPointer] = cpu->A;
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cpu->StackPointer--;
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break;
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case 0x32:
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// PSHB - Push B to the Stack.
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cpu->Data[cpu->StackPointer] = cpu->B;
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cpu->StackPointer--;
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break;
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case 0x33: {
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// PSHD - Push the selected Data Pointer Address to the Stack.
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// Order, high byte, low byte
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// This ordering makes it easier to add offsets with register math.
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uint16_t pushed = *selectDataPointer(cpu);
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cpu->Data[cpu->StackPointer] = (pushed >> 8) & 0xFF;
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cpu->StackPointer--;
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cpu->Data[cpu->StackPointer] = pushed & 0xFF;
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cpu->StackPointer--;
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}
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break;
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case 0x34:
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// POPA - Pop A from the Stack.
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cpu->StackPointer++;
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cpu->A = cpu->Data[cpu->StackPointer];
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break;
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case 0x35:
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// POPB - Pop B from the Stack.
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cpu->StackPointer++;
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cpu->B = cpu->Data[cpu->StackPointer];
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break;
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case 0x36: {
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// POPD - Pop a Data Address from the Stack into the selected Data Pointer.
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uint16_t *popped = selectDataPointer(cpu);
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cpu->StackPointer++;
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*popped = (uint16_t)cpu->Data[cpu->StackPointer];
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cpu->StackPointer++;
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*popped |= (uint16_t)cpu->Data[cpu->StackPointer] << 8;
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}
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break;
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//
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// 4x - Data Operations:
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//
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case 0x40:
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// INCD - Increment the selected Data Pointer.
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(*selectDataPointer(cpu))++;
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break;
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case 0x41:
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// DECD - Decrement the selected Data Pointer.
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(*selectDataPointer(cpu))--;
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break;
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case 0x42:
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// LDA - Load A from Data.
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cpu->A = cpu->Data[*selectDataPointer(cpu)];
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break;
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case 0x43:
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// LDB - Load B from Data.
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cpu->B = cpu->Data[*selectDataPointer(cpu)];
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break;
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case 0x44:
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// STQ - Store Q into Data.
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cpu->Data[*selectDataPointer(cpu)] = cpu->Q;
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break;
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case 0x45:
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// STA - Store A into Data.
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cpu->Data[*selectDataPointer(cpu)] = cpu->A;
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break;
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case 0x46:
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// STB - Store B into Data.
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cpu->Data[*selectDataPointer(cpu)] = cpu->B;
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break;
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case 0x47: {
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// SETD - Set the selected Data Pointer.
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uint16_t *destination = selectDataPointer(cpu);
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cpu->ProgramCounter++;
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uint16_t Address;
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Address = (uint16_t)cpu->Program[cpu->ProgramCounter] << 8; // Cast the 8 bits to a 16 bit value and shift them to the high byte.
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cpu->ProgramCounter++;
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Address |= (uint16_t)cpu->Program[cpu->ProgramCounter];
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*destination = Address;
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}
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break;
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case 0x48: {
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// DPUP - Offset the selected Data Pointer up by the value of the next byte of Program Memory.
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uint16_t *target = selectDataPointer(cpu);
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cpu->ProgramCounter++;
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*target += cpu->Program[cpu->ProgramCounter];
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}
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break;
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case 0x49: {
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// DPDN - Offset the selected Data Pointer down by the value of the next byte of Program Memory.
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uint16_t *target = selectDataPointer(cpu);
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cpu->ProgramCounter++;
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*target -= cpu->Program[cpu->ProgramCounter];
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}
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break;
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//
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// Dx - Output Operations:
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//
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case 0xD0:
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// OUTQ - Write the value of Q to an output port.
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cpu->ProgramCounter++;
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OutputHandler(cpu->Q, cpu->Program[cpu->ProgramCounter]);
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break;
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case 0xD1:
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// OUTA - Write the value of A to an output port.
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cpu->ProgramCounter++;
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OutputHandler(cpu->A, cpu->Program[cpu->ProgramCounter]);
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break;
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case 0xD2:
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// OUTB - Write the value of B to an output port.
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cpu->ProgramCounter++;
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OutputHandler(cpu->B, cpu->Program[cpu->ProgramCounter]);
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break;
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//
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// Ex - Input Operations:
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//
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case 0xE0:
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// INA - Read an Input to A.
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cpu->ProgramCounter++;
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cpu->A = InputHandler(cpu->Program[cpu->ProgramCounter]);
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break;
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case 0xE1:
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// INB - Read an Input to B.
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cpu->ProgramCounter++;
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cpu->B = InputHandler(cpu->Program[cpu->ProgramCounter]);
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break;
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//
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// Fx - Special Operations:
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//
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case 0xF0:
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// NOP - Do nothing.
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break;
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case 0xFF:
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// HALT - Set the Halt Bit of the Status Register.
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cpu->Status |= 0x80;
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break;
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default:
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// Unknown Instruction.
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return 1;
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}
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return 0;
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}
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void stepCPU(CPURegisters *cpu) {
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if (!(cpu->Status & 0x80)) {
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// The CPU is not halted, so do a cycle.
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executeOperation(cpu->Program[cpu->ProgramCounter], cpu);
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cpu->ProgramCounter++;
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}
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}
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