86 lines
1.4 KiB
Go
86 lines
1.4 KiB
Go
package cpu
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import "fmt"
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const ARCH = "MOS6502"
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// bits for the status register
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const (
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FlagCarry = 1 << 0 // Bit 0
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FlagZero = 1 << 1 // Bit 1
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FlagInterrupt = 1 << 2 // Bit 2
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FlagDecimal = 1 << 3 // Bit 3
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FlagBreak = 1 << 4 // Bit 4
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FlagUnused = 1 << 5 // Bit 5
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FlagOverflow = 1 << 6 // Bit 6
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FlagNegative = 1 << 7 // Bit 7
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)
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type Bus interface {
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Read(address uint16) uint8
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}
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type CPU struct {
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A uint8 // Accumulator
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X uint8 // X Register
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Y uint8 // Y Register
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PC uint16 // Program Counter
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SP uint8 // Stack Pointer
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Status uint8 // Status Flags
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Bus Bus // Connection to the Bus
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}
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func (c *CPU) Reset() {
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c.A = 0
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c.X = 0
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c.Y = 0
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c.SP = 0xFD
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c.Status = 0x24
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c.PC = 0x0000
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}
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// Small helper func to flip the status register bits
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func (c *CPU) setFlag(flag uint8, set bool) {
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if set {
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c.Status |= flag
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} else {
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c.Status &= ^flag
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}
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}
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// fetch -> decode -> execute cycle
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func (c *CPU) Step() {
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// fetch with program counter and bus memory
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opcode := c.Bus.Read(c.PC)
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fmt.Printf("Executing at PC: %04X | Opcode: %02X\n", c.PC, opcode)
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// decode + exe need to split this out to handle all instructions etc
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// Look at : https://www.ahl27.com/posts/2023/01/6502-emu1/
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switch opcode {
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case 0xEA: //NOP
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// does nothing its a no op
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default:
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fmt.Printf("Unknown opcode: %02X\n", opcode)
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}
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// Move the Program counter
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c.PC++
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}
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