mirror of
https://github.com/ariejan/i6502.git
synced 2024-09-28 14:55:07 +00:00
95 lines
2.0 KiB
Go
95 lines
2.0 KiB
Go
package i6502
|
|
|
|
import "fmt"
|
|
|
|
type Cpu struct {
|
|
bus *AddressBus // The address bus
|
|
|
|
PC uint16 // 16-bit program counter
|
|
P byte // Status Register
|
|
|
|
A byte // Accumulator
|
|
X byte // X index register
|
|
Y byte // Y index register
|
|
}
|
|
|
|
const (
|
|
ResetVector = 0xFFFC // 0xFFFC-FFFD
|
|
)
|
|
|
|
// Create an new Cpu instance with the specified AddressBus
|
|
func NewCpu(bus *AddressBus) (*Cpu, error) {
|
|
return &Cpu{bus: bus}, nil
|
|
}
|
|
|
|
func (c *Cpu) HasAddressBus() bool {
|
|
return c.bus != nil
|
|
}
|
|
|
|
// Reset the CPU, emulating the RESB pin.
|
|
func (c *Cpu) Reset() {
|
|
c.PC = c.bus.Read16(ResetVector)
|
|
c.P = 0x34
|
|
}
|
|
|
|
// Load the specified program data at the given memory location
|
|
// and point the Program Counter to the beginning of the program
|
|
func (c *Cpu) LoadProgram(data []byte, location uint16) {
|
|
for i, b := range data {
|
|
c.bus.Write(location+uint16(i), b)
|
|
}
|
|
|
|
c.PC = location
|
|
}
|
|
|
|
// Execute the instruction pointed to by the Program Counter (PC)
|
|
func (c *Cpu) Step() {
|
|
instruction := c.readNextInstruction()
|
|
c.PC += uint16(instruction.Size)
|
|
// fmt.Println(instruction)
|
|
c.execute(instruction)
|
|
}
|
|
|
|
func (c *Cpu) execute(instruction Instruction) {
|
|
switch instruction.opcodeId {
|
|
case adc:
|
|
c.ADC(instruction)
|
|
}
|
|
}
|
|
|
|
func (c *Cpu) resolveOperand(in Instruction) uint8 {
|
|
switch in.addressingId {
|
|
case immediate:
|
|
return in.Op8
|
|
default:
|
|
return c.bus.Read(c.memoryAddress(in))
|
|
}
|
|
}
|
|
|
|
func (c *Cpu) memoryAddress(in Instruction) uint16 {
|
|
switch in.addressingId {
|
|
case absolute:
|
|
return in.Op16
|
|
case absoluteX:
|
|
return in.Op16 + uint16(c.X)
|
|
case absoluteY:
|
|
return in.Op16 + uint16(c.Y)
|
|
case indirect:
|
|
return c.bus.Read16(in.Op16)
|
|
case indirectX:
|
|
return c.bus.Read16(uint16(in.Op8 + c.X))
|
|
case indirectY:
|
|
return c.bus.Read16(uint16(in.Op8)) + uint16(c.Y)
|
|
case relative:
|
|
panic("Relative addressing not yet implemented.")
|
|
case zeropage:
|
|
return uint16(in.Op8)
|
|
case zeropageX:
|
|
return uint16(in.Op8 + c.X)
|
|
case zeropageY:
|
|
return uint16(in.Op8 + c.Y)
|
|
default:
|
|
panic(fmt.Errorf("Unhandled addressing mode. Are you sure you are running a 6502 ROM?"))
|
|
}
|
|
}
|