rocknes

rocknes is a NES emulator, written from scratch in Python. I have always admired CPU emulation, and the best way to understand how it works is to build one.

6502 CPU
- Addressing modes
  - Implicit
  - Accumulator
  - Immediate
  - Zero Page
  - Zero Page,X
  - Zero Page,Y
  - Relative
  - Absolute
  - Absolute,X
  - Absolute,Y
  - Indirect
  - Indexed Indirect
  - Indirect Indexed
- Instruction Set
  - ADC
  - AND
  - ASL
  - BCC
  - BCS
  - BEQ
  - BIT
  - BMI
  - BNE
  - BPL
  - BRK
  - BVC
  - BVS
  - CLC
  - CLD
  - CLI
  - CLV
  - CMP
  - CPX
  - CPY
  - DEC
  - DEX
  - DEY
  - EOR
  - INC
  - INX
  - INY
  - JMP
  - JSR
  - LDA
  - LDX
  - LDY
  - LSR
  - NOP
  - ORA
  - PHA
  - PHP
  - PLA
  - PLP
  - ROL
  - ROR
  - RTI
  - RTS
  - SBC
  - SEC
  - SED
  - SEI
  - STA
  - STX
  - STY
  - TAX
  - TAY
  - TSX
  - TXA
  - TXS
  - TYA
References

6502 CPU

Addressing Modes

Relative

Used in relative branch instructions. Signed offset is added to the program counter. Offsets range from -128 to 127. The offset is relative to the first byte of the next instruction. For example, 90 00 is a BCC with no effect.

Instruction Set

ADC

Add with carry.

AND

Logical and.

ASL

Arithmetic shift left.

BCC

Branch if carry clear.

Addressing mode	Opcode	Bytes	Cycles
Relative	90	2	2 (3 if branch taken, 4 if page boundary crossed)

BCS

Branch if carry set.

Addressing mode	Opcode	Bytes	Cycles
Relative	B0	2	2 (3 if branch taken, 4 if page boundary crossed)

BEQ

Branch if equal (when zero flag is set).

Addressing mode	Opcode	Bytes	Cycles
Relative	F0	2	2 (3 if branch taken, 4 if page boundary crossed)

BIT

Bit test.

BMI

Branch if minus (when negative flag is set).

Addressing mode	Opcode	Bytes	Cycles
Relative	30	2	2 (3 if branch taken, 4 if page boundary crossed)

BNE

Branch if not equal (when zero flag is not set).

Addressing mode	Opcode	Bytes	Cycles
Relative	D0	2	2 (3 if branch taken, 4 if page boundary crossed)

BPL

Branch if positive (when negative flag is not set).

Addressing mode	Opcode	Bytes	Cycles
Relative	10	2	2 (3 if branch taken, 4 if page boundary crossed)

BRK

Force interrupt.

BVC

Branch if overflow clear (when overflow flag is not set).

Addressing mode	Opcode	Bytes	Cycles
Relative	50	2	2 (3 if branch taken, 4 if page boundary crossed)

BVS

Branch if overflow set (when overflow flag is set).

Addressing mode	Opcode	Bytes	Cycles
Relative	70	2	2 (3 if branch taken, 4 if page boundary crossed)

CLC

Clear carry flag.

CLD

Clear decimal mode.

CLI

Clear interrupt disable.

CLV

Clear overflow flag.

CMP

Compare.

CPX

Compare X register.

CPY

Compare Y register.

DEC

Decrement memory.

DEX

Decrement X register.

DEY

Decrement Y register.

EOR

Exclusive or.

INC

Increment memory.

INX

Increment X register.

INY

Increment Y register.

JMP

Jump.

JSR

Jump to subroutine.

LDA

Load accumulator.

LDX

Load X register.

LDY

Load Y register.

LSR

Logical shift right

NOP

No operation.

ORA

Logical inclusive or.

PHA

Push accumulator.

PHP

Push processor status.

PLA

Pull accumulator.

PLP

Pull processor status.

ROL

Rotate left.

ROR

Rotate right.

RTI

Return from interrupt.

RTS

Return from subroutine.

SBC

Subtract with carry.

SEC

Set carry flag.

SED

Set decimal flag.

SEI

Set interrupt disable.

STA

Store accumulator.

STX

Store X register.

STY

Store Y register.

TAX

Transfer accumulator to X.

TAY

Transfer accumulator to Y.

TSX

Transfer stack pointer to X.

TXA

Transfer X to accumulator.

TXS

Transfer X to stack pointer.

TYA

Transfer Y to accumulator.

References

http://www.obelisk.me.uk/6502/reference.html https://www.masswerk.at/6502/6502_instruction_set.html https://en.wikibooks.org/wiki/6502_Assembly