6502
The MOS Technology 6502 is an 8-bit microprocessor designed by Chuck Peddle for MOS Technology in 1975. When it was introduced it was the least expensive full featured CPU on the market by far, at about 1/6th the price, or less, of competing designs from larger companies such as Motorola and Intel. It was nevertheless faster than most of them, and, along with the Zilog Z80, sparked off a series of computer projects that would eventually result in the home computer revolution of the 1980s. The 6502 design was originally second-sourced by Rockwell and Synertek and later licensed to a number of companies; it is still made for embedded systems.
Originally the CPC was destined to be designed around the 6502 processor. But when Amstrad approached Locomotive Software to develop a Basic for it with a very tight deadline, Locomotive PLC, who already had a Z80 Basic in the works, urged and convinced Amstrad to switch to the Z80.
Contents
Description
The 6502 microprocessor is an 8-bit CPU with an 8-bit ALU and a 16-bit address bus capable of direct access to 64KB of memory space. Like the Z80, the 6502 is also a little-endian CPU, meaning it stores 16-bit values with the least significant byte first, followed by the most significant byte. The 6502 has 151 instructions, which are composed of 56 distinct opcodes across various addressing modes.
Although it lacks the raw processing power of processors like the Intel 80x86 or the Motorola 68000 series, the 6502 was known for its efficiency and affordability, making it a popular choice for embedded systems and early home computers. Its simple design contributed to lower manufacturing costs and simplified integration.
The 6502 chip is made up of 4528 transistors (3510 enhancement transistors and 1018 depletion pullup transistors). It comes in a 40-pin DIP package. It has been produced by various manufacturers and used in a wide range of applications, from early gaming consoles like the Atari VCS and Nintendo Entertainment System to personal computers like the Apple II and Commodore 64.
Registers
| Register | Size | Description | Notes |
|---|---|---|---|
| A (Accumulator) | 8-bit | Main register for arithmetic, logic, and data transfer | Most operations use this register |
| X (Index Register X) | 8-bit | Used for indexing memory and loop counters | Can be used for addressing modes like Indexed Indirect, Zero Page Indexed, and Absolute Indexed |
| Y (Index Register Y) | 8-bit | Used for indexing memory and loop counters | Often used in Absolute and Zero Page Indexed addressing |
| P (Processor Status) | 8-bit |
|
The status register is modified by arithmetic and logic operations, as well as interrupts |
| S (Stack Pointer) | 8-bit | Points to the current location in the stack | Stack is located in page 1 ($0100-$01FF), 8-bit S is offset to this base |
| PC (Program Counter) | 16-bit | Points to the next instruction to be executed | Automatically increments as instructions are executed |
Memory Access
The address space that the 6502 uses is split into pages. There are 256 pages and each page is 256 bytes in size, ranging from page 0 to page 255.
In order to make up for the lack of registers, the 6502 includes a zero page addressing mode ($0000-$00FF) that uses only 1 address byte in the instruction instead of the 2 that are needed to address the full 64 KB of memory. This provides fast access to the first 256 bytes of RAM by using shorter instructions.
The stack is permanently located in page 1 ($0100 to $01FF) and managed by the 8-bit stack pointer (S), with an initial value of $FF. It grows downward as data is pushed onto the stack.
Instructions PHA and PHP push the accumulator and processor status onto the stack, while PLA and PLP pull them back. Subroutine calls with JSR store the return address on the stack, and RTS retrieves it to continue execution. Similarly, interrupts (BRK) push the program counter and status, while RTI restores them. The stack has a 256-byte limit, and overflow occurs if not managed properly.
BRK/IRQ/NMI/RESET
On a RESET, the CPU loads the vector from $FFFC/$FFFD into the program counter and continues fetching instructions from there.
On an NMI, the CPU pushes the low byte and the high byte of the program counter as well as the processor status onto the stack, disables interrupts and loads the vector from $FFFA/$FFFB into the program counter and continues fetching instructions from there.
On an IRQ, the CPU does the same as in the NMI case, but uses the vector at $FFFE/$FFFF.
On a BRK instruction, the CPU does the same as in the IRQ case, but sets bit #4 (B flag) in the copy of the status register that is saved on the stack.
6502 Instruction set
Standard instructions
| Mnemonic | Addressing Modes | Flags | Operation | Description | |||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| No arg | A | #$nn | $nnnn | $nnnn,X | $nnnn,Y | ($nnnn) | $nn | $nn,X | $nn,Y | ($nn,X) | ($nn),Y | rel | N | V | - | B | D | I | Z | C | |||
| ADC | 69 | 6D | 7D | 79 | 65 | 75 | 61 | 71 | N | V | - | - | - | - | Z | C | A + M + CF → A, CF | Add Memory to Accumulator with Carry | |||||
| AND | 29 | 2D | 3D | 39 | 25 | 35 | 21 | 31 | N | - | - | - | - | - | Z | - | A ∧ M → A | "AND" Memory with Accumulator | |||||
| ASL | 0A | 0E | 1E | 06 | 16 | N | - | - | - | - | - | Z | C | CF ← /M7...M0/ ← 0 | Arithmetic Shift Left | ||||||||
| BCC | 90 | - | - | - | - | - | - | - | - | Branch on CF = 0 | Branch on Carry Clear | ||||||||||||
| BCS | B0 | - | - | - | - | - | - | - | - | Branch on CF = 1 | Branch on Carry Set | ||||||||||||
| BEQ | F0 | - | - | - | - | - | - | - | - | Branch on ZF = 1 | Branch on Result Zero | ||||||||||||
| BIT | 2C | 24 | N | V | - | - | - | - | Z | - | A ∧ M, M7 → NF, M6 → VF | Test Bits in Memory with Accumulator | |||||||||||
| BMI | 30 | - | - | - | - | - | - | - | - | Branch on NF = 1 | Branch on Result Minus | ||||||||||||
| BNE | D0 | - | - | - | - | - | - | - | - | Branch on ZF = 0 | Branch on Result Not Zero | ||||||||||||
| BPL | 10 | - | - | - | - | - | - | - | - | Branch on NF = 0 | Branch on Result Plus | ||||||||||||
| BRK | 00 | - | - | - | - | - | 1 | - | - | PC + 2↓, [FFFE] → PCL, [FFFF] → PCH | Break Command | ||||||||||||
| BVC | 50 | - | - | - | - | - | - | - | - | Branch on VF = 0 | Branch on Overflow Clear | ||||||||||||
| BVS | 70 | - | - | - | - | - | - | - | - | Branch on VF = 1 | Branch on Overflow Set | ||||||||||||
| CLC | 18 | - | - | - | - | - | - | - | 0 | 0 → CF | Clear Carry Flag | ||||||||||||
| CLD | D8 | - | - | - | - | 0 | - | - | - | 0 → DF | Clear Decimal Mode | ||||||||||||
| CLI | 58 | - | - | - | - | - | 0 | - | - | 0 → IF | Clear Interrupt Disable | ||||||||||||
| CLV | B8 | - | 0 | - | - | - | - | - | - | 0 → VF | Clear Overflow Flag | ||||||||||||
| CMP | C9 | CD | DD | D9 | C5 | D5 | C1 | D1 | N | - | - | - | - | - | Z | C | A - M | Compare Memory and Accumulator | |||||
| CPX | E0 | EC | E4 | N | - | - | - | - | - | Z | C | X - M | Compare Index Register X To Memory | ||||||||||
| CPY | C0 | CC | C4 | N | - | - | - | - | - | Z | C | Y - M | Compare Index Register Y To Memory | ||||||||||
| DEC | CE | DE | C6 | D6 | N | - | - | - | - | - | Z | - | M - 1 → M | Decrement Memory By One | |||||||||
| DEX | CA | N | - | - | - | - | - | Z | - | X - 1 → X | Decrement Index Register X By One | ||||||||||||
| DEY | 88 | N | - | - | - | - | - | Z | - | Y - 1 → Y | Decrement Index Register Y By One | ||||||||||||
| EOR | 49 | 4D | 5D | 59 | 45 | 55 | 41 | 51 | N | - | - | - | - | - | Z | - | A ⊻ M → A | "Exclusive OR" Memory with Accumulator | |||||
| INC | EE | FE | E6 | F6 | N | - | - | - | - | - | Z | - | M + 1 → M | Increment Memory By One | |||||||||
| INX | E8 | N | - | - | - | - | - | Z | - | X + 1 → X | Increment Index Register X By One | ||||||||||||
| INY | C8 | N | - | - | - | - | - | Z | - | Y + 1 → Y | Increment Index Register Y By One | ||||||||||||
| JMP | 4C | 6C | - | - | - | - | - | - | - | - | [PC + 1] → PCL, [PC + 2] → PCH | JMP Indirect | |||||||||||
| JSR | 20 | - | - | - | - | - | - | - | - | PC + 2↓, [PC + 1] → PCL, [PC + 2] → PCH | Jump To Subroutine | ||||||||||||
| LDA | A9 | AD | BD | B9 | A5 | B5 | A1 | B1 | N | - | - | - | - | - | Z | - | M → A | Load Accumulator with Memory | |||||
| LDX | A2 | AE | BE | A6 | B6 | N | - | - | - | - | - | Z | - | M → X | Load Index Register X From Memory | ||||||||
| LDY | A0 | AC | BC | A4 | B4 | N | - | - | - | - | - | Z | - | M → Y | Load Index Register Y From Memory | ||||||||
| LSR | 4A | 4E | 5E | 46 | 56 | 0 | - | - | - | - | - | Z | C | 0 → /M7...M0/ → CF | Logical Shift Right | ||||||||
| NOP | EA | - | - | - | - | - | - | - | - | No operation | No Operation | ||||||||||||
| ORA | 09 | 0D | 1D | 19 | 05 | 15 | 01 | 11 | N | - | - | - | - | - | Z | - | A ∨ M → A | "OR" Memory with Accumulator | |||||
| PHA | 48 | - | - | - | - | - | - | - | - | A↓ | Push Accumulator On Stack | ||||||||||||
| PHP | 08 | - | - | - | 1 | - | - | - | - | P↓ | Push Processor Status on Stack | ||||||||||||
| PLA | 68 | N | - | - | - | - | - | Z | - | (S)↑ → A | Pull Accumulator From Stack | ||||||||||||
| PLP | 28 | N | V | - | B | D | I | Z | C | (S)↑ → P | Pull Processor Status From Stack | ||||||||||||
| ROL | 2A | 2E | 3E | 26 | 36 | N | - | - | - | - | - | Z | C | CF ← /M7...M0/ ← CF | Rotate One Bit Left (Memory or Accumulator) | ||||||||
| ROR | 6A | 6E | 7E | 66 | 76 | N | - | - | - | - | - | Z | C | CF → /M7...M0/ → CF | Rotate One Bit Right (Memory or Accumulator) | ||||||||
| RTI | 40 | N | V | - | B | D | I | Z | C | (S)↑ → P, (S)↑ → PCL, (S)↑ → PCH | Return From Interrupt | ||||||||||||
| RTS | 60 | - | - | - | - | - | - | - | - | (S)↑ → PCL, (S)↑ → PCH, PC + 1 → PC | Return From Subroutine | ||||||||||||
| SBC | E9 | ED | FD | F9 | E5 | F5 | E1 | F1 | N | V | - | - | - | - | Z | C | A - M - (1 - CF) → A | Subtract Memory from Accumulator with Borrow | |||||
| SEC | 38 | - | - | - | - | - | - | - | 1 | 1 → CF | Set Carry Flag | ||||||||||||
| SED | F8 | - | - | - | - | 1 | - | - | - | 1 → DF | Set Decimal Mode | ||||||||||||
| SEI | 78 | - | - | - | - | - | 1 | - | - | 1 → IF | Set Interrupt Disable | ||||||||||||
| STA | 8D | 9D | 99 | 85 | 95 | 81 | 91 | - | - | - | - | - | - | - | - | A → M | Store Accumulator in Memory | ||||||
| STX | 8E | 86 | 96 | - | - | - | - | - | - | - | - | X → M | Store Index X in Memory | ||||||||||
| STY | 8C | 84 | 94 | - | - | - | - | - | - | - | - | Y → M | Store Index Y in Memory | ||||||||||
| TAX | AA | N | - | - | - | - | - | Z | - | A → X | Transfer Accumulator to Index X | ||||||||||||
| TAY | A8 | N | - | - | - | - | - | Z | - | A → Y | Transfer Accumulator to Index Y | ||||||||||||
| TSX | BA | N | - | - | - | - | - | Z | - | S → X | Transfer Stack Pointer to Index X | ||||||||||||
| TXA | 8A | N | - | - | - | - | - | Z | - | X → A | Transfer Index X to Accumulator | ||||||||||||
| TXS | 9A | - | - | - | - | - | - | - | - | X → S | Transfer Index X to Stack Pointer | ||||||||||||
| TYA | 98 | N | - | - | - | - | - | Z | - | Y → A | Transfer Index Y to Accumulator | ||||||||||||
Illegal instructions
The opcodes in bold are unstable. Only 2 of those 7 opcodes (8B, AB) are actually unstable in the sense that they may produce a truly unpredictable result. The other 5 opcodes actually produce predictable results – but the conditions under which they do that and the produced results are a bit unexpected.
| Mnemonic | Addressing Modes | Flags | Operation | Description | ||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| No arg | #$nn | $nnnn | $nnnn,X | $nnnn,Y | $nn | $nn,X | $nn,Y | ($nn,X) | ($nn),Y | N | V | - | B | D | I | Z | C | |||
| ANC (ANC2) | 0B, 2B | N | - | - | - | - | - | Z | C | A ∧ M → A, NF → CF | "AND" Memory with Accumulator then Move Negative Flag to Carry Flag | |||||||||
| ARR | 6B | N | V | - | - | - | - | Z | C | (A ∧ M) / 2 → A | "AND" Accumulator then Rotate Right | |||||||||
| ASR (ALR) | 4B | 0 | - | - | - | - | - | Z | C | (A ∧ M) / 2 → A | "AND" then Logical Shift Right | |||||||||
| DCP (DCM) | CF | DF | DB | C7 | D7 | C3 | D3 | N | - | - | - | - | - | Z | C | M - 1 → M, A - M | Decrement Memory By One then Compare with Accumulator | |||
| ISC (ISB, INS) | EF | FF | FB | E7 | F7 | E3 | F3 | N | V | - | - | - | - | Z | C | M + 1 → M, A - M → A | Increment Memory By One then SBC then Subtract Memory from Accumulator with Borrow | |||
| JAM (KIL, HLT) | 02, 12, 22,
32, 42, 52, 62, 72, 92, B2, D2, F2 |
- | - | - | - | - | - | - | - | Stop execution | Halt the CPU | |||||||||
| LAS (LAR) | BB | N | - | - | - | - | - | Z | - | M ∧ S → A, X, S | "AND" Memory with Stack Pointer | |||||||||
| LAX (LXA) | AB | AF | BF | A7 | B7 | A3 | B3 | N | - | - | - | - | - | Z | - | M → A, X | Load Accumulator and Index Register X From Memory | |||
| NOP (DOP, TOP) | 1A, 3A, 5A,
7A, DA, FA |
80, 82, 89,
C2, E2 |
0C | 1C, 3C, 5C,
7C, DC, FC |
04, 44, 64 | 14, 34, 54,
74, D4, F4 |
- | - | - | - | - | - | - | - | No operation | No Operation | ||||
| RLA | 2F | 3F | 3B | 27 | 37 | 23 | 33 | N | - | - | - | - | - | Z | C | CF ← /M7...M0/ ← CF, A ∧ M → A | Rotate Left then "AND" with Accumulator | |||
| RRA | 6F | 7F | 7B | 67 | 77 | 63 | 73 | N | V | - | - | - | - | Z | C | CF → /M7...M0/ → CF, A + M + CF → A | Rotate Right and Add Memory to Accumulator | |||
| SAX (AXS, AAX) | 8F | 87 | 97 | 83 | - | - | - | - | - | - | - | - | A ∧ X → M | Store Accumulator "AND" Index Register X in Memory | ||||||
| SBC (USBC) | EB | N | V | - | - | - | - | Z | C | A - M - ~CF → A | Subtract Memory from Accumulator with Borrow | |||||||||
| SBX (AXS, SAX) | CB | N | - | - | - | - | - | Z | C | (A ∧ X) - M → X | Subtract Memory from Accumulator "AND" Index Register X | |||||||||
| SHA (AHX, AXA) | 9F | 93 | - | - | - | - | - | - | - | - | A ∧ X ∧ V → M | Store Accumulator "AND" Index Register X "AND" Value | ||||||||
| SHS (TAS, XAS) | 9B | - | - | - | - | - | - | - | - | A ∧ X → S, S ∧ (H + 1) → M | Transfer Accumulator "AND" Index Register X to Stack Pointer then Store Stack Pointer "AND" Hi-Byte In Memory | |||||||||
| SHX (SXA, XAS) | 9E | - | - | - | - | - | - | - | - | X ∧ (H + 1) → M | Store Index Register X "AND" Value | |||||||||
| SHY (SYA, SAY) | 9C | - | - | - | - | - | - | - | - | Y ∧ (H + 1) → M | Store Index Register Y "AND" Value | |||||||||
| SLO (ASO) | 0F | 1F | 1B | 07 | 17 | 03 | 13 | N | - | - | - | - | - | Z | C | M * 2 → M, A ∨ M → A | Arithmetic Shift Left then "OR" Memory with Accumulator | |||
| SRE (LSE) | 4F | 5F | 5B | 47 | 57 | 43 | 53 | N | - | - | - | - | - | Z | C | M / 2 → M, A ⊻ M → A | Logical Shift Right then "Exclusive OR" Memory with Accumulator | |||
| XAA (ANE) | 8B | N | - | - | - | - | - | Z | - | (A ∨ V) ∧ X ∧ M → A | Non-deterministic Operation of Accumulator, Index Register X, Memory and Bus Contents | |||||||||
Opcode matrix
| Addressing modes: A - accumulator, # - immediate, zpg - zero page, abs - absolute, ind - indirect, rel - relative. Uncolored cells are illegal opcodes. | ||||||||||||||||
| High nibble | Low nibble | |||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | A | B | C | D | E | F | |
| 0 | BRK | ORA (ind,X) | JAM | SLO (ind,X) | NOP zpg | ORA zpg | ASL zpg | SLO zpg | PHP | ORA # | ASL A | ANC # | NOP abs | ORA abs | ASL abs | SLO abs |
| 1 | BPL rel | ORA (ind),Y | JAM | SLO (ind),Y | NOP zpg,X | ORA zpg,X | ASL zpg,X | SLO zpg,X | CLC | ORA abs,Y | NOP | SLO abs,Y | NOP abs,X | ORA abs,X | ASL abs,X | SLO abs,X |
| 2 | JSR abs | AND (ind,X) | JAM | RLA (ind,X) | BIT zpg | AND zpg | ROL zpg | RLA zpg | PLP | AND # | ROL A | ANC # | BIT abs | AND abs | ROL abs | RLA abs |
| 3 | BMI rel | AND (ind),Y | JAM | RLA (ind),Y | NOP zpg,X | AND zpg,X | ROL zpg,X | RLA zpg,X | SEC | AND abs,Y | NOP | RLA abs,Y | NOP abs,X | AND abs,X | ROL abs,X | RLA abs,X |
| 4 | RTI | EOR (ind,X) | JAM | SRE (ind,X) | NOP zpg | EOR zpg | LSR zpg | SRE zpg | PHA | EOR # | LSR A | ALR # | JMP abs | EOR abs | LSR abs | SRE abs |
| 5 | BVC rel | EOR (ind),Y | JAM | SRE (ind),Y | NOP zpg,X | EOR zpg,X | LSR zpg,X | SRE zpg,X | CLI | EOR abs,Y | NOP | SRE abs,Y | NOP abs,X | EOR abs,X | LSR abs,X | SRE abs,X |
| 6 | RTS | ADC (ind,X) | JAM | RRA (ind,X) | NOP zpg | ADC zpg | ROR zpg | RRA zpg | PLA | ADC # | ROR A | ARR # | JMP (ind) | ADC abs | ROR abs | RRA abs |
| 7 | BVS rel | ADC (ind),Y | JAM | RRA (ind),Y | NOP zpg,X | ADC zpg,X | ROR zpg,X | RRA zpg,X | SEI | ADC abs,Y | NOP | RRA abs,Y | NOP abs,X | ADC abs,X | ROR abs,X | RRA abs,X |
| 8 | NOP # | STA (ind,X) | NOP # | SAX (ind,X) | STY zpg | STA zpg | STX zpg | SAX zpg | DEY | NOP # | TXA | ANE # | STY abs | STA abs | STX abs | SAX abs |
| 9 | BCC rel | STA (ind),Y | JAM | SHA (ind),Y | STY zpg,X | STA zpg,X | STX zpg,Y | SAX zpg,Y | TYA | STA abs,Y | TXS | TAS abs,Y | SHY abs,X | STA abs,X | SHX abs,Y | SHA abs,Y |
| A | LDY # | LDA (ind,X) | LDX # | LAX (ind,X) | LDY zpg | LDA zpg | LDX zpg | LAX zpg | TAY | LDA # | TAX | LXA # | LDY abs | LDA abs | LDX abs | LAX abs |
| B | BCS rel | LDA (ind),Y | JAM | LAX (ind),Y | LDY zpg,X | LDA zpg,X | LDX zpg,Y | LAX zpg,Y | CLV | LDA abs,Y | TSX | LAS abs,Y | LDY abs,X | LDA abs,X | LDX abs,Y | LAX abs,Y |
| C | CPY # | CMP (ind,X) | NOP # | DCP (ind,X) | CPY zpg | CMP zpg | DEC zpg | DCP zpg | INY | CMP # | DEX | AXS # | CPY abs | CMP abs | DEC abs | DCP abs |
| D | BNE rel | CMP (ind),Y | JAM | DCP (ind),Y | NOP zpg,X | CMP zpg,X | DEC zpg,X | DCP zpg,X | CLD | CMP abs,Y | NOP | DCP abs,Y | NOP abs,X | CMP abs,X | DEC abs,X | DCP abs,X |
| E | CPX # | SBC (ind,X) | NOP # | ISC (ind,X) | CPX zpg | SBC zpg | INC zpg | ISC zpg | INX | SBC # | NOP | SBC # | CPX abs | SBC abs | INC abs | ISC abs |
| F | BEQ rel | SBC (ind),Y | JAM | ISC (ind),Y | NOP zpg,X | SBC zpg,X | INC zpg,X | ISC zpg,X | SED | SBC abs,Y | NOP | ISC abs,Y | NOP abs,X | SBC abs,X | INC abs,X | ISC abs,X |
Block Diagram
