6502

The 6502 CPU

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.

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.

BCD operations are limited to addition and subtraction using the ADC and SBC instructions.

On NMOS, when Decimal Mode is on, the ADC and SBC instructions update NF, VF and ZF based on the binary result before the decimal correction is applied. Only CF is updated correctly. On CMOS, all the flags are updated correctly, at the cost of 1 additional cycle.

On NMOS, DF is not defined after RESET. On CMOS, DF is automatically cleared on RESET.

On NMOS, DF is unchanged when entering an interrupt of any kind. This can cause unexpected bugs in the interrupt handler if Decimal Mode is on when an interrupt occurs. On CMOS, DF is automatically cleared on interrupt. Upon returning from an interrupt, the processor restores the status register from the stack, including DF.

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-$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. The stack has a 256-byte limit, and overflow occurs if not managed properly.

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.

All I/O operations are memory-mapped. There are no port-based I/O instructions.

The 6502 divides each clock cycle into two phases (ϕ1 and ϕ2):

• During the ϕ1 half-cycle, no bus access occurs. This phase is dedicated to internal CPU operations.
• During the ϕ2 half-cycle, the CPU accesses the external bus for memory reads/writes or I/O operations.

The use of half-cycles ensures that memory and I/O devices have predictable timing windows when the CPU will access the bus, while still allowing the CPU to perform internal operations in parallel.

Unlike most microprocessors, the 6502 does not make memory accesses on an "as needed" basis. It always does a fetch or store on every single clock cycle. When there isn't anything to be fetched or stored, a "garbage" fetch or store occurs. This is mainly of importance with the memory-mapped I/O devices:

• On NMOS, when adding a carry to the MSB of an address, a fetch occurs at a garbage address. On CMOS, the last byte of the instruction is refetched.
• On NMOS, when doing a fetch-modify-store instruction (INC, DEC, ASL, LSR, ROL, ROR), garbage is stored into the location during the "modify" cycle... followed by the "real" store cycle which stores the correct data. On CMOS, a second fetch is performed instead of a garbage store.

The 6510 CPU adds an AEC pin that the VIC-II uses to kick the CPU off the bus.

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.

The priority sequence for interrupts, from top priority to bottom, is as follows: RESET, BRK, NMI, IRQ. Source at chapter 7.19

On NMOS, NMI or IRQ that happens during specific moments of BRK execution can cause BRK to be effectively skipped. On CMOS, this situation is correctly handled by executing BRK and then servicing the interrupt.

Standard instructions

Cycles are shown in parenthesis for each opcode. p=1 if page is crossed. t=1 if branch is taken.

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.

• On NMOS, an indirect JMP will behave unexpectedly when the indirect address crosses a page boundary, because the 6502 does not add the carry to calculate the address of the high byte. For example, JMP ($19FF) will use the contents of $19FF and $1900 for the JMP address. On CMOS, this issue was fixed, at the cost of 1 additional cycle. In our example, JMP ($19FF) will use the contents of $19FF and $2000 for the JMP address.

• Some instructions, particularly those involving branches or indexed addressing modes, incur an extra cycle if the processor has to cross a memory page boundary. This is problematic for time-sensitive code.

• Conditional jumps are only 8-bit relative. And unconditional jumps are only 16-bit absolute.

• LDX absolute,Y and LDY absolute,X instructions exist but not the corresponding instructions STX absolute,Y and STY absolute,X.

• The CLV (Clear Overflow Flag) instruction exist but not the SEV (Set Overflow Flag) instruction.

• The ROR instruction didn't exist in the very earliest (pre-1977) chips.

• The NOP instruction takes 2 full-cycles. This is the minimum amount of cycles an instruction can take.

• The 65C02 fixed multiple bugs of the original NMOS 6502, but also removed access to all illegal instructions. And it added some extra instructions. In fact, there are multiple implementations of the 65C02 (WDC 65C02, WDC 65C02S, Rockwell R65C02, CSG 65CE02, ...), each with its own variant of the instruction set.

• 6502 at the English-language Wikipedia
• 6502 Instruction Set
• 6502 Family CPU Reference
• The 6502/65C02/65C816 Instruction Set Decoded
• Media:SY6500 - SY65C02 datasheet.pdf - provides a detailed breakdown of the machine cycles
• Media:6502 (65xx) Microprocessor Instant Reference Card.pdf
• Media:6502-dead-cycles.pdf
• No more secrets - NMOS 6510 Unintended Opcodes