6502

The MOS Technologies 6502 processor was introduced in the mid-1970s to fill the need for a affordable general-purpose CPU. Its low cost (US$25 at introduction, less than C$0.89 now) was less than one-sixth of competing CPUs, and it had very simple circuitry requirements which made it simple and inexpensive to incorporate it into products. The 6602 (or a slight variation) was therefore used in many home and personal computers, such as the Apple II; the Commodore PET, Vic-20, and C64; the Atari 400 and 800; the BBC Micro; and games such as the Nintendo Entertainment System (NES), Atari 5200, and Atari 6200. A number of variations of this processor have been produced, using different semiconductor processes, integrated peripherals, instruction and data-width extensions, and pinouts. Several different versions are still in production for various embedded applications, and it remains a popular chip for homebrew system builders.

Memory

The 6502 is an 8-bit processor with a 16-bit address bus. It is therefore able to access 64 kilobytes (2¹⁶ bytes). Since each 16-bit address is comprised of two 8-bit bytes, memory can be viewed as 256 pages of 256 bytes each.

Each pointer in memory is stored in two consecutive memory locations, with the lowest-value byte stored first; this is known as Little Endian order. Thus, a pointer at memory location $0010, which points to memory location $ABCD, would be stored like this:

 Memory $0010: $CD
 Memory $0011: $AB

Some pages have special, pre-defined purposes:

Page	Name	Starting address	Ending address	Purpose
00	Zero Page	$0000	$00FF	Variables requiring fast access
01	Stack	$0100	$01FF	Values are pushed to, and pulled (popped) from, this region in first-in last-out (FIFO) order. The stack descends as it is used - more recently-pushed values are stored at lower addresses than older values. The stack wraps around, so if more than 256 bytes are pushed, the oldest values will be overwritten.
FF	Vector Table	$FF00	$FFFF	The last 6 bytes of this page contain three 2-byte addresses. $FE contains a pointer to code which is run when an interrupt request is received; $FC contains a pointer to code which is run when the CPU is reset (including when it is first started); and $FA contains a pointer to code which is run when a non-maskable interrupt (NMI) is received.

In addition, each system built using the 6502 would have hardware devices, such as the video system, keyboard, and communication interfaces, occupying a portion of the address space.

Registers

There are three general-purpose registers:

• Accumulator (A) - the main register for math operations.
• X Index (X) - a register which can be used for limited math operations as well as indexed addressing modes, where an index value is added to a base address for memory operations.
• Y Index (Y) - a register similar to the X register. Some index operations may only be performed with a specific index register (X or Y, but not interchangeably).

There are also three special-purpose registers:

• Program Counter (PC) - a pointer to the currently-executing instruction in memory.
• Stack Pointer (S or SP) - a pointer to the current position in the stack
• Processor Status (P or PS) - a collection of bits (flags) which indicate or control aspects of the processor mode and status:
  • C - Carry - Used to carry or borrow during addition and subtraction operations. If set (=1) at the start of an add-with-carry (ADC) operation, an additional 1 will be added to the result; if cleared (=0) at the start of a subtract-with-carry instruction (SBC), an additional 1 will be subtracted from the result. This flag will be set or cleared to indicate if an (unsigned) addition overflowed (result > 255) or the (unsigned) subtraction underflowed (result < 0)
  • Z - Zero flag - indicates that an operation produced a zero result. Since comparison instructions (CMP, CPX, CPY for comparisions involving the A, X, or Y registers respectively) are actually subtractions, comparing two equal numbers by subtraction will result in a zero value, setting this flag.
  • I - Interrupt disable
  • D - Decimal mode - bytes are interpreted as two-digit decimal values instead of 8-bit binary values when doing math
  • B - Break - Indicates a software interrupt (BRK instruction) has caused a non-maskable interrupt (NMI), rather than a hardware NMI.
  • V - Overflow - Set when a math operation overflows (result > 127) or underflows (result < -128) a one-byte signed result
  • S - Negative Sign - set when an operation produces a negative result (bit 7 is set in the result)

Instruction Set

The 6502 instruction set consist of a number of single-byte opcodes, each of which is followed by 0, 1, or 2 bytes of arguments. Each opcode corresponded to an instruction, which consists of an operation and an addressing mode. 6502 Assembly Language uses 3-letter menomics to specify the operation, and argument syntax to specify the addressing mode. For example:

 LDA #$05  ; load the accumulator with the number 5
 LDA $05   ; load the accumulator with the contents of memory location 5 (in the zero page)
 LDA $f005 ; load the accumulator with the contents of memory location $f005

See the references (below) for the full details of the 6502 instruction set.

6502 Emulator

Since 6502 systems are no longer very common, a web-based 6502 Emulator is available for assembling, testing, and debugging 6502 Assembly code.

Resources

• 6502.cdot.systems, the 6502 emulator we use in this course
• Wikipedia entry for 6502
• 6502.org
• Visual 6502 - a project to physically disassemble and analyze the 6502 chip, including photographs of the chip die and a visual simulation of voltages on the chip
• Easy 6502 (tutorial using an earlier version of the 6502 emulator we use in this course)
• 6502 Opcodes with Register Definitions
• 6502 Opcodes with Detailed Operation Information
• MOnSter 6502 - a large-scale, transistor-level implementation of the 6502, with lots of LEDs!