The MOS Technologies 6502 process 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.
Memory
The 6502 (and its variants) 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 $10, which pointed to memory location $1234, would be stored like this:
Memory $10 -> $34 Memory $11 -> $12
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 the 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. |
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) - a collection of bits which indicate or control aspects of the processor mode and status:
- C - Carry - Used to indicate a carry or borrow during addition/subtraction
- Z - Zero flag - indicates that an operation produced a zero or equal result
- 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 rather than a hardware interrupt has occurred
- V - Overflow - Set when a math operation overflows a one-byte result
- S - Negative Sign - set when an operation produces a negative result