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.

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