[[Category:SPO600]][[Category:Computer Architecture]][[Category:6502]][[Image:MOS_6502_1.jpg|thumb|400px|right|The MOS 6502 processor. Image credit: Christian Bassow - [https://creativecommons.org/licenses/by-sa/4.0 CC-BY-SA 4.0] ]]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 6502 (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. {{Admon/tip|Hex notation - $XX|In most 6502 documentation, including this page, the <code>$</code> prefix indicates hexadecimal notation. On other systems, this may be designated by a <code>0x</code> prefix.}}

||01||Stack||$0100||$01FF||Values are pushed to, and pulled (popped) from, this region in first-in last-out (FIFOFILO) 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. (Note that the 6502 BRK instruction is counted as an NMI, and the B status flag can be used to determine if a hardware NMI or BRK instruction was received).

** S N - Negative Sign - set when an operation produces a negative result (bit 7 is set in the result)

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

LDA $05 ; load the accumulator with the contents of memory location 5 ($05 in the zero page($0005)