[[Category:SPO600]][[Category:Computer Architecture]][[Category:Assembly Language6502]][[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. It The 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 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. (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).

* Processor Status (Por PS) - a collection of bits (flags) which indicate or control aspects of the processor mode and status:** C - Carry - Used to indicate a carry or borrow during addition/and subtractionoperations. 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 resultin a zero value, setting this flag.

** B - Break - Indicates a software interrupt (BRK instruction) has caused a non-maskable interrupt (NMI), rather than a hardware interrupt has occurredNMI.** V - Overflow - Set when a math operation overflows (result > 127) or underflows (result < -128) a one-byte signed result** 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)

== References Resources == * Resources[http://6502.cdot.systems 6502.cdot.systems], the 6502 emulator we use in this course** [https://en.wikipedia.org/wiki/MOS_Technology_6502 Wikipedia entry for 6502]** [http://6502.org/ 6502.org]** [http://www.visual6502.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** [https://skilldrick.github.io/easy6502/ Easy 6502](tutorial using an earlier version of the 6502 emulator we use in this course)** [http://www.6502.org/tutorials/6502opcodes.html 6502 Opcodes with Register Definitions]** [https://www.masswerk.at/6502/6502_instruction_set.html 6502 Opcodes with Detailed Operation Information]*[https://www.pagetable.com/c64ref/6502/?tab=2 6502 instructions via the "Ultimate Commodore 64 Reference" site]* [https://monster6502.com/ MOnSter 6502] - a large-scale, transistor-level implementation of the 6502, with lots of LEDs!