Editing MOS Technology 6502 (section)

===Addressing===
The chip uses the index and stack registers effectively with several [[addressing mode]]s, including a fast "direct page" or "zero page" mode, similar to that found on the [[PDP-8]], that accesses memory locations from addresses 0 to 255 with a single 8-bit address (saving the cycle normally required to fetch the high-order byte of the address){{mdash}}code for the 6502 uses the zero page much as code for other processors would use registers. On some 6502-based microcomputers with an operating system, the [[operating system]] uses most of zero page, leaving only a handful of locations for the user.

Addressing modes also include ''implied'' (1-byte instructions); ''absolute'' (3 bytes); ''indexed absolute'' (3 bytes); ''indexed zero-page'' (2 bytes); ''relative'' (2 bytes); ''accumulator'' (1); ''indirect,x'' and ''indirect,y'' (2); and ''immediate'' (2). Absolute mode is a general-purpose mode. Branch instructions use a signed 8-bit offset relative to the instruction after the branch; the numerical range −128..127 therefore translates to 128 bytes backward and 127 bytes forward from the instruction following the branch (which is 126 bytes backward and 129 bytes forward from the start of the branch instruction). Accumulator mode operates on the accumulator register and does not need any [[operand]] data. Immediate mode uses an 8-bit [[literal (computer programming)|literal]] operand.