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MOS 6502AD 4585 top

A MOS 6502 processor in a DIP-40 plastic package. The four-digit date code indicates it was made in the 45th week of 1985.

The ideaEdit

It was to create a cheep and efficient 8-bit CPU that could be used in cheaper and more portable commuters, possibly even ending the need for massive and costly mainframe of the day computers like the IBM System/370!

Electrical engineeringEdit


6502 Pin configuration (40-Pin Dual in-line package.). Author: Bill Bertram.


The 6502 is a little-endian 8-bit processor with a 16-bit address bus. The original versions were fabricated using an 8 µm[50] process technology chip with an advertised die size of 153 x 168 mils (3.9 x 4.3 mm) or an area of 16.6 mm2.

The internal logic runs at the same speed as the external clock rate, but despite the slow clock speeds (typically in the neighborhood of 1 to 2 MHz), the 6502's performance was competitive with other contemporary CPUs using significantly faster clocks. This is partly due to a simplistic state machine implemented by combinatorial (clockless) logic to a greater extent than in many other designs; the two phase clock (supplying two synchronizations per cycle) can thereby control the whole machine-cycle directly. Typical instructions might take half as many cycles to complete on the 6502 than contemporary designs. Like most simple CPUs of the era, the dynamic NMOS 6502 chip is not sequenced by a microcode ROM but uses a PLA (which occupied about 15 percent of the chip area) for instruction decoding and sequencing. Like most eight-bit microprocessors, the chip does some limited overlapping of fetching and execution.

The low clock frequency moderated the speed requirement of memory and peripherals attached to the CPU, as only about 50 percent of the clock cycle was available for memory access (due to the asynchronous design, this percentage varied strongly among chip versions). This was critical at a time when affordable memory had access times in the range 250 - 450 ns. The original NMOS 6502 was minimalistically engineered and efficiently manufactured and therefore cheap—an important factor in getting design wins in the very price-sensitive game console and home computer markets. Like its precursor, the Motorola 6800, the 6502 has very few registers. At the time the processor was designed, the number of transistors that could be economically put on a chip was very constrained (around a few thousand), so it made sense to rely on RAM instead of allocating expensive NMOS chip area for CPU registers.

The MOS Technology 6502's electrical architectureEdit



The then Western standard silicon\germanium etching process.


General historyEdit

The MOS Technology 6502 (typically called the "sixty-five-oh-two" or "six-five-oh-two") is an 8-bit microprocessor that was designed by a small team led by Chuck Peddle for MOS Technology. It was notably cheaper than it's peers. Along with the Zilog Z80 CPU it sparked a series of projects that resulted in the home computer revolution of the early 1980s.

The 6502 was designed by many of the same engineers, led by Tom Bennett, that had designed the Motorola 6800 microprocessor family in 1971. Motorola's target customers were established electronics companies such as Hewlett-Packard, Tektronix, TRW and Chrysler.

Popular home video game consoles and computers, such as Atari, Apple II, Nintendo Entertainment System, Commodore PET and others, used the 6502 or variations of the basic design. Soon after the 6502's introduction, MOS Technology was purchased outright by Commodore International, who continued to sell the microprocessor and licenses to other manufacturers. In the early days of the 6502, it was second-sourced by Rockwell and Synertek, and later licensed to other companies. It is estimated that hundreds of millions of it's later CMOS descendants are still in regular use globally. It was one of the most popular microchips ever made.

The earliest revisions of the 6502, such as those shipped with some KIM-1 computers, had a severe bug in the ROR (rotate right memory or accumulator) instruction. The operation of ROR in these chips is effectively an LSR instruction that does not affect the carry bit in the status register. MOS left the instruction out of chip documentation entirely because of the defect, promising that ROR would appear on 6502 chips starting in 1976. The vast majority of 6502 chips in existence today do not exhibit this bug.

Cold War impactEdit

It was better than any European, Soviet or Japanese product of the time. It showed how behind the times Soviet semi-conductor technology had become in the 1970s and the Soviets knew it.


MOS Technology 6502.
Category. Statistic.
Designed in. 1972.
Made in. 1975.
Transistors per chip. 3,510.
Power supply. Low.
Still in use. Only in a few old computers kept by private individuals, company archives and museums.
Nationality. American.


A 40 pin epoxy Dual in-line package.

Also seeEdit

  1. Science
  2. Integrated circuits
  3. Ferranti
  4. Radio
  5. TO-92 transistor packing unit shell
  6. TO-18 transistor packing unit shell.


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