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Schematic of a MOS 6502 [1]

The 6502 is an 8-bit microprocessor that was designed by Chuck Peddle and Bill Mensch for MOS Technology in 1975. Peddle and Mensch were previously involved in designing the Motorola 6800. The 6502's performance was actually competitive with other CPUs using significantly faster clocks, partly due to a simplistic state machine implemented by combinatorial logic to a greater extent than in many other designs. Therefor it was a popular processor for dedicated chess computers as well as general purpose systems such as the single board computer KIM-1, and early home computers, notably Apple II, Atari 8-bit, Commodore PET and Commodore 64, which actually uses a close derivative of the 6502, the 6510.

Register Files

Like its precursor, the Motorola 6800 (but unlike Intel 8080) the 6502 has very few registers. There was one 8-bit accumulator register (A), two 8-bit index registers (X and Y), an 8-bit processor status register (P), an 8-bit stack pointer (S), and a 16-bit program counter (PC).


Unlike Motorola 6800, the 6502 is a little-endian processor, concerning the byte-order of 16-bit words in memory.


By Dan and Kathe Spracklen from their Oral History [2] on the advantages of the 6502 compared to 8080/Z80 while porting Sargon:

Danny Spracklen: Yes, the instruction set was a little more efficient, I think, and more- it was more, if you could say, RISC like. ...
It was a simpler instruction set, but it actually- the cycle times on the individual instructions were very fast, like one microsecond, instead of like 4 or 5 microseconds, on the Z-80.
Kathleen Spracklen: Yes. And it had the short branch. You can’t underestimate the value that the short branch offered. They had the ability- if you could branch within a one byte distance of where you were, and using it as a sign number, positive or negative, you had an extremely fast branching instruction. So if you could make loops very, very small and tight, you could really run with high, high speed. 



MC650x Internal Architecture [3]


TK20 [4]

The TurboKit TK20 by German engineering company Schaetzle+Bsteh was a discrete 6502 compatible 18 MHz high speed CPU with own fast shadow memory build from more than 100 integrated circuits, dedicated for general purpose and chess computers with 6502 processor, connected via a ribbon cable to the 40 pin processor socket of the host system [5] [6] [7]. The TK 20 was used by Ed Schröder with his dedicated Rebel programs at the WCCC 1986 and various World Microcomputer Chess Championship, and also by Ulf Rathsman and his Y! programs. Further, the dedicated Saitek Leonardo Turbo mini series was extended by Schaetzle+Bsteh with an 18 MHz integrated TurboKit [8].

See also


Selected Publications


For instance, how to flip the board in 6502 Assembly [10] :

Forum Posts

External Links

6502.org Forum


  1. Schematic of a MOS 6502, carefully reverse engineered over a long period of time by the chip enthusiast who runs BreakNES, showing how the classic chip works, MOS Technology 6502 - Wikipédia.fr
  2. Gardner Hendrie (2005). Oral History of Kathe and Dan Spracklen. pdf from The Computer History Museum
  3. Foltyn Presentation - Microprocessor 6502
  4. Image from ICT 2006 and 13th Chess Computer Users (Gebruikers) tournament old CSVN site Photo Gallery 24, Photos by Eric van Reem and Kees Sio
  5. TurboKit – Schachcomputer.info Wiki
  6. Schaetzle+Bsteh High Speed Processor System TK20 TurboKit MTK20 Multi TurboKit. pdf hosted by Hein Veldhuis
  7. TK20 Turbokit by Chessbama, Hiarcs Forum, December 09, 2014
  8. Saitek Leonardo TurboKit from Schachcomputer.info Wiki (German)
  9. 6502.org: Books
  10. 6502 MICROPROCESSOR Instruction Set Summary

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