Fairchild F8

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Fairchild F8 or F3850 microprocessor.

The Fairchild F8 was an 8-bit microprocessor created by Fairchild Semiconductor announced in 1974, shipped in 1975.[1][2]


The processor itself had no address bus — memory addresses were kept in each co-processor's own address counter and were manipulated through five control signals, reducing the number of pins and the associated cost. It also featured 64 bytes of scratchpad memory, accessed by the ISAR register in cells (register windows) of eight, which meant external RAM was not always needed for small applications. In addition, the 2-chip processor did not need support chips, unlike others which needed seven or more.

The use of the ISAR register allowed a subroutine to be entered without saving registers, the ISAR would just be changed, speeding execution. Special purpose registers were usually stored in the second cell (regs 8-15), and only the first sixteen registers could be accessed directly. The windowing concept was useful, but only the register pointed to by the ISAR could be accessed — to access other registers, the ISAR was incremented or decremented through the window.

The F8 ran at 1–2 MHz, yielding a 0.5 μs cycle time.


The F8 was released in a single-chip implementation (the Mostek 3870) in 1977. According to the CPU Museum, "in 1977 the F8 was the world´s leading microprocessor in terms of CPU sales."[3]

The F8 was used in the Fairchild Channel F Video Entertainment System in 1976 and in the VideoBrain Computer system in 1977.




  1. ^ "CPU of the Day: Fairchild F8 Microprocessor". The CPU Shack Museum. June 8, 2013. Retrieved August 18, 2013. 
  2. ^ "The Fairchild F8". The Antique Chip Collector's Page. Retrieved August 18, 2013. 
  3. ^ 8-bit Microprocessors - F8 (3850) at the Wayback Machine (archived July 17, 2011)

This article is based on material taken from the Free On-line Dictionary of Computing prior to 1 November 2008 and incorporated under the "relicensing" terms of the GFDL, version 1.3 or later.

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