Fairchild F8

Fairchild F8 or F3850 microprocessor.

The Fairchild F8 was an 8-bit microprocessor created by Fairchild Semiconductor. It was introduced in 1975 and was "the world's leading microprocessor in terms of CPU sales" in 1977.

Features

The processor itself had no address bus — memory addresses were kept in each co-processor's own address counter and was 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 MHz-2 MHz, yielding a .5uS cycle time.

History

The F8 was released in a single-chip implementation (the Mostek 3870) in 1977.

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

The F8 inspired other similar CPUs, such as the Intel 8048.

Designers

One of the people responsible for developing it was Robert Noyce who co-founded Fairchild but then left in 1968 to found Intel.

References

• "8-bit Microprocessors - F8 (3850)", cpu-museum.com.
• Datasheet: http://datasheets.chipdb.org/Fairchild/F8/fairchild-3850.pdf

External links

• Free On-Line Dictionary of Computing
• The Great CPU List

This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.