COSMAC ELF

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COSMAC ELF Prototype with PIXIE Graphics Display

COSMAC ELF on display at the Computer History Museum. (Below the Altair 8800 computer and next to the TV Typewriter.)

The COSMAC ELF was an RCA 1802 microprocessor-based computer based on a series of construction articles in Popular Electronics magazine in 1976 and 1977. Through the back pages of electronics magazines, both Netronics and Quest Electronics offered low-priced kits that were based on this design. The system was a very early personal computer. It was operated without built-in ROMs and programs were entered directly with help of the CPU integrated dma.

It featured two seven-segment display digits for output and a set of toggle switches or a hexadecimal keypad for input. The base configuration had 256 bytes of RAM, but expansion boards could raise that to 4096 or 32,768 bytes of RAM.

It was much like RCA COSMAC VIP without the CDP 1861 "Pixie" video chip, although some ELF's did have video chips installed.

A simple circuit used the DMA feature of the 1802 to permit entry of programs and data into RAM through the hex keyboard. Typing two hexadecimal keys on the keyboard and pressing the “I” key would enter a byte into RAM and display it on the pair of hex LEDs, then advance the DMA counter to the next location. Typically, a small cassette loader or monitor program would be entered in this way to permit loading of larger programs.

A 3.58 MHz crystal oscillator was divided down to drive the microprocessor and a companion RCA 1861 “PIXIE” video generator IC. NTSC compatible monochrome video output could be generated using DMA operations interleaved with carefully arranged 1802 opcodes. The maximum resolution by the 1861 was 64h by 128v pixels. By changing the placement of instructions in the video display subroutine, pixel rows could be repeated to obtain lower resolutions, allowing the video display to be used even with 256 bytes of RAM.

A one-bit output from the microprocessor, the Q line, could be driven by software to produce sounds through an attached speaker or to save programs in RAM to a cassette recorder. Branch instructions in the 1802 instruction set could read the state of the EF1 through EF4 input lines, which were used to read the I keypad button and programs from the cassette recorder, along with input from peripherals such as a light pen.

A series of newsletters and small booklets offered by Netronics and Quest contained 1802 machine language and CHIP-8 programs, along with schematics for expanding the ELF and adding peripherals, including a light pen.

Tiny BASIC, a version of BASIC offered by Tom Pittman, could be used to write small BASIC programs on the ELF that could display through the PIXIE display or TV-Typewriter hardware.

In August 2006, Nuts and Volts maganize, along with Spare Time Gizmos released a project to build the "Cosmac Elf 2000," based on the original ELF, with some newer and easier to find components.

[edit] References

Construction Articles

Weisbecker, Joseph (August 1976). "Build the COSMAC ELF (Part 1)". Popular Electronics 10 (2): 33-38. Ziff Davis.
Weisbecker, Joseph (September 1976). "Build the COSMAC ELF (Part 2)". Popular Electronics 10 (3): 37-40. Ziff Davis.
Weisbecker, Joseph (March 1977). "Build the COSMAC ELF (Part 3)". Popular Electronics 11 (3): 63-67. Ziff Davis.
Weisbecker, Joseph (July 1977). "Build the COSMAC ELF (Part 4 PIXIE Graphics Display)". Popular Electronics 12 (1): 41-46. Ziff Davis.