COSMAC Elf with Pixie Graphics Display
|Memory||256 bytes of RAM; expandable|
The COSMAC Elf was an RCA 1802 microprocessor-based computer described in 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 single-board personal computer. It was operated without built-in ROMs and programs were entered directly with help of the CPU integrated DMA using 8 toggle switches and an Input push button.
It featured two hexadecimal LED displays 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, with an upper limit of 64K.
The original Elf design used a crystal with a frequency in the range of 1 to 2 MHz with the 1802's built in oscillator circuit.
A simple circuit used the DMA feature of the 1802 to permit entry of programs and data into RAM through the toggle switches. Entering a byte via the toggle switches and pressing the "input" button would enter a byte into RAM and display it on the pair of hex LEDs, then advance the DMA counter to the next location. A "memory protect" switch could be used to disable memory alteration. If an error was made in program entry, it could be corrected by turning on memory protect, turning off load mode (thus resetting the program counter to zero), turning on load mode, and pressing "input" to advance to the address of the incorrect data. After turning off memory protect, the correct value could be entered.
The fourth article of the series presented modifications to use a companion RCA 1861 “Pixie” video generator IC (CDP1861). The Pixie required a 1.76 MHz clock, and since that was an uncommon crystal frequency, usually a readily available 3.579545 MHz colorburst crystal was instead used in a separate oscillator circuit with a divide-by-two circuit to drive the clock inputs of both the microprocessor and Pixie. Monochrome video output (with timing roughly approximating NTSC standard) could be generated using DMA operations interleaved with carefully arranged 1802 opcodes as instructions in software. The maximum resolution by the 1861 was 64h by 128v pixels. By changing the placement of instructions in the video display control and interrupt routines, pixel rows could be repeated to obtain lower resolutions, allowing the video display to be used with 256 bytes of RAM (64×32 square pixels).
A one-bit output from the microprocessor, the Q line, could be driven by software to produce sounds through an attached speaker, to save programs in RAM to a cassette recorder, and for serial I/O output. 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 (input) momentary pushbutton (typically EF4), programs from the cassette recorder through interface circuitry, serial I/O input, and input from peripherals such as a light pen. There are also seven 8-bit I/O ports available for decoding and interfacing.
The original Elf computers were essentially home-built versions of the RCA Microkit, Microtutor I and Microtutor II, which were RCA's demonstration boards for their CDP1801 2-chip predecessor and single-chip CDP1802 microprocessors.
Enhanced kits such as the Netronics Elf II and Quest Super Elf added features such as keypads for data entry, serial I/O, cassette interface, and the CDP1861 "Pixie" video chip. RCA later introduced their own expanded version as the COSMAC VIP.
In August 2006, Nuts and Volts magazine, 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 and enhanced features, modules, and functionality, including the STG1861 Pixie Graphics Replacement board that is functionally equivalent to the now-rare RCA CDP1861 integrated circuit.
Various other hobbyist systems can be found on the Internet, including hardware emulators using FPGA and modern microcontrollers.
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. Other, similar information and hobbyist software projects can be found on the Internet.
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 low-resolution monochrome graphics display or TV-Typewriter hardware.
Other languages available are noted at the RCA 1802 wikipedia entry, including interpreters, compilers and assemblers.
Game cartridges for the RCA Studio II contain Chip-8 games, which can run on other 1802 systems. File dumps of these games can be found on the Internet.
- Weisbecker, Joseph (August 1976). "Build the COSMAC Elf (Part 1)". Popular Electronics (Ziff Davis) 10 (2): 33–38.
- Weisbecker, Joseph (September 1976). "Build the COSMAC Elf (Part 2)". Popular Electronics (Ziff Davis) 10 (3): 37–40.
- Weisbecker, Joseph (March 1977). "Build the COSMAC Elf (Part 3)". Popular Electronics (Ziff Davis) 11 (3): 63–67.
- Weisbecker, Joseph (July 1977). "Build the COSMAC Elf (Part 4 Pixie Graphics Display)". Popular Electronics (Ziff Davis) 12 (1): 41–46.