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Source: COSMAC ELF

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, enhanced 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 byte data value output and a set of 8 toggle switches for input. (a hexadecimal keypad was an optional extension) The base configuration had 256 bytes of RAM, but expansion projects could raise that to a power of two-based memory store, with an upper limit of 64K address space.
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. The resulting 1.7897725 MHz clock was close enough for the hardware to work. 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 of the 1861 was 64h by 128v rectangular 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 single bit value 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.