Editing COSMAC ELF (section)

==Description==

It featured two [[hexadecimal]] [[Light-emitting diode|LED]] displays for byte data value output and a set of 8 [[toggle switch]]es for input. (a hexadecimal [[keypad]] was an optional extension) The base configuration had 256 [[byte]]s 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&nbsp;MHz with the 1802's built in [[electronic oscillator|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 CDP1861|RCA 1861]] “Pixie” video generator [[integrated circuit|IC]] (CDP1861).  The Pixie required a 1.76&nbsp;MHz clock, and since that was an uncommon crystal frequency, usually a readily available 3.579545&nbsp;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&nbsp;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 [[opcode]]s 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 pixel]]s).

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 communications|serial]] [[I/O]] output.  [[Branch instruction]]s 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.