Hackaday · Maken

COSMAC Elf Turns 50

In late 1976, an article in Popular Electronics described how to build a "powerful, expandable" computer for $80. The computer was based on the RCA 1802 CPU, designed by RCA employee Joseph Weisbecker.

The RCA 1802 was the second generation of the CPU, but the first to be an all-in-one chip. Weisbecker began developing the CPU as a personal project, initially creating a version called "Fred" with 100 TTL chips, intending it for video game use.

From a hacker's perspective, the 1802 offered features like built-in DMA, enabling a low-cost video display that read directly from memory. It also had 16 registers, each usable for various functions like the program counter or memory index. However, it lacked a direct stack for function calls and returns, requiring programmers to use specific registers for subroutines, a method detailed in the "standard call return technique" (SCRT) routines in the manual.

Despite its limitations, people continue to build 1802-based computers and use simulators. The CPU's instruction set is noted for its regularity, suggesting a single designer for the architecture.

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