Signetics 2650A

The Signetics 2650 was an 8-bit microprocessor introduced in mid-1975. According to Adam Osborne's book An Introduction to Microprocessors Vol 2: Some Real Products, it was "the most minicomputer-like" of the microprocessors available at the time.

Signetics sold development boards e.g. the PC1001 and its successor, the PC1500 "Adaptable Board Computer", ranging in price from A$165 to A$400. The chip by itself sold for around A$20. Several hardware construction projects and programming articles were published in magazines such as Electronics Australia and Elektor and related kits were sold by electronics stores. These factors led to its use by a number of hobbyists in many countries such as Australia, U.S.A., United Kingdom, the Netherlands and Germany.

The chip contained seven 8-bit general purpose registers, although only four were visible at any time. It was limited to a 15-bit address space (thereby addressing a maximum of 32 KB of memory), since the upper bit of a 16-bit memory reference was reserved to indicate that the indirect memory addressing mode was to be used (a minicomputer-like feature).

The address space was further limited by the use of another two bits of the address to indicate the indexing mode for all logical and arithmetic (i.e. non branch) instructions. This divided the address space into four 8 KB 'pages', only the current (i.e. the page the program was running in) could be addressed directly, the others only by indirect addressing.

While there were nine different addressing modes, the lack of any 16-bit registers and the 13–15-bit address space prevented widespread use. Despite this, an operating system ("2650 DOS") was available, along with 8 KB and 12 KB BASIC interpreters (sold by Central Data Corporation USA), and many games of the Hunt the Wumpus style. Most programs were written in assembly language.

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