Capricorn (microprocessor)

The Capricorn family of microprocessors was developed by Hewlett-Packard in the late 1970s for the HP series 80 scientific microcomputers. Capricorn was first used in the HP-85 desktop BASIC computer, introduced in January 1980. Steve Wozniak was inspired to build the Apple to be a computer like the HP 9830, and in 1976 he offered HP rights to the Apple computer. He was turned down and was given a release. When the calculator division started an 8 bit computer projected called Capricorn, he left for Apple when he wasn't allowed to work on that project.

Architecture

The Capricorn is a microprogrammed CPU containing 64 eight-bit registers, an eight-bit arithmetic logic unit (ALU), a shifter and control logic. One register pair is defined as the program counter, another pair as the stack pointer, and one more pair as an index pointer for internal operations. There is no dedicated accumulator — any general register can be used for ALU results because the register file is designed to allow two read and one write operations at the same time. Any pair of registers can be used as a 16-bit index register.

The ALU can work either in binary or binary-coded decimal (BCD) mode. Variable-length instructions let the programmer treat data in the upper 32 registers as entities between one and eight bytes in length — for example, two eight-byte values (e.g. mantissa of a floating-point number) can be added using a single instruction. This feature reduces the number of loops that need to be programmed.

The CPU has an interrupt mechanism with up to 127 vectors. For direct memory access, the CPU can be halted by an external device.

Implementation

The Capricorn CPU was implemented as a silicon-gate NMOS logic circuit (4.93×4.01 mm) in a 28-pin dual in-line package, with an 8-bit, multiplexed external bus. The CPU chip consumed 330 mW at 625 kHz.

CPU timing is controlled by four non-overlapping clock phases with 200 nanosecond width and 200 nanosecond spacing, for an overall clock cycle of 1.6 microseconds, equivalent to 625 kHz clock frequency.

The complete system included support chips co-designed with the CPU, such as a dynamic memory controller, keyboard controller with timers, printer controller and CRT controller. A special buffer chip connected to the expansion slots.

References

A Custom LSI Approach to a Personal Computer. Todd R. Lynch, Hewlett-Packard Journal, August 1980, Volume 31, No.8, pp. 18.

Tim Mikkelsen. "What is the cpu for HP-87?". Retrieved 2010-02-12.