Mostek 5065

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Mostek 5065: a simple overview

The MK5065 was an 8-bit microprocessor made by Mostek and released in February 1974. It was based on an original Motorola design created for Olivetti’s calculator and was licensed to Mostek for non-calculator uses. It stood out for its three separate register sets, which allowed very fast interrupt handling, and a wait-for-data mode that helped with direct memory access (DMA).

Key points
- Market position: Competing with the Fairchild F8; the 5065 was priced around $58 in medium-volume purchases, but it didn’t see wide use. Mostek later licensed the F8 design and dropped the 5065 from its catalog.
- Architecture: 8-bit data, 15-bit address space (up to 32 KB). It used a multiplexed 16-pin bus (high and low bytes) for addresses and data, making memory accesses take two cycles.
- Interrupts and levels: Three levels of registers (Level 1, Level 2, Level 3) for fast interrupt handling. Two external interrupt pins (INT1 and INT2) could be enabled or disabled in software. On interrupt, the current instruction finished, and the processor switched to the appropriate register level.
- Stack and memory: The first 256 memory locations (zero page) acted as a small stack, holding up to 128 16-bit addresses. A pointer register tracked the top of the stack.
- Addressing and instructions: About 51 instructions and 81 opcodes. Most instructions used two-byte opcode-operand formats; some used a one-byte form. Direct vs indirect addressing was selectable, and indirect addressing could chain. Branches and subroutine calls used a 12-bit address format.
- I/O: I/O was not memory-mapped; dedicated I/O instructions used a four-bit field as an ad-hoc data payload. I/O cycles involved a four-step process to exchange data with peripherals.
- Peripherals and timing: It required an external clock chip (MK5009). Instruction times ranged roughly from 3 to 16 microseconds, and a 1 MHz clock was possible.
- Packages and tools: Packaged in a 40-pin DIP. The system offered an evaluation setup called GEMS-8 (General Evaluation Microprocessor System) including ROM, RAM, and a UART. The ROM contained a small development tool and debugger; the full kit was priced around a thousand dollars.

In short, the Mostek 5065 was an early, interrupt-friendly 8-bit processor with a compact but versatile design. Its rapid interrupt handling and DMA support were notable, but it was soon eclipsed by newer NMOS designs like the Fairchild F8, and by 1975 Mostek focused on the F8-based MK3850.