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Book created: 12-24-2017


HER/12 is a hobbyist project which goal is to design and build a mini-computer using 1970's technologies. Built from discrete SSI and MSI integrated circuits rather than VLSI microprocessor, the machine is meant to taste old and serve well for the years to come.


Click to selectAbout this project
HER/12 is my second attempt to produce a minicomputer based on discrete ICs. The first attempt, namely "Heritage/1", was a 16 bit minicomputer and failed mostly because of wrong decisions made in terms of implementation. HER/12 inherits most of the ideas developed for the former, but in a more realistic manner.

The HER/12 project started by the end of January 2018 and this web site has been used since then as a systematic documentation tool. The site constitutes a complete manual of the machine, written ahead of time; such a practice has proven right as it helped me to organize ideas and mature the design long before implementation.

Once the architecture was defined well enough, a functional simulator software (HERSIM) was written.
This proven useful too. Thanks to HERSIM, the computer design could be fine tuned in cold before getting into the expensive and time consuming task of building and testing the actual circuits.

Click to selectComputer Main Features

  • 12-bits architecture clocked at 12 MHz.

  • Console support for Single-Step execution, Halt-On-Fail and Halt-On-Break-Point

  • One Accumulator (A), three aux registers (B, C, D) and one Index register (IX).

  • ALU supports 2's complement and BCD arithmetic.

  • Fast Ram (35ns) organized in 4K by 12-bit words.

  • Memory space broken into 32 pages (128-words each) for direct addressing.

  • Page-Zero direct addressing

  • Indirect addressing via any of the registers: A,B,C,D,IX,PC,SP.

  • Average instruction completes (fetch + exec) in 3 clock periods (250 ns), equivalent to 4 million instructions per second.

  • Instruction decoding is made with hard-wired logic (no micro-programming)

  • I/O accessible via 16 channels (no external bus)

  • Memory expansion possible through RAM-DISK peripheral via I/O channel.



Click to selectDesign considerations
The HER/12 minicomputer assumes a historical retro-context located between mid 1960s and early 1970s and it is meant to be a minicomputer in the historical sense.

The following constraints have been established by design:

* Strict 12-bits architecture.
* Only SSI and MSI integrated circuits of the series 7400 must be employed.
* Not use of ROM memory chips of any kind.
* Peripherals connect to the CPU by the mean of "I/O Channels" rather than external bus.
* Construction must be agile and low cost.

The machine is designed to achieve high performance within these limitations by optimizing instruction encoding and execution for minimum clock cycles. Also, sufficient resources to programmers has been put in place, such as indirect addressing, one index register and hardware support for BCD arithmetic.

I have to admit that using fast static RAM semiconductor memory is historically incorrect. Memory were very slow in the 70s; the one employed by HER/12, in contrast, is as fast as the CPU electronics itself.

Click to selectProject Plan and current status
Being this a long-term effort, the project has been divided into manageable phases. Those are:

* Phase #1: Design of the architecture.
* Phase #2: Functional Simulation of the CPU in Software.
* Phase #3: Design and unit-testing of the electronics.
* Phase #4: Building of the physical CPU equipment.
* Phase #5: Peripherals.

The project is currently (feb 2018) in Phase #2.

Click to selectCredits
Author    : Armando Acosta
Location  : Miami, FL. USA
Background: Electrical Engineering
Email     : develacosta@gmail.com.

Comments and suggestions are welcome.
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