Back to BASICS

In March of 2017, I began to follow a project on Hackaday.io for a microcomputer constructed entirely out of common TTL integrated circuits.  TTL stands for transistor-transistor logic, and appeared first in the very late 1960s.  Several of the iconic minicomputers of the early 1970s were constructed from TTL integrated circuits – including the Data General Nova and the DEC PDP-11. Remarkably the descendants of this family of logic devices are still available today, using modern high speed CMOS process so that they are much faster yet consume a lot less power.  In real terms they are also a lot cheaper than they were –  with typical devices costing between $0.50 and $1.00.

The Gigatron TTL Microcomputer has evolved from the breadboard computer featured in Hackaday.  It was released as a DIY soldering kit last March and could play simple video games – typical of the late 1970s and early 1980s.  It has 64 colour graphics and sound similar to the Acorn Electron or Sinclair Spectrum which appeared at that time. However, all of these features are done completely without a microprocessor – all of the control logic, arithmetic and input – output is done using readily available TTL chips.

Gigatron uses just 35 TTL chips – and these ate simple logic functions such as counters, multiplexers, decoders, registers and adders. There is nothing used here that would not have been available back in 1975.  However Gigatron does take advantage of decades of memory evolution, and uses modern ROM and RAM chips- which would have been very expensive – but now available for just a couple of dollars each.

Gigatron produces a 64 colour video signal that can be displayed on any  VGA compatible monitor.

The Gigatron kit contains all the components you need to make a fully fledged computer – and sells for under $200.  Full details are available on the gigatron.io website. There is a User forum – linked from the Gigatron website.

In the last year,  the Gigatron has benefited from both hardware and software upgrades. Today I received by post the new V3 ROM which includes a BASIC interpreter and a small adaptor pcb, which allows you to use a PS/2 keyboard with the Gigatron.

Writing in BASIC takes me back about 40 years to when I first encountered microcomputers at high-school.  It’s an easy language to learn and opens up the architecture of the Gigatron for further exploration.

Whilst relatively simple in hardware terms, there is considerable sophistication in the software which allows it to perform the colour graphics and sound. Fundamentally it is an 8-bit Harvard architecture, executing native machine language out of ROM, but on top of this is built a 16-bit Von Neuman virtual machine – which executes code out of RAM – whilst seamlessly taking care of all of the critical video timings.

The virtual cpu – or vCPU has just 34 core instructions which allow 16-bit operations to be performed.  These operations execute in typically 14 to 28 clock cycles (6.25MHz clock) – but nevertheless are still about 5 times the performance of a 6502 or Z80 – which were the usual choice of microprocessor in the late 1970s and early 1980s.  Indeed the Gigatron is even faster than the first IBM PC of 1981 – although it cannot address as much memory.

There is scope to increase the clock speed of the Gigatron possibly to 16 or 20MHz- and I am currently doing some experimentation  – which will become the subject of a future post.

I am also intrigued with the possibility of developing a Forth-like language based on the 34 instructions of the vCPU. Like the old days of BASIC – the interpreted language would be self-hosted on the Gigatron – and not rely on modern PCs, python or sophisticated tools for its development.

 

 

 

 

 

 

 

 

About monsonite

mostly human
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