Open-Source Turbo Card

Speeding the Electron

The Electron is in many ways an elegant machine, but it is not, even by the standards of the day, a particularly fast one. 

The 6502 CPU only achieves the full 2MHz clock when executing code from ROM. Code in RAM runs at a maximum of 1MHz, while in screen modes 0, 1 and 2, where the ULA steals memory cycles for large amounts of time, the 6502 slows to around 0.5MHz.

Back in the 80s several companies developed 'turbo cards' which, when fitted to the Elk, would permit code stored in the lower 8K of system RAM to execute a 2MHz, boosting performance. 

These cards were usually complex designs that could be difficult to install, and thus were not hugely popular back in the day. This means they are now rare and can be hard to find.

Some time ago I decided to try creating a turbo card that uses modern programmable logic and SMD components to keep the design as simple and cheap as possible.  The Open-Source Turbo Card (OSTC) is the result. It's a small, simple board that fits in place of the 6502 CPU. Most Electrons do not have a socketed CPU, so fitting an OSTC requires the CPU to be desoldered and a socket soldered to the motherboard. This kind of work can be done by any good electronics repair shop fairly easily.

When fitted the OSTC behaves like an old-school turbo card, with the exception that it speeds up the first 12K of RAM rather than 8K. A toggle switch can be fitted to enable or disable the OSTC.

So, how fast is the OSTC? The screens below show the ClockSP benchmark, which tests the speed of BASIC execution relative to a 2MHz BBC Model B, running on an Electron in screen mode 1 (320x256x4). A stock Electron is on the left, an OSTC fitted Elk on the right.

As can be seen, the Electron's performance jumps from 0.55MHz to 1.85MHz, a speed up of more than 3.3x.

Buying an OSTC

The OSTC is, as the name states, an open source project. All the files required to build your own can be found at the Github link below. I do not at present sell pre-built OSTCs, as I'm very busy with the ElkSD and MasterSD devices. However, the for sale section of the Startdot forum is a good place to look if you would like to purchase a pre-built OSTC.

Stardot forum

OSTC on Github