Quite a bit of work has been going on behind the scenes on the FPGA based accelerator card, so it’s time for an update. TG68 is now running stable in 68020 mode at a clock frequency of 6x the motherboard clock (about 42 MHz), and an instruction cache has been implemented that can feed the core with 0 tick latency in the event of a cache hit. This has added up to some pretty impressive performance, with AIBB’s integer maths test running at more than double the speed of a 25 MHz 68040 A4000.

Being reasonably content with performance, and expecting a further increase on the final hardware anyway due to the use of a faster FPGA, we moved on to one of the other key features that this board will have at launch – support for booting from the on-board micro SD.

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The Amiga is still a popular platform with enthusiasts, with the vibrant add-on scene still seeing new accelerator cards being developed. Most of these are based on the long obsolete faster derivatives of the 68000, such as the 68030 and 68060. These are getting increasingly hard to find for a reasonable price.

For the last few weeks I’ve been tinkering with hooking an FPGA up to an A500+ in place of its 68000 CPU with a view to using the open-source TG68 core as a fast 68020. This core is already proven in projects such as Minimig and MISTer, so it seemed like a logical choice. Rather than going straight to a custom PCB I made the decision to use an existing FPGA development board that I had on hand, along with a simple level shifting interface put together on stripboard. This has proved to work perfectly well for testing, although the grounding did need to be beefed up with copper tape to get things working reliably. A proper PCB with an FPGA integrated will be designed in due course.

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It’s been a while since I last did any retro-computing stuff, but when someone contacted me recently asking for permission to use some of my VHDL in an open-source accelerator project for the Amiga 1200 my attention was grabbed.

The A1200 was the second Amiga I owned back in the 90s.  This is a Motorola 68EC020 based machine running at 14.2 MHz – twice the clock speed of the older Amigas.  As luck would have it I still had the A1200 in storage, and after replacing some capacitors and cleaning things up a bit it was back in full working order, so I agreed to help out…

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Spectrum +2A boot menu

I’ve been promising for years to put the source for my FPGA retro-computing bits up on GitHub.  Well I’ve finally gone and done it.

These are essentially just the 2011 releases that are already available for download from here, but with the full development history intact.  I know that others have been doing some excellent work based on both of these projects, both in terms of adding new features and porting to other boards.  If anyone feels like forking and patching in their changes I will gladly accept pull requests in order to keep things together.

You can clone the BBC Micro from here, and the Spectrum from here, and if you missed the accompanying write-ups and videos you can find them up the top under Retro FPGA.