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Briel Altair 8800micro Revisited
As some of you will know, I built a Briel Altair 8800micro which recreates the experience of the original Altair 8800 on modern components. The kit was fin to build, and very functional, with 32K RAM and an SD Card for loading and saving programs. The only downside was that it couldn't run CP/M as CP/M requires 64K RAM and some mass storage such as floppy disks.
A couple of months ago Vince Briel - the creator of this and other retro-styled computer kits, announced the availability of a limited run of 88-DSK RAM drive upgrades for beta testing. These add another 32K RAM and two battery backed RAM drives that emulate the 88-DSK floppy controller and drives. I missed the initial run of 20 kits, but got in on the second run.
The kit arrived promptly, consisting of a PCB, a handful of chips and passive components and a battery pack. I got stuck into it yesterday, and completed assembly up to the point of inserting the chips in about an hour.
After checking the board for shorts and bad joints I plugged it in and fired it up on the bench.
Running Altair (Microsoft) 8K BASIC reported 59206 bytes free memory, so the RAM upgrade was at least visible.
Loading disk images into the RAM drives off SD card is slow - it all comes through the serial terminal at fairly modest speed - a full disk image is 510K and takes 5 minutes or so to load. Once loaded you can boot from floppy by loading the bootloader binary - something that would have been toggled in or loaded from paper tape on the original. The terminal provides a way of auto-loading and arbitrary binary, so this is ideal for the boot loader. Before I knew it, I had CP/M 2.2 running :)
Initially I had problems with the RAM drives being wiped or corrupted every time I cycled power, but this was soon resolved through discovery of the small "On-Off" switch on the battery pack :)
And then magic happened...