The AlphaStation 500 is a workstation from Digital, circa 1996. Mine is a 500 MHz model and has an Alpha 21164A processor (aka EV56). And the way it boots is weird.

On your common-or-garden PC, there has always been some kind of ROM chip. It holds a piece of firmware known as the BIOS. This ROM chip is available at a well-known location in the processor’s address space (remembering that any PC processor boots up in 16-bit, 8088 compatible mode, with a 1 MiB address space, just like an IBM PC 5150) and the processor just starts executing code in it after reset.

The Alpha (or at least this AlphaStation 500 - although I think they mostly worked like this) is different.

There is a serial ROM (SROM) on the main board, and after reset, some logic internal to the CPU kicks in to generate a clock pulse. This drives an (external) counter, which provides consecutive addresses into the ROM. The 1-bit output of the ROM is sent into the processor, in sync with the clock pulse. The Alpha processor stores this data in its internal Instruction Cache. Once the code load is complete, the processor then executes the code, from its cache. The code is just enough to get the processor to configure its external memory bus, and find a memory-mapped parallel ROM, which it will then hand-over execution to.

But wait, how do you find a 1-bit wide ROM chip? Aren’t most ROM chips at least 8 bits wide? Yes, they are.

Well that’s certainly an interesting take on booting. The rest of the post is about trying to decipher an SROM dump in order to work out why a system might not be booting. It even includes some DEC Alpha assembly—that’s not something you see everyday.