Emulating The AT&T 3B2 Computer

On November 29, 2014, I posted the following message to the Classic Computer Mailing List.

Folks,

For some reason I got it in my head that writing an AT&T 3B2
emulator might be a good idea. That idea has pretty much
been derailed by lack of documentation.

I have been unable to find any detailed technical description
of any 3B2 systems. Visual inspection of a 3B2 300 main board
reveals the following major components:

  - WE32100 CPU
  - WE32101 MMU
  - 4 x D2764A EPROM
  - TMS2797NL floppy disk controller
  - PD7261A hard disk controller
  - SCN2681A dual UART
  - AM9517A Multimode DMA Controller

How these are addressed is anybody's guess.

To even dream of doing an emulator, I at least need to know
the system memory map -- what physical addresses these devices
map to. Without that it's pretty pointless to get started.

If anyone has access to this kind of information, please drop
me a line. Otherwise, I'll just put this one on the far-back
burner!

-Seth

When I sent that message, I had no idea that it would lead to almost four years of effort, and perhaps if I had known I would have given up. But thankfully I didn’t, and today I’m happy to say that the effort was, at long last, successful. The 3B2/400 emulator works well enough that I have released it to the world and rolled it back into the parent SIMH source tree.

Because this project required so much reverse engineering, and because documentation about the 3B2 is still so scarce and hard to come by, I wanted to take the time to document how the emulator came about.

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