FPGA VAX update, now DIY TTL computers

[Tony Duell]

> > More seriously, I have a machine with 16K * 48bit words of memory that
> > can be written under program control, but which can't be read back by
> > said program. This memory has nothing to do with any peripheral
> > device.....
> 
> Is it, say, a microcode store, on maybe a PERQ of some sort?

Of course. The processor can write to the control store, it can execute 
the control store's contents as microinstructions, but it can't read it 
back as 48 bit words.

Of course at the hwardware level the control store is being read when the 
instructions are executed, the RAMs do have conventional data outputs, etc.

I should have made it more difficult by specifying I had a 24 bit machine 
wit said 16K*48 bit memory. Firstly some people would probably have 
assumed that there was significance in the fact that the memory width was 
twice the word size (there isn't, all classic-PERQs have 48 bit wide 
microcode), and secondly, most PERQs are 20 bit machines, the 24 bit one 
is moderately uncommon (to put it mildly)

-tony