Modifying microcode

[Robert Armstrong]

On 02/06/18 15:17, allison via cctech wrote:
>
> It was my understanding from using the 730 that there was limited
> (really limited) microcode
> enough to load the WCS as the tu58 was a serial device (standard tu58)
> and the 730 had to
> unpack and stuff the WCS.� You need little to do that but far from even
> PDP11 instruction set.
> The Microcode was loaded was the "what made it a VAX stuff".
>
> Allison

  What you're describing is the 750, not the 730.  Unlike the 78x and 730/725, the 750 had no console front end processor.  The microcode did it all, including loading any additional microcode from TU58.  

  As others have said, the 730 had an 8085 CFE processor that loaded all the VAX microcode.  The main CPU had no ROMs and could do absolutely nothing until the CFE loaded the microcode.

  FWIW, the CFE in the 730 had a 2K ROM and (I think) about 8K of RAM.  The first thing the CFE did after power on, before it did anything with the VAX CPU, was to load the rest of the 8085 code from the TU58 into that CFE RAM.  The 2K of ROM had just enough code to type characters on the console and to talk to the TU58, and even the CFE couldn't really do much until the rest of its code was loaded from the TU58.

  Once that was done, the CFE command line interpreter executed a script (one of four possible ones, depending on the HW configuration) and that script had the CFE commands to load the VAX microcode.

  FWIW, when you first turn on the 730 it types "CONV11" on the console, followed by the rest of the startup dialog.  If you pay attention, there's a tiny pause between the "CON" and the "V11".  That's because the first part was typed by the 8085 ROM just to let you know that the CFE was alive, and the version number was actually typed by the RAM part of the CFE code after it was loaded from the tape.

Bob