Booting an IBM MP 3000 S/390 System

Al Kossow aek at
Thu Aug 6 09:33:38 CDT 2015

On 8/6/15 6:16 AM, geneb wrote:
> One thing I don't understand - why can't the machine boot on its own?
> Why would IBM design a computer that required another computer just to
> boot it?

Main processor microcode is in RAM. Putting microcode in ram and having 
a small computer load it was actually pretty common in the 70's and 80's 
in larger systems since then you didn't have to manage the hassle of 
patching microcode in ROM.

Apple ended up putting a small TI microcontroller in the G5 because it 
also couldn't boot on its own. There was a bunch of volatile state you
had to set up before it would fetch its first instruction.

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