[Info-vax] OT: Halt and Catch Fire

[Chris Scheers]

glen herrmannsfeldt wrote:
> Neil Rieck <n.rieck at sympatico.ca> wrote:
>> On Tuesday, June 2, 2015 at 11:25:53 PM UTC-4, JF Mezei wrote:
>>> Yeah, I know , "HCF, Halt and Catch Fire"  was an IBM 370 instruction in
>>> many "hacked" university documents.
>  
> (snip) 
>> As others have already pointed out, there was no HCF instruction 
>> but I have been told by people who worked on those machines that 
>> you could not stop the CPU on some IBM machines: Halt really meant 
>> the CPU went into a tight NOP loop (which is were the catch-fire 
>> myth comes from).
> 
> You stop the CPU from executing instructions by setting the WAIT
> bit in the PSW.  360's were leased, with the charge depending on
> how much you used the system. There is a usage meter that runs when
> the CPU is running, that is, when the WAIT bit is not set. 
> 
> When there is nothing else to do, the CPU goes to a WAIT state, until
> the next interrupt. 
> 
> There are some error conditions, mostly during IPL when things
> aren't going well, called disabled WAIT states.  If you set WAIT
> and disable all interrupts, then the system stops.

IIRC, the HCF instruction was a joke in the IBM world.  Along with such 
things as "Execute Operator" and the even more desirable "Execute 
Programmer".  <grin>

The 6800 microprocessor did have an undocumented instruction that 
basically turned the address bus into a 16 bit counter.  This has since 
been referred to as HCF.

-- 
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Chris Scheers, Applied Synergy, Inc.

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