Re: 6800 hidden opcodes?

On Tue, 14 Apr 1998 17:05:13 -0700, Clay Cowgill <ClayC@diamondmm.com> wrote:

>> 6502s have this opcode as well - and on some 6502s it really can
>> destroy
>> the CPU. (or so I've heard anyway...) It does something similar to the
>> 6800's - toggling bus lines - so it may be that this is true for the
>> 6800 as well.
>>
> From and old copy of "The New Hacker's Dictionary":
>
>HCF /H-C-F/ n. Mnemonic for 'Halt and Catch Fire', any of several
>undocumented and semi-mythical machine instructions with destructive
>side-effects, supposedly included for test purposes on several
>well-known architectures going as far back as the IBM 360.

When I was attending a local Jr College they had a IBM 360 clone (A Univac --
well actually the 360 was a Univac clone since Univac sued IBM for use of their
instruction set and won.)

There were these "SVC" or "Service Call" instructions that called parts of the
operating system/hardware.

There were 256 SVC. The assembler wouldn't allow certain codes to be assembled,
but of course nothing stop you from just typing them in, in hex.

I remember SVC 90 was pretty cool. It brought the system down *hard*. It
cleared all registers, cleared the drum, clear all virtual memory, and cleared
the core. It didn't even allow for the normal "Abnormal Termination" dumped
that allowed the sysop to step through and find out who crashed the system. It
also almost got me kick out of school. Those were two powerful bytes!

FWIW: The system operator didn't really have to step through the code to find
out what happened, I think the sequence of events was:

 Execute SVC 90
 System crashes *BAD*
 Zonn: "Fuck!"
 Lab telephone: *Ring*
 Zonn [picks up phone]: "Uh, Hey Rob what's going on?"
 Rob: "Something told me you were in the Lab, why don't you come down here, I
need to talk with you..."

That would have been fine except a friend of mine at the time had to show off
the SVC 90 on two more occasions, almost getting both of us thrown out of
school.

He was one of those "Psycho Nerds" who claimed: "We have the right to execute
any opcode we want! If it's available on the architecture then, damn it!, it's
ours to use!" The administration had no idea what he was talking about... I'm
still not sure I do...

-Zonn

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Received on Tue Apr 14 17:49:34 1998