🚀 go-pugleaf

In article <3307@gmdzi.UUCP> strobl@gmdzi.UUCP (Wolfgang Strobl) writes:
>The Motorola 6800 processor had an undocumented instruction
>(code 0x9d), which transformed the cpu into a fast binary counter
>on the address bus. It was otherwise dead, not even a NMI could
>awake it. I got this information (and the mnemonic HCF, Halt and
>Catch Fire for the instruction) from an article in Byte:
>
>   G. Wheeler, "Undocumented M6800 instructions", Byte, 
>   December 1977, page 46-48
>
>The purpose of this instruction was perhaps some kind of 
>factory test. I don't think it could do any hardware damage.

Right you are.  It was more popularly called WTD (Walk The Dog),
and is unbelievably handy when debugging the address bus, memory
systems, and so on.  It generated READ cycles at incrementing addresses,
wrapping from $FFFF to $0000, and repeating forever until *RESET
was asserted.  It made it really easy to find things like stuck address
bits, open bus signals, incorrect memory decoding, and the like.

It's been almost 15 years, but if memory serves :-) there were two
kinds of Walk The Dog instructions.  One, I think $9C, started at
address 0.  The other, I think $9D, took a 16-bit operand which was
the address to start walking from.

Jumping into data, or into the operand of an instruction, quite
often would eventually cause a WTD instruction to be executed, and
then we were off and walking.

>
>Wolfgang Strobl
>#include <std.disclaimer.hpp>

-Eric

-- 
Eric Black	"Garbage in, Gospel out"
Atherton Technology, 1333 Bordeaux Dr., Sunnyvale, CA, 94089
  Email: er...@Atherton.COM     Voice: +1 408 734 9822