🚀 go-pugleaf

Instruction restart on a CISC can cause grevious problems when there
are side effects to reading or writing certain addresses.  Consider,
for instance, the following instruction which reads a device register
and stores it into memory.  (This is 68xxx code.)

	movb	a0@(2),_memloc

This instruction could fault after the read of the device register
if either _memloc or the tail end of the instruction isn't resident.
Many devices will clear interrupt bits when you read their status
register or shift in the next byte when you read their data
register, so repeating the entire instruction doesn't give equivalent
results.  One could code this (in C) as

	register  char temp;

	temp = dev->reg;
	memloc = temp;

and hope the compiler doesn't optimize too agressively, or maybe
(someday) use "volatile" to indicate to the compiler that dev->reg
is special, but I wouldn't bet any current compilers take instruction
restart into account when generating code for volatile data fetches.

For the moment this isn't a major problem, since Unix device drivers
usually run in physical memory (so any fault that occurs is fatal,
and won't be rerun), but I long for the day when this isn't necessarily
so.