🚀 go-pugleaf

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	"The bottom line for an I/O instruction is that it represents a
	synchronization point from the perspective of the hardware.
	That is, all unconfirmed operations have to be verified before
	the I/O operation can take place."

This is a sufficient but not a necessary condition.  It's more
restrictive than it needs to be.

By way of counter-example:  the M88k has a lot of pipelining, and some
of the floating-point exceptions are imprecise.  I might very well have
code like this:

	fmul.sss	r2,r3,r4	; start a floating multiply
	ld		r5,r6,0		; start an I/O read

and, while the load is underway, the FPU decides to fire off a floating
underflow exception.  That's fine, I expected this and my floating
exception handler substitutes zero, cleans out the pipes, and returns.
I don't need to idle for four cycles waiting for the multiply to finish
just on the rare chance that it will underflow.

If for some reason I want synchronization, I'll use the provided
instructions to suspend execution until all the pipelines have drained.
But I don't need to do this on every I/O op.

  -=- Andrew Klossner   (uunet!tektronix!frip.WV.TEK!andrew)    [UUCP]
                        (andrew%frip.wv.tek.com@relay.cs.net)   [ARPA]