Re: Unidentified subject!
Dave Lowry:
Could someone explain to me how instructions are sequenced? Why is
the nop *before* the +? I would expect the sequence to go:
+ nop nop nop <instruction fetch>
or:
--- --- --- + <instruction fetch>
So that nops are required *after* the +, or an instruction fetch to
allow for carry prop. Thanks.
The issue is setup time.
The MuP21 computes all arithmetic and logic results in parallel, all
the time. [After all, the hardware is just going to sit there
anyways, why not let it do its work?] The corresponding instructions
latch the proper result and (sometimes) shift the stack around.
The problem with + (and +n or whatever it's called -- the multiply
step) is that they are implemented using ripple carry. So, after
modifying the TOS, it sometimes takes a few extra nanoseconds for the
carry to propagate through this result of adding TOS and 2nd from top.
So, if you do + too soon, you may get a result where carry hasn't
completed.
In principle, you might be able to get away with + without carry
propagation time (if you knew that carry would only have to propagate
7 bits or less). A special case of this is -OR which is addition with
carry completely disabled (or, more properly, the processor grabs the
pre-carry result rather than the post-carry result).
Raul D. Miller
