[Info-vax] Y3K for PDP-11 Operating Systems

[G Cornelius]

Johnny Billquist wrote:
> On 2011-05-10 07.29, Bob Koehler wrote:
>>     I never did understand why the VAX C RTL did a LOCC first, instead
>>     of a MOVTUC with a straight-through translation table and null escape
>>     character.  I assumed MOVTUC was even slower.
> 
> 
> I'm pretty sure that on later machines, MOVTUC is an emulated
> instruction, so you don't want to use it unless you really want the
> special features. For the simple C string case, having that translation
> table is a small memory cost, but it also hurts performance.
> But from a purity point of view, yes, MOVTUC should also be able to do it.

References to a 1-1 MOVTUC table are basically wasted memory
accesses, and, in fact, will be mostly cache misses until a
significant proportion of the table has been loaded.  Not to
mention having flushed information out of cache, resulting in
likely cache misses later.  Also: these are per-byte cache misses,
where actual data fetches should load four, eight, or even more
soon to be used bytes into cache whenever a miss occurs.

George