[HECnet] DU11 vs. DUV11

[Brian Schenkenberger, VAXman-]

Johnny Billquist <bqt at softjar.se> writes: 

{...snip...}
One of them is a simple optimization for the PDT-11. It's not strictly
neccesary, but apparently DEC thought the time gain was enough to make
it worth exploiting. Code looks like this: 

              .IF DF   L$$SI1 

              MOVB       @(R5)+,-(SP)       ;;; COPY CHARACTER FOR WORD MOVE             
MOV         (SP)+,(R4)           ;;;   (SAVES 85 USECS ON PDT-11) 

              .IFF       ; DF L$$SI1 

              MOVB       @(R5)+,(R4)         ;;; OUTPUT A CHARACTER 

              .ENDC     ; DF L$$SI1 

OK, I'll bite.   Why is moving a character in the deferred location in
R5 to the stack and then, from the stack to the address in R4 faster 
than just going from the deferred R5 location to the R4 address?

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