On Mon, 17 Feb 2014 20:05:12 -0500, Gerhard Postpischil wrote: >On 2/17/2014 7:34 PM, Bernd Oppolzer wrote: >> My question is: if we had such an instruction, how would this fit into the >> overall machine concept? And: are there some performance benefits, >> or are there some problems with this approach, which I do not see? >> >> I'm sure, that some historical machines had such concepts ... > >The IBM 704/709/709x machines had several SKIP instruction types >(Compare[one or two skips], Compare Logical[1 or 2 skips], Test Sense >Switch); using the latter I could create a print string (0/1 for 6 >switches) with fourteen instructions. However, all instructions were the >same length, which avoided problems. On zOS machines you'd need to >specify the skip amount to make them usable in general, or use macros? > The Data General Nova had no condition code register; rather every RR instruction had a condition mask selecting the condition on which the next instruction would be skipped or not. Also, a bit which suppressed loading the target register: "Don't load; just test." One might code a fairly long sequence of interleaved RR instructions beginning with a conditional skip and followed by any number of unconditional skips until the paths finally merged at the bottom.
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