assembly - What is a branch taken stall? -


i'm analyzing simple assembly program winmips64 , in 1 moment program has branch taken stall, don't know why , type of stall is. have been searching on internet , found related "prediction stalls", didn't understand it.

in picture can see 1 of moment when branch taken stall produced.

thanks in advance :)

enter image description here

a branch taken stall stall when branch taken :-)

the core reason instruction prefetch, multiple instructions in stream processed in parallel. see pipeline picture

in simplest form, while instruction n in stream executing, next (n+1) decoding. machine executes 1 instruction ever clock on average, in reality instruction takes 2 clock (one clock decode, 1 clock execute). netburst architecture (pentium 4/d) notorious having deep pipelines. (and horrible penalties if mispredicted)

if branch (go different instruction next), instruction on target address hasn't decoded yet. therefore must decoded first, , execution of instruction takes 2 clocks. clock branch stall penalty.

more modern cpus try minimize these penalties guestimating possible branch targets possible , prefetch instructions speculatively. maximize chance branch prefetched calculate chance branch taken


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