SATA, PATA, or anything else: if it has to cross the PCI bus, a simple readX()/writeX() can stall the CPU for the equivalent of hundreds of instructions. I agree with Jeff, it is always worth even moderately complex logic to avoid I/O.
Note that an isolated write{bwl}() *may* be almost free in most
cases, due to write buffers between the CPU and the bus.
But those buffers are of limited depth (typically 3/4 entries),
and a stall there often causes a 0.5us (or more) delay.When measuring PATA hardware, I found the delay was often between 600ns and 1200ns (0.6us to 1.2us), per readX()/writeX(). With SATA, it will likely be around 11 PCI clocks, or say 363ns on most current platforms.
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