For implementers, backpressure adds complexity without providing guarantees. The machinery to track queue sizes, compute desiredSize, and invoke pull() at the right times must all be implemented correctly. However, since these signals are advisory, all that work doesn't actually prevent the problems backpressure is supposed to solve.
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Three microcode cycles for the writeback alone. That's acceptable because segment loads are already expensive multi-cycle operations, and the designers likely expected them to be infrequent -- most programs load their segments once at startup and never touch them again. Page translations happen on every memory access, so the same approach would be ruinous. Hence the fully autonomous hardware walker.