What surprised me was that this entire walk is fully hardware-driven -- no microcode involvement at all. The state machine reads the page directory entry, reads the page table entry, checks permissions, and writes back the Accessed and Dirty bits, all autonomously. Since it's hardware-driven, it runs in parallel with the microcode and needs its own memory bus arbitration -- the paging unit must share the bus with both data accesses from the microcode and prefetch requests from the instruction queue.
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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.