For segments, the job is split between hardware and microcode. When LD_DESCRIPTOR fires its protection test at 5CD, the Test PLA OR's the A-bit (bit 8 of the descriptor high DWORD) into a temporary register called PROTUN. Then the microcode at PROT_TESTS_PASSED takes over and writes the modified value back to the GDT or LDT in memory:
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The problem gets worse in pipelines. When you chain multiple transforms — say, parse, transform, then serialize — each TransformStream has its own internal readable and writable buffers. If implementers follow the spec strictly, data cascades through these buffers in a push-oriented fashion: the source pushes to transform A, which pushes to transform B, which pushes to transform C, each accumulating data in intermediate buffers before the final consumer has even started pulling. With three transforms, you can have six internal buffers filling up simultaneously.
GC thrashing in SSR: Batched chunks (Uint8Array[]) amortize async overhead. Sync pipelines via Stream.pullSync() eliminate promise allocation entirely for CPU-bound workloads.