Nehalem

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Nehalem microarchitecture

• Reintroduction of SMT (HyperThreading)
• Store-forwarding aliasing issue on 4k strides.
• Double-pumped FP SSE + integer SSE/x87 + load + store units
• Fetch up to 16 bytes of aligned instructions from cache per cycle.
• Up to 4 instructions, no more than 1 complex (this does not necessarily mean 1 µop), decoded per cycle. 64-bit macro-fusion
• Instructions with more than 4 µops are fed from MSROM, and will take more than one cycle in the Instruction Decoder Queue.
• Forwarding results between integer, integer SIMD, and FP units adds latency compared to forwards within the domain.
• One register may be written per cycle.
• 48 load buffers, 32 store buffers, 10 fill buffers.
• 36 reservation stations (up from 32), 128 ROB entries (up from 96).
• Calltrace cache of 16 entries.
• 2-way loop end BTB for every 16 bytes, 4-way general BTB.
• Loop Stream Detector replays from IDQ if the loop consists of:
  • 4 16-byte icache fetches or less
  • 28 total µops or less
  • 4 taken branches or less, none of them a RET
  • preferably more than 64 iterations?
• Be sure to use register parameter-passing conventions, not the stack, to avoid stalls on store-forward of high-latency floating point stores.
• Peak issue rate of 1 128-bit load and 1 128-bit store per cycle.