Revision as of 02:39, 20 January 2021

Merced was the first Itanium microarchitecture designed by Intel.

Architecture

• 10 stage pipeline
  • IPG: Get next instruction pointer
  • FET: Fetch from instruction cache
  • ROT: Instruction rotation, decoupling buffer
  • EXP: Instruction dispersal
  • REN: Register remapping
  • WLD: Word line decode
  • REG: Register file read
  • EXE: Execute
  • DET: Exception detection
  • WRB: Writeback
• Branch Predictor
  • Early zero bubble predictor using Target Address Registers controlled by the compiler
  • Two-level predictor with 4 bits of local history and a 512 entry prediction table
  • Indirect branches handled with 64 entry Multiway Branch Prediction Table
  • 64-entry Target Address Cache
  • 8 entry Return Address Stack
  • Branch predictor can resteer at ROT stage using the loop exit predictor, or compiler provided prediction hints for the third slot in a bundle.
  • Branch predictor can resteer at EXP stage for any branch
• 16K 4-way L1 Instruction Cache
• Fetch and Decode
  • Two bundles, each containing three instructions, fetched from the instruction cache every cycle
  • No decoder necessary
• Execution Engine
  • Instructions from bundles dispersed to issue ports
  • Scoreboarding resolves dependencies with compiler hints
  • FPU can be accessed from the integer side by floating point get and set instructions.
    • Transfer from FPU to integer side takes two clocks
    • Transfer from integer side to FPU takes 9 clocks
• Memory Subsystem
  • L1D has two cycle latency
  • Software can issue "advanced loads", which go into a Advanced Load Address Table that checks for conflicting stores. Software needs to check the ALAT before using the load result. If there's a conflict, a software handler has to reissue the conflicting load.
  • The FPU is directly fed by the dual-ported L2 cache, with 9 cycle load latency.