The reorder buffer and issue queue are two different structures. The
reorder buffer holds onto instructions that are in flight in order to
maintain the ordering of all instructions. This allows for precise
interrupts and recovery to specific instructions. The issue queue holds
instructions that are waiting to be issued, and is used to schedule
ready instructions for execution given the functional units available.
Although the 21264 HRM doesn't seem to make a specific mention of the
ROB, it's implied that it's 80 entries large (section 2.1.1.5). The
issue queue is only 20 entries for integers and 15 entries for floating
point instructions. A quick search on the web seems to confirm these
numbers.
Kevin
James Srinivasan wrote:
Please can someone explain what the reorder buffer size (rob_size)
parameter refers to in the encumbered CPU model? I've been reading over
the Alpha 21264 docs with few mentions of a reorder buffer but quite a
few mentions of an issue queue which sounds similar but not identical,
Thanks,
James
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