Page 185 - A Practical Guide from Design Planning to Manufacturing

158 Chapter Five

would never have fetched. There must be a way of discarding the improper
results produced by these instructions. All the performance enhancing
microarchitecture features work to improve performance while main-
taining software compatibility. The true out-of-order nature of the
processor must be concealed from the program, and the illusion of
in-order execution maintained. The most common way to accomplish
this is to use a reorder buffer (ROB).
A reorder buffer takes out-of-order instructions that have completed
execution and puts them back into the original program order. When
instructions are being fetched and are still in order each one is allocated
an entry in the ROB in the order the instructions are supposed to be exe-
cuted. After the instructions are executed out of order, the ROB is updated
to track which have been completed. The results produced by any instruc-
tion are guaranteed to be correct only if all earlier instructions have
completed successfully. Only at this point are the results committed and
the instruction truly finished. This is called retiring an instruction.
Before retirement any result produced by an instruction is specula-
tive. New instructions will use the latest speculative result, but the
most recent committed results must also be maintained in case the
speculative results must be discarded. Only when an instruction retires
are the committed results updated with that instruction’s result.
Figure 5-18 shows an example of the ROB and the RAT at three dif-
ferent moments in time as a set of three instructions is retired. The code

Reorder buffer Register alias table
Ready Physical Architectural
to retire destination destination Physical Architectural
register register
ADD R3, R2, R1 Oldest No R3 AX
BNZ label No - - R10 AX
SUB R6, R5, R4 R15 BX
Yes R6 BX

Retired
Yes R3 AX
Add completes R3 AX
Oldest No - -
execution R15 BX
Yes R6 BX

Branch completes Yes R3 AX
R3 AX
and was Yes - -
mispredicted R15 BX
Oldest Yes R6 BX X
Discarded
Figure 5-18 Instruction retirement.