Sun Microelectronics
264
UltraSPARC User’s Manual
•
Breaking the group and scheduling the ALU instruction with the next group.
Notice that this may not lengthen the critical path (in terms of number of
cycles executed) if the next group can accommodate this extra instruction
without adding any new group.
16.2.2.5 Impact of Instruction Alignment on PDU
There is one branch prediction entry for every two instructions in the I-Cache.
Each entry, consisting of a two-bit field, indicates if the branch is predicted taken
or not-taken (the state machine is described in Section 16.2.6 ). In addition to the
branch prediction field, there is a next field associated with every four instruc-
tions. The next field contains the index of the line and the associativity number
(or way) of the line that should be fetched next. For sequential code, the next field
points to the next line in the I-Cache. If a predicted taken branch is among the
four instructions, the next field contains the index of the target of the branch.
The following cases represent situations when the prediction bits and/or the next
field do not operate optimally:
1.
When the target of a branch is word 1 or word 3 of an I-Cache line
(Figure 16-2) and the fourth instruction to be fetched (instruction 4 and 6
respectively) is a branch, the branch prediction bits from the wrong pair of
instructions are used.
Figure 16-2
Odd Fetch to an I-Cache Line
2.
If a group of four instructions (instructions 0-3 or instructions 4-7) contains
two branches and can be entered at a different position than the beginning
of the group (other than instruction 0 and 4 respectively), the next field will
contain the update from the latest branch taken in this group of four
instructions, which may not be the one associated with the branch of
interest (Figure 16-3).
Figure 16-3
Next Field Aliasing Between Two Branches
0
1
2
3
4
5
6
7
Odd Fetches
Next Field
Branch
Branch
Entry Point
Entry Point
Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com