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2–24 Internal Architecture
21264/EV68A Hardware Reference Manual
Special Cases of Alpha Instruction Execution
2.6.3 Prefetch, Evict Next: LDQ and HW_LDQ Instructions
The 21264/EV68A processes this instruction like a normal prefetch transaction (Read-
BlkSpec command), with one exception—if the load misses the Dcache, the addressed
cache block is allocated into the Dcache, but the Dcache set allocation pointer is left
pointing to this block. The next miss to the same Dcache line will evict the block. For
example, this instruction might be used when software is reading an array that is known
to fit in the offchip Bcache, but will not fit into the onchip Dcache. In this case, the
instruction ensures that the hardware provides the desired prefetch function without dis-
placing useful cache blocks stored in the other set within the Dcache.
The HW_LDQ instruction construct equates to the HW_LD instruction with the LEN
field set. See Table 6–3.
2.7 Special Cases of Alpha Instruction Execution
This section describes the mechanisms that the 21264/EV68A uses to process irregular
instructions in the Alpha instruction set, and cases in which the 21264/EV68A pro-
cesses instructions in a non-intuitive way.
2.7.1 Load Hit Speculation
The latency of integer load instructions that hit in the Dcache is three cycles. Figure 2–
9 shows the pipeline timing for these integer load instructions. In Figure 2–9:
Figure 2–9 Pipeline Timing for Integer Load Instructions
There are two cycles in which the IQ may speculatively issue instructions that use load
data before Dcache hit information is known. Any instructions that are issued by the IQ
within this 2-cycle speculative window are kept in the IQ with their requests inhibited
until the load instruction’s hit condition is known, even if they are not dependent on the
load operation. If the load instruction hits, then these instructions are removed from the
queue. If the load instruction misses, then the execution of these instructions is aborted
and the instructions are allowed to request service again.
Symbol Meaning
Q Issue queue
R Register file read
EExecute
D Dcache access
BDatabusactive
1Cycle Number
ILD
Instruction 1
Instruction 2
2 3 4 5 6 7 8
QREDB
QR
Q
Hit
FM-05814.AI4