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21264/EV68A Hardware Reference Manual
Internal Architecture 2–23
Load Instructions to R31 and F31
2.6 Load Instructions to R31 and F31
This section describes how the 21264/EV68A processes software-directed prefetch
transactions and load instructions with a destination of R31 and F31.
Prefetches allocate a MAF entry. How the MAF entry is allocated is what distinguishes
the type of prefetch. A normal prefetch is equivalent to a normal load MAF (that is, a
MAF entry that puts the block into the Dcache in a readable state). A prefetch with
modify intent is equivalent to a normal store MAF (that is, a MAF entry that puts the
block into the Dcache in a writeable state). A prefetch, evict next, is equivalent to a nor-
mal load MAF, with the additional behavior described in Section 2.6.3.
A prefetch is not performed if the prefetch hits in the Dcache (as if it were a normal
load).
Load operations to R31 and F31 may generate exceptions. These exceptions must be
dismissed by PALcode.
The following sections describe the operational prefetch behavior of these instructions.
2.6.1 Normal Prefetch: LDBU, LDF, LDG, LDL, LDT, LDWU, HW_LDL Instructions
The 21264/EV68A processes these instructions as normal cache line prefetches. If the
load instruction hits the Dcache, the instruction is dismissed, otherwise the addressed
cache block is allocated into the Dcache.
The HW_LDL instruction construct equates to the HW_LD instruction with the LEN
field clear. See Table 6–3.
2.6.2 Prefetch with Modify Intent: LDS Instruction
The 21264/EV68A processes an LDS instruction, with F31 as the destination, as a
prefetch with modify intent transaction (ReadBlkMod command). If the transaction hits
a dirty Dcache block, the instruction is dismissed. Otherwise, the addressed cache block
is allocated into the Dcache for write access, with its dirty and modified bits set.
Table 2–6 Instructions Retired Without Execution
Instruction Type Notes
INTA, INTL, INTM, INTS All with R31 as destination.
FLTI, FLTL, FLTV All with F31 as destination. MT_FPCR is not included
because it has no destination—it is never removed from the
pipeline.
LDQ_U All with R31 as destination.
MISC TRAPB and EXCB are always removed. Others are never
removed.
FLTS All (SQRT, ITOF) with F31 as destination.