Compaq EV68A Load Instructions to R31 and F31, Prefetch with Modify Intent LDS Instruction

Load Instructions to R31 and F31

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.

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.

21264/EV68A Hardware Reference Manual Internal Architecture 2–23