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User’s Manual
IBM PowerPC 750GX and 750GL RISC Microprocessor
Programming Model
Page 68 of 377
gx_02.fm.(1.2)
March 27, 2006
22 SPD
Speculative cache access disable
0 Speculative bus accesses to nonguarded space (G = 0) from both the instruction
and data caches are enabled.
1 Speculative bus accesses to nonguarded space in both caches are disabled.
23 IFEM
Enable M bit on bus for instruction fetches.
0 M bit disabled. Instruction fetches are treated as nonglobal on the bus.
1 Instruction fetches reflect the M bit from the WIM settings.
24 SGE
Store gathering enable
0 Store gathering is disabled.
1 Integer store gathering is performed for write-through to nonguarded space or for
cache-inhibited stores to nonguarded space for 4-byte, word-aligned stores. The
load store unit (LSU) combines stores to form a double word that is sent out on
the 60x bus as a single-beat operation. Stores are gathered only if successive,
eligible stores are queued and pending. Store gathering is performed regardless
of address order or endian mode. The store-gathering feature is enabled by set-
ting the HID0[SGE] bit (bit 24).
25 DCFA
Data-cache flush assist. (Force data cache to ignore invalid sets on miss replacement
selection.)
0 The data-cache flush assist facility is disabled.
1 The miss replacement algorithm ignores invalid entries and follows the replace-
ment sequence defined by the PLRU bits. This reduces the series of uniquely
addressed load or Data Cache Block Zero (dcbz) instructions to eight per set.
The bit should be set just before beginning a cache flush routine, and should be
cleared when the series of instructions completes.
26 BTIC
Branch target instruction-cache enable—used to enable use of the 64-entry branch
instruction cache.
0 The BTIC is disabled, the contents are invalidated, and the BTIC behaves as if it
were empty. New entries cannot be added until the BTIC is enabled.
1 The BTIC is enabled, and new entries can be added.
27 — Not used. Defined as FBIOB on earlier 603-type processors.
28 ABE
Address broadcast enable—controls whether certain address-only operations (such as
cache operations, Enforce In-Order Execution of I/O [eieio], and Synchronization [sync])
are broadcast on the 60x bus.
0 Address-only operations affect only local L1 and L2 caches and are not broad-
cast.
1 Address-only operations are broadcast on the 60x bus. Affected instructions are
eieio, sync, Data Cache Block Invalidate (dcbi), Data Cache Block Flush (dcbf),
and Data Cache Block Store (dcbst). A sync instruction completes only after a
successful broadcast. Execution of eieio causes a broadcast that can be used to
prevent any external devices, such as a bus bridge chip, from store gathering.
Note: A Data Cache Block Set to Zero (dcbz) instruction (with M = 1, coherency
required) always broadcasts on the 60x bus regardless of the setting of this bit. An
Instruction Cache Block Invalidate (icbi) is never broadcast. No cache operations, except
dcbz, are snooped by the 750GX regardless of whether the ABE is set. Bus activity
caused by these instructions results directly from performing the operation on the 750GX
cache.
Bits Field Name Description
1. For additional information, see Section 11.9, Checkstops, on page 361.
2. For additional information about power-saving modes, see
Table 10-2, HID0 Power Saving Mode Bit Settings, on page 337.