IntelSoftwareDevelopersManual

52 preventing caching to prevent the l1 and l2 caches

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Unformatted text preview: cache of these processors. The P6 family processors do not provide these pins because the L2 cache in internal to the chip package. • 9.5.1. Precedence of Cache Controls (P6 Family Processor) In the P6 family processors, the cache control flags and MTRRs operate hierarchically for restricting caching. That is, if the CD flag is set, caching is prevented globally (see Table 9-4). If the CD flag is clear, either the PCD flags and/or the MTRRs can be used to restrict caching. If there is an overlap of page-level caching control and MTRR caching control, the mechanism that prevents caching has precedence. For example, if an MTRR makes a region of system memory uncachable, a PCD flag cannot be used to enable caching for a page in that region. The converse is also true; that is, if the PCD flag is set, an MTRR cannot be used to make a region of system memory cacheable. In cases where there is a overlap in the assignment of the write-back and write-through caching policies to a page and a region of memory, the write-through policy takes precedence. The writecombining policy (which can only be assigned through an MTRR) takes precedence over either write-through or write-back. Table 9-5 describes the mapping from MTRR memory types and page-level caching attributes to effective memory types, when normal caching is in effect (the CD and NW flags in control register CR0 are clear). Combinations that appear in gray are implementation-defined and may be implemented differently on future Intel Architecture processors. System designers are encouraged to avoid these implementation-defined combinations. When normal caching is in effect, the effective memory type is determined using the following rules: 1. If the PCD and PWT attributes for the page are both 0, then the effective memory type is identical to the MTRR-defined memory type. 2. If the PCD flag is set, then the effective memory type is UC. 3. If the PCD flag is clear and the PWT flag is set, the effective memory type is WT for the WB memory type and the MTRR-defined memory type for all other memory types. 4. Setting the PCD and PWT flags to opposite values is considered model-specific for the WP and WC memory types and architecturally-defined for the WB, WT, and UC memory types. 9-13 MEMORY CACHE CONTROL Table 9-5. Effective Memory Type Depending on MTRR, PCD, and PWT Settings MTRR Memory Type UC WC PCD Value X 0 0 1 1 WT 0 1 WP 0 0 1 1 WB 0 0 1 NOTE: This table assumes that the CD and NW flags in register CR0 are set to 0. The effective memory types in the grey areas are implementation defined and may be different in future Intel Architecture processors. PWT Value X 0 1 0 1 X X 0 1 0 1 0 1 X Effective Memory Type UC WC WC WC UC WT UC WP WP WC UC WB WT UC 9.5.2. Preventing Caching To prevent the L1 and L2 caches from performing caching operations after they have been enabled and have received cache fills, perform the following steps: 1. Enter the no-fill cache mode. (Set the CD flag in control register CR0 to 1 and the NW flag to 0. 2. Flush all caches using the WBINVD instruction. 3. Disable the MTRRs and set the default memory type to uncached or set all MTRRs for the uncached memory type. For more information, see the discussion of the TYPE field and the E flag in Section 9.12.2.1., “MTRRdefType Register”. The caches must be flushed when the CD flag is cleared to insure system memory coherency. If the caches are not flushed in step 2, cache hits on reads will still occur and data will be read from valid cache lines. 9-14 MEMORY CACHE CONTROL 9.6. CACHE MANAGEMENT INSTRUCTIONS The INVD and WBINVD instructions are used to invalidate the contents of the L1 and L2 caches. The INVD instruction invalidates all internal cache entries, then generates a specialfunction bus cycle that indicates that external caches also should be invalidated. The INVD instruction should be used with care. It does not force a write-back of modified cache lines; therefore, data stored in the caches and not written back to system memory will be lost. Unless there is a specific requirement or benefit to invalidating the caches without writing back the modified lines (such as, during testing or fault recovery where cache coherency with main memory is not a concern), software should use the WBINVD instruction. The WBINVD instruction first writes back any modified lines in all the internal caches, then invalidates the contents of both L1 and L2 caches. It ensures that cache coherency with main memory is maintained regardless of the write policy in effect (that is, write-through or writeback). Following this operation, the WBINVD instruction generates one (P6 family processors) or two (Pentium® and Intel486™ processors) special-function bus cycles to indicate to external cache controllers that write-back of modified data followed by invalidation of external caches should occur. 9.7. SELF-MODIFYING CODE A write to a memory location in a code segment that is currently cached in the processor causes the associated cache line (or lines) to be invalidated. This check is based on the physical address of...
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