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Unformatted text preview: uses the default memory type. 9.12.5. MTRR Initialization On a hardware reset, a P6 family processor clears the valid flags in the variable-range MTRRs and clears the E flag in the MTRRdefType register to disable all MTRRs. All other bits in the MTRRs are undefined. Prior to initializing the MTRRs, software (normally the system BIOS) must initialize all fixed-range and variable-range MTRR registers fields to 0. Software can then initialize the MTRRs according to the types of memory known to it, including memory on devices that it auto-configures. This initialization is expected to occur prior to booting the operating system. See Section 9.12.8., “Multiple-Processor Considerations”, for information on initializing MTRRs in multiple-processor systems. 9.12.6. Remapping Memory Types A system designer may re-map memory types to tune performance or because a future processor may not implement all memory types supported by the P6 family processors. The following rules support coherent memory-type re-mappings: 1. A memory type should not be mapped into another memory type that has a weaker memory ordering model. For example, the uncacheable type cannot be mapped into any other type, and the write-back, write-through, and write-protected types cannot be mapped into the weakly ordered write-combining type. 2. A memory type that does not delay writes should not be mapped into a memory type that does delay writes, because applications of such a memory type may rely on its writethrough behavior. Accordingly, the write-back type cannot be mapped into the writethrough type. 9-27 MEMORY CACHE CONTROL 3. A memory type that views write data as not necessarily stored and read back by a subsequent read, such as the write-protected type, can only be mapped to another type with the same behaviour (and there are no others for the P6 family processors) or to the uncacheable type. In many specific cases, a system designer can have additional information about how a memory type is used, allowing additional mappings. For example, write-through memory with no associated write side effects can be mapped into write-back memory. 9.12.7. MTRR Maintenance Programming Interface The operating system maintains the MTRRs after booting and sets up or changes the memory types for memory-mapped devices. The operating system should provide a driver and application programming interface (API) to access and set the MTRRs. The function calls MemTypeGet() and MemTypeSet() define this interface. MEMTYPEGET() FUNCTION The MemTypeGet() function returns the memory type of the physical memory range specified by the parameters base and size. The base address is the starting physical address and the size is the number of bytes for the memory range. The function automatically aligns the base address and size to 4-KByte boundaries. Pseudocode for the MemTypeGet() function is given in Example 9-2. Example 9-2. MemTypeGet() Pseudocode #define MIXED_TYPES -1 /* 0 < MIXED_TYPES || MIXED_TYPES > 256 */ IF CPU_FEATURES.MTRR /* processor supports MTRRs */ THEN Align BASE and SIZE to 4-KByte boundary; IF (BASE + SIZE) wrap 64-GByte address space THEN return INVALID; FI; IF MTRRdefType.E = 0 THEN return UC; FI; FirstType ← Get4KMemType (BASE); /* Obtains memory type for first 4-KByte range */ /* See Get4KMemType (4KByteRange) in Example 9-3 */ FOR each additional 4-KByte range specified in SIZE NextType ← Get4KMemType (4KByteRange); IF NextType ≠ FirstType THEN return MixedTypes; FI; ROF; return FirstType; 9-28 MEMORY CACHE CONTROL ELSE return UNSUPPORTED; FI; If the processor does not support MTRRs, the function returns UNSUPPORTED. If the MTRRs are not enabled, then the UC memory type is returned. If more than one memory type corresponds to the specified range, a status of MIXED_TYPES is returned. Otherwise, the memory type defined for the range (UC, WC, WT, WB, or WP) is returned. The pseudocode for the Get4KMemType() function in Example 9-3 obtains the memory type for a single 4-KByte range at a given physical address. The sample code determines whether an PHY_ADDRESS falls within a fixed range by comparing the address with the known fixed ranges: 0 to 7FFFFH (64-KByte regions), 80000H to BFFFFH (16-KByte regions), and C0000H to FFFFFH (4-KByte regions). If an address falls within one of these ranges, the appropriate bits within one of its MTRRs determine the memory type. Example 9-3. Get4KMemType() Pseudocode IF MTRRcap.FIX AND MTRRdefType.FE /* fixed registers enabled */ THEN IF PHY_ADDRESS is within a fixed range return MTRRfixed.Type; FI; FOR each variable-range MTRR in MTRRcap.VCNT IF MTRRphysMask.V = 0 THEN continue; FI; IF (PHY_ADDRESS AND MTRRphysMask.Mask) = (MTRRphysBase.Base AND MTRRphysMask.Mask) THEN return MTRRphysBase.Type; FI; ROF; return MTRRdefType.Type; MEMTYPESET() FUNCTION The MemTypeSet() function in Example 9-4 sets a MTRR for the physical memory range specified by the parameters base and size to the type specified by type. The base address and size are multiples...
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This note was uploaded on 06/07/2013 for the course ECE 1234 taught by Professor Kwhon during the Spring '10 term at University of California, Berkeley.

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