MERCURY127
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PhysMask and PhysBase are used together to determine whether a target physical-address falls within the specified address range. PhysMask is logically ANDed with PhysBase and separately ANDed with the upper 40 bits of the target physical-address. If the results of the two operations are identical, the target physical-address falls within the specified memory range. The pseudo-code for the operation is:
Код: MaskBase = PhysMask AND PhysBase
MaskTarget = PhysMask AND Target_Address[51:12]
IF MaskBase == MaskTarget
target address is in range
ELSE
target address is not in range |
Variable Range Size and Alignment. The size and alignment of variable memory-ranges (MTRRs) and I/O ranges (IORRs) are restricted as follows:
• The boundary on which a variable range is aligned must be equal to the range size. For example, a memory range of 16 Mbytes must be aligned on a 16-Mbyte boundary.
• The range size must be a power of 2 (2 n , 52 > n > 11), with a minimum allowable size of 4 Kbytes.
For example, 4 Mbytes and 8 Mbytes are allowable memory range sizes, but 6 Mbytes is not allowable.
PhysMask and PhysBase Values. Software can calculate the PhysMask value using the following procedure:
1. Subtract the memory-range physical base-address from the upper physical-address of the memory range.
2. Subtract the value calculated in Step 1 from the physical memory size.
3. Truncate the lower 12 bits of the result in Step 2 to create the PhysMask value to be loaded into the MTRRphysMaskn register. Truncation is performed by right-shifting the value 12 bits.
For example, assume a 32-Mbyte memory range is specified within the 52-bit physical address space, starting at address 200_0000h. The upper address of the range is 3FF_FFFFh. Following the process outlined above yields:
Код: 1. 3FF_FFFFh–200_0000h = 1FF_FFFFh
2. F_FFFF_FFFF_FFFF–1FF_FFFFh = F_FFFF_FE00_0000h
3. Right shift (F_FFFF_FE00_0000h) by 12 = FF_FFFF_E000h |
In this example, the 40-bit value loaded into the PhysMask field is FF_FFFF_E000h.
Software must also truncate the lower 12 bits of the physical base-address before loading it into the PhysBase field. In the example above, the 40-bit PhysBase field is 00_0000_2000h.
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