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Memory Management Unit
4-12
M68060 USER’S MANUAL
MOTOROLA
have an encoding of U-bit = 0, M-bit = 1, and PDT field = 01 or 11. This encoding indicates
that the page descriptor is resident, not used, and modified. The processor’s table search
algorithm never leaves a descriptor in this state. This state is possible through direct manip-
ulation by the operating system for this specific instance.
4.2.2 Descriptors
There are three types of descriptors used in the translation tables, root, pointer, and page.
Root table descriptors are used in root-level tables and pointer table descriptors are used in
pointer-level tables. Descriptors in the page-level tables contain either a page descriptor for
the translation or an indirect descriptor that points to a memory location containing the page
descriptor. The P-bit in the TCR selects the page size as either 4 or 8 Kbytes.
4.2.2.1 TABLE DESCRIPTORS.
Figure 4-10 illustrates the formats of the root and pointer
table descriptors.
4.2.2.2 PAGE DESCRIPTORS.
Figure 4-11 illustrates the page descriptors for both
4-Kbyte and 8-Kbyte page sizes. Refer to
Section 5 Caches
for details concerning caching
page descriptors.
31 9876543210
POINTER TABLE ADDRESS XXXXXUW UDT
ROOT TABLE DESCRIPTOR (ROOT LEVEL)
31 9876543210
PAGE TABLE ADDRESS XXXXXUW UDT
POINTER TABLE DESCRIPTOR (POINTER LEVEL)
Figure 4-10. Table Descriptor Formats
31 1211109876543210
PHYSICAL ADDRESS UR G U1 U0 S CM M U W PDT
4K PAGE DESCRIPTOR (PAGE LEVEL)
31 131211109876543210
PHYSICAL ADDRESS UR UR G U1 U0 S CM M U W PDT
8K PAGE DESCRIPTOR (PAGE LEVEL)
31 76543210
DESCRIPTOR ADDRESS PDT
INDIRECT PAGE DESCRIPTOR (PAGE LEVEL)
Figure 4-11. Page Descriptor Formats
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