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Memory Management Unit
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.
31
ROOT TABLE DESCRIPTOR (ROOT LEVEL)
31
POINTER TABLE DESCRIPTOR (POINTER LEVEL)
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
4K PAGE DESCRIPTOR (PAGE LEVEL)
31
8K PAGE DESCRIPTOR (PAGE LEVEL)
31
INDIRECT PAGE DESCRIPTOR (PAGE LEVEL)
4-12
POINTER TABLE ADDRESS
PAGE TABLE ADDRESS
Figure 4-10. Table Descriptor Formats
PHYSICAL ADDRESS
PHYSICAL ADDRESS
DESCRIPTOR ADDRESS
Figure 4-11. Page Descriptor Formats
M68060 USER'S MANUAL
9
8
7
X
X
9
8
7
X
X
12 11 10
9
8
7
UR G U1 U0
S
13 12 11 10
9
8
7
UR UR G U1 U0
S
7
6
5
4
3
2
1
0
X
X
X
U
W
UDT
6
5
4
3
2
1
0
X
X
X
U
W
UDT
6
5
4
3
2
1
0
CM
M
U
W
PDT
6
5
4
3
2
1
0
CM
M
U
W
PDT
6
5
4
3
2
1
0
PDT
MOTOROLA
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