Digi NS9750 User Manual
Page 114
M e m o r y M a n a g e m e n t U n i t ( M M U )
9 0
N S 9 7 5 0 H a r d w a r e R e f e r e n c e
Figure 32: Second-level descriptor
A second-level descriptor defines a tiny, small, or large page descriptor, or is invalid:
A large page descriptor provides the base address of a 64 KB block of
memory.
A small page descriptor provides the base address of a 4 KB block of
memory.
A tiny page descriptor provides the base address of a 1 KB block of memory.
Coarse page tables provide base addresses for either small or large pages. Large page
descriptors must be repeated in 16 consecutive entries. Small page descriptors must
be repeated in each consecutive entry.
Fine page tables provide base addresses for large, small, or tiny pages. Large page
descriptors must be repeated in 64 consecutive entries. Small page descriptors must
be repeated in four consecutive entries. Tiny page descriptors must be repeated in
each consecutive entry.
Table 37 describes the second-level descriptor bit assignments.
31
16 15
12 11 10
9
8
0
1
2
3
4
5
0
0
0
0
1
1
1
1
C
B
AP0
Large page base address
Small page base address
Tiny page base address
7
6
C
C
B
B
AP0
AP
AP1
AP1
AP2
AP2
AP3
AP3
Fault
Large page
Small page
Tiny page
Bits
Large
Small
Tiny
Description
[31:16]
[31:12]
[31:10]
Form the corresponding bits of the physical address.
[15:12]
---
[9:6]
SHOULD BE ZERO
Table 37: Second-level descriptor bits