Epson ARM.POWERED ARM720T - Manual (Page 107)

7: Memory Management Unit
ARM720T CORE CPU MANUAL EPSON 7-11
A level two descriptor defines a tiny, a small, or a large page descriptor, or is invalid:
• a large page descriptor provides the base address of a 64KB block of memory
• a small page descriptor provides the base address of a 4KB block of memory
• a tiny page descriptor provides the base address of a 1KB 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 and tiny page descriptors must be repeated in each consecutive entry.
Level two descriptor bit assignments are described in Table 7-7.
The two least significant bits of the level two descriptor indicate the descriptor type as shown
in Table 7-8.
Note:
Tiny pages do not support subpage permissions and therefore only have one set of
access permission bits.
Table 7-7 Level two descriptor bits
Bits
Description
Large Small Tiny
31:16 31:12 31:10 These bits form the corresponding bits of the
physical address
15:12 - 9:6 Should Be Zero
11:4 11:4 5:4 Access permission bits. Domain access control
on page 7-17 and Fault checking sequence on
page 7-19 show how to interpret the access
permission bits
3:2 3:2 3:2 These bits, C and B, indicate whether the area of
memory mapped by this page is treated as
cachable or noncachable, and bufferable or
nonbufferable. (The system is always
write-through.)
1:0 1:0 1:0 These bits indicate the page size and validity and
are interpreted as shown in Table 7-8
Table 7-8 Interpreting page table entry bits [1:0]
Value Meaning Description
0 0 Invalid Generates a page translation fault
0 1 Large page Indicates that this is a 64KB page
1 0 Small page Indicates that this is a 4KB page
1 1 Tiny page Indicates that this is a 1KB page