
2-54
PRELIMINARY
Memory Addressing Methods
Advanci ng the S tandar ds
Table 2-27.
Directory and Page Table Entry (DTE and PTE) Bit Definitions
BIT POSITION
FIELD NAME
DESCRIPTION
31-12
BASE
ADDRESS
Specifies the base address of the page or page table.
11-9
--
Undefined and available to the programmer.
8-7
--
Reserved and not available to the programmer.
6
D
Dirty Bit. If set, indicates that a write access has occurred to the page (PTE only,
undefined in DTE).
5
A
Accessed Flag. If set, indicates that a read access or write access has occurred
to the page.
4
PCD
Page Caching Disable Flag. If set, indicates that the page is not cacheable in
the on-chip cache.
3
PWT
Page Write-Through Flag. If set, indicates that writes to the page or page tables
that hit in the on-chip cache must update both the cache and external memory.
2
U/S
User/Supervisor Attribute. If set (user), page is accessible at privilege level 3. If
clear (supervisor), page is accessible only when CPL
≤ 2.
1
W/R
Write/Read Attribute. If set (write), page is writable. If clear (read), page is
read only.
0
P
Present Flag. If set, indicates that the page is present in RAM memory, and
validates the remaining DTE/PTE bits. If clear, indicates that the page is not
present in memory and the remaining DTE/PTE bits can be used by the
programmer.
For a TLB hit, the TLB eliminates accesses to
external directory and page tables.
The L1 TLB is a small cache optimized for speed
whereas the L2 TLB is a much larger cache opti-
mized for capacity. The L2 TLB is a proper
superset of the L1 TLB.
The TLB must be flushed by the software when
entries in the page tables are changed. Both the
L1 and L2 TLBs are flushed whenever the CR3
register is loaded. A particular page can be
flushed from the TLBs by using the INVLPG
instruction.
2.12.4.1 Translation Lookaside
Buffer Testing
The L1 and L2 Translation Lookaside Buffers
(TLBs) can be tested by writing, then reading
from the same TLB location. The operation to be
performed is determined by the command
(CMD) field (Table 2-28, Page 2-54) in the TR6
register.
Table 2-28. CMD Field
CMD
OPERATION
LINEAR
ADDRESS BITS
x00
Write to L1
15 - 12
x01
Write to L2
17 - 12
010
Read from L1 X port
15 -12
011
Read from L2 X port
17 -12
110
Read from L1 Y port
15 - 12
110
Read from L2 Y port
17 - 12