Tlb Operation - Freescale Semiconductor MPC850 User Manual

Data/Instruction Fetch
Compare address
TLB reload (read page
description from external
memory to TLB)

8.3.3 TLB Operation

Each TLB contains pointers to pages in physical memory where data is indexed by the EPN.
TLBs entries can have different page sizes. The entry page size determines which EA bits
are compared and how many of its lsbs pass untranslated as physical address bits.
For a 4-Kbyte page, four subpage validity flags are supported, allowing any combination of
1-Kbyte subpages to be mapped. For any other page size, all of these flags should have the
same value. Programming non-4-Kbyte pages with different valid bits is a programming
error. Subpage validity flags can be manipulated to implement 1–4 Kbyte pages or any other
combination of 1-Kbyte subpages. However, all subpages of an effective page frame must
map to the same physical page. During translation, the EA, the privilege level (MSR[PR]),
and CASID are provided to the TLB, as shown in Figure 8-3. In the TLB, the EA and
CASID are compared with each entry's EPN and ASID. The CASID is compared only
when the matching entry is programmed as unshared. See Table 8-12 and Table 8-13.
32-bit EA is generated
with TLB (0 clock penalty)
entries
TLB
Yes
Hit
?
No
(20–23 clock penalty
@ one wait-state
external memory)
Use page description from TLB
Figure 8-2. Flow of Load/Store Access
Chapter 8. Memory Management Unit
Is page
No
valid
?
Yes
Access permitted
No
by page protection
?
Yes
Address Translation
TLB error exception