Digi NS9215, Hardware Reference Manual, Page: 126 / 517

W O R K I N G W I T H T H E C P U
TLB structure
126
Hardware Reference NS9215
Care must be taken if the translated address differs from the untranslated address,
because several instructions following the enabling of the MMU might have been
prefetched with MMU off (
VA=MVA=PA
). If this happens, enabling the MMU can be
considered as a branch with delayed execution. A similar situation occurs when the
MMU is disabled. Consider this code sequence:
MRC p15, 0, R1, c1, C0, 0 ; Read control register
ORR R1, #0x1 ; Set M bit
MCR p15, 0,R1,C1, C0,0 ; Write control register and enable MMU
Fetch Flat
Fetch Flat
Fetch Translated
Note:
Because the same register (R1: Control register) controls the enabling of
ICache, DCache, and the MMU, all three can be enabled using a single
MCR
instruction.
Disabling the
MMU
Clear bit 0 (the M bit) in the R1: Control register to disable the MMU.
Note:
If the MMU is enabled, then disabled, then subsequently re-enabled, the
contents of the TLB are preserved. If these are now invalid, the TLB must be
invalidated before re-enabling the MMU (see “R8:TLB Operations register” on
page 97).
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
T L B s t r u c t u r e
The MMU runs a single unified TLB used for both data accesses and instruction
fetches. The TLB is divided into two parts:
An eight-entry fully-associative part used exclusively for holding locked down
TLB entries.
A set-associative part for all other entries.
Whether an entry is placed in the set-associative part or lockdown part of the TLB
depends on the state of the TLB Lockdown register when the entry is written into
the TLB (see “R10:TLB Lockdown register” on page 101).
When an entry has been written into the lockdown part of the TLB, it can be
removed only by being overwritten explicitly or, when the MVA matches the locked
down entry, by an MVA-based TLB invalidate operation.
The structure of the set-associative part of the TLB does not form part of the
programmer’s model for the ARM926EJ-S processor. No assumptions must be made