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Memory Management Unit (MMU)
4-14
Freescale Semiconductor
4.3.1.2.9
Changes to ACRs and CACR
New ACR and CACR bits improve address granularity and supervisor mode protection and memory
functionality for physical- and virtual- address environments.
4.3.1.2.10
ACR Address Improvements
The ACR registers provide a 16-Mbyte address window. For a given request address, if the ACR is valid
and the request mode matches the mode specified in the supervisor mode field (ACR
n
[S]), hit
determination is specified as:
ACRx_Hit = 0;
if ((address[31:24] and ~ACR
n
[23:16]) == (ACR
n
[31:24] and ~ACR
n
[23:16]))
ACRx_Hit = 1;
With this hit function, ACRs can assign address attributes for user or supervisor requests to memory spaces
of at least 16 Mbytes (through the address mask). With the MMU definition, the ACR hit function is
improved by the address mask mode bit (ACR
n
[AMM]), which supports finer address granularity. See
.
The revised hit determination becomes:
ACRx_Hit = 0;
if (ACR
n
[10] == 1)
if ((address[31–24] == ACR
n
[31–24])) &&
((address[23–20] and ~ACR
n
[19–16]) == (ACR
n
[23–20] and ~ACR
n
[19–16])))
ACRx_Hit = 1;
else if (address[31–24] and ~ACR
n
[23–16]) == (ACR
n
[31–24] and ~ACR
n
[23–16]))
ACRx_Hit = 1;
Table 4-10. New ACR and CACR Bits
Field
Description
ACR
n
[10]
AMM
Address mask mode. Determines access to the associated address space.
0 The ACR hit function is the same as previous versions, allowing control of a 16-Mbyte or greater memory
region.
1 The upper 8 bits of the address and ACR are compared without a mask function; bits 23–20 of the address
and ACR are compared masked by ACR[19–16], allowing control of a 1- to 16-Mbyte region.
Reset value is 0.
ACR
n
[3]
SP
Supervisor protect. Determines access to the associated address space.
0 Supervisor and user access allowed.
1 Only supervisor access allowed. Attempted user access causes an access error exception.
Reset value is 0.
CACR[23]
DDSP
Default data supervisor protect. Determines access to the associated data space.
0 Supervisor and user access allowed.
1 Only supervisor access allowed. Attempted user access causes an access error exception.
Reset value is 0.
CACR[7]
DISP
Default instruction supervisor protect. Determines access to the associated instruction space.
0 Supervisor and user access allowed.
1 Only supervisor access allowed. Attempted user access causes access error exception
Reset value is 0.
Содержание MCF54455
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