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Memory Management Unit
4-20
M68060 USER’S MANUAL
MOTOROLA
updated before the MC68060 allows a page to be accessed. Table 4-1 lists the page
descriptor update operations for each combination of U-bit, M-bit, write-protected, and read
or write access type.
An alternate address space access is a special case that is immediately used as a physical
address without translation. Because the MC68060 implements a merged instruction and
data space, instruction address spaces (SFC/DFC = $6 or $2) using the MOVES instruction
are converted into data references (SFC/DFC = $5 or $1). The data memory unit handles
these translated accesses as normal data accesses. If the access fails due to an ATC fault
or a physical bus error, the resulting access error stack frame contains the converted func-
tion code in the TM field for the faulted access. If the MOVES instruction is used to write
instruction address space, then to maintain cache coherency, the corresponding addresses
must be invalidated in the instruction cache. The SFC and DFC values and results for nor-
mal (TT = 0) and for MOVES (TT = 10) accesses are listed in Table 4-2.
4.2.6 Address Translation Protection
The MC68060 MMUs provide separate translation tables for supervisor and user address
spaces. The translation tables contain both mapping and protection information. Each table
and page descriptor includes a write-protect (W) bit that can be set to provide write protec-
Table 4-1. Updating U-Bit and M-Bit for Page Descriptors
Previous Status WP Bit Access
Type
Page Descriptor
Update Operation
New Status
U-Bit M-Bit U-Bit M-Bit
00
X Read
Locked RMW Access to Set U 1 0
0 1 Locked RMW Access to Set U 1 1
1 0 None 1 0
1 1 None 1 1
00
0
Write
Write to Set U and M 1 1
0 1 Write to Set U 1 1
1 0 Write to Set M 1 1
1 1 None 1 1
00
1
None 0 0
0 1 None 0 1
1 0 None 1 0
1 1 None 1 1
NOTE: WP indicates the accumulated write-protect status.
Table 4-2. SFC and DFC Values
SFC/DFC Value Results
TT TM
000 10 000
001 00 001
010 00 001
011 10 011
100 10 100
101 00 101
110 00 101
111 10 111
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