Updating U-Bit And M-Bit For; Sfc And Dfc Values - Motorola M68060 User Manual

Memory Management Unit
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.
Table 4-1. Updating U-Bit and M-Bit for Page Descriptors
Previous Status
U-Bit M-Bit
0
0
1
1
0
0
1
1
0
0
1
1
NOTE: WP indicates the accumulated write-protect status.
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-
4-20
Access
WP Bit
Type
0
1
X Read
0
1
0
1
0
0
1
Write
0
1
1
0
1
Table 4-2. SFC and DFC Values
SFC/DFC Value
000
001
010
011
100
101
110
111
M68060 USER'S MANUAL
Page Descriptor
Update Operation
Locked RMW Access to Set U
Locked RMW Access to Set U
None
None
Write to Set U and M
Write to Set U
Write to Set M
None
None
None
None
None
Results
TT TM
10 000
00 001
00 001
10 011
10 100
00 101
00 101
10 111
New Status
U-Bit M-Bit
1 0
1 1
1 0
1 1
1 1
1 1
1 1
1 1
0 0
0 1
1 0
1 1
MOTOROLA