3- 34 M68040 USER'S MANUAL MOTOROLA
MC68030 and MC68851 cause F-line unimplemented instruction exceptions if executed in
either supervisor or user mode by the M68040.
3.7.4 Register Programming Considerations
If the entries in the ATCs are no longer valid when a reset operation occurs (as is normally
expected), an explicit flush operation must be specified by the system software. The
assertion of RSTI disables translation by clearing the E-bits of the TCR, DTTRx, and
ITTRx, but it does not flush the ATCs. Reading or writing any of the MMU registers (URP,
SRP, TCR, MMUSR, DTTR0, DTTR1, ITTR0, ITTR1) does not flush the ATCs. Since a
write to these registers can cause some or all the address translations to change, the write
should be followed by a PFLUSH operation to flush the ATCs if necessary.
The status bits in the MMUSR indicate conditions to which the operating system should
respond. In a typical access error exception handler, the flowchart illustrated in Figure
3-23 can be used to determine the cause of an MMU fault. The PTEST instruction sets
the bits in the MMUSR appropriately, and the program can branch to the appropriate code
segment for the condition.