MC68030 Datasheet, PDF (357/602 Pages) Motorola, Inc – ENHANCED 32-BIT MICROPROCESSOR

Memory Management Unit
The table search performed by the PTEST instruction may end in a bus error termination.
Either the address translation tables are not correctly built or main memory has failed (either
a transient or permanent failure).
A supervisor protection violation or a write protection violation usually indicates that the task
generating the exception attempted to access an area of the virtual address space that is
not part of the task's address space. The operating system usually recovers from such an
error by terminating (aborting) the task.
When the PTEST instruction returns the invalid status, the bus error is a page fault, and the
operating system must identify the specific type of page fault. When the limit violation bit
returned by the PTEST instruction is set, the task that took the exception was trying to
access a page that has not been allocated. The example system aborts the task in this case.
In other systems, this is an implicit request for more virtual memory, particularly if the
reference is in a stack area.
When no limit violation occurred, a descriptor is invalid. Typically, the descriptor contains
software flags that provide relevant information. The example operating system checks to
see if the invalid descriptor is in an upper level or a lower level table. When the descriptor is
in the upper level table, the task was attempting to access unallocated virtual memory, and
the system aborts the task. When the descriptor is in a lower level table, the system checks
software flags to identify the invalid descriptor.
When the software flags indicate that the descriptor corresponds to an unallocated page,
the system aborts the task. When the descriptor refers to a virgin page (allocated, but not
yet accessed) and the request for the page was a read request, the page is actually invalid
because the read operation reads unknown data. However, the example operating system
does not consider the type of request, but assigns a physical page frame to the page and
writes the page descriptor to the page table. Some systems clear virgin pages to zero.
MOTOROLA
MC68030 USER’S MANUAL
9-69