Motorola MC68030 User Manual page 378

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The translation table structure consists of:
CRP. upper level table in the task control block, which contains 32 long
pointers:
[0]. lower level table common to all tasks; maps all operating system
areas (first 4 Mbytes of virtual space). This common table contains
512 short-page entries (2K bytes).
[1], lower level table for first 16 Mbytes of user program/data/stack area.
[31]. lower level table for last 16 Mbytes (of 496 total) of user program/
data/stack area.
The user program can only access virtual addresses starting at 16 Mbytes
and extending upward to the limit of 512 Mbytes. The code, the data, and
the stacks for user tasks are allocated in this area of virtual memory. Super-
visor programs can access the entire virtual map; they can access addresses
that directly access the I/O ports as well as the entire physical memory at
untranslated addresses. The address tables are set up so that virtual ad-
dresses are equal to the physical addresses for the supervisor between 1 and
3 Mbytes. Folding the physical address space into the virtual space greatly
simplifies operations that use physical addresses. The folding does not nec-
essarily mean that the virtual addresses are the same as the physical ad-
dresses. For example, the boot/diagnostic ROM at physical address zero could
be assigned a virtual address of 3 Mbytes. However, any external bus masters
or circuitry (such as breakpoint registers) resident on the physical side of the
bus must have physical addresses. This requires the overhead of operating
system code to perform address translation.
This virtual memory map provides supervisor addresses that are unique with
respect to user addresses; all supervisor routines can directly access any
user area without being restricted to certain instructions or addressing modes.
The separate user and supervisor maps suggest that two root pointe'rs should
be used, one for the supervisor map and one for the user map. However,
the supervisor must be able to access user translation tables for proper access
to user data items. With separate root pointers, the supervisor table structure
must be linked to that of the user. To do this requires an additional level of
table lookup (function code level) for the supervisor address table.
This example uses a simpler scheme instead. Only the CPU root pointer is
used, and, for each task, the first entry of the upper level table (for the
supervisor portion, the first 16 Mbytes of virtual address space) points to the
same lower level table. This common lower level table has supervisor pro-
MC68030 USER'S MANUAL
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