MC68030 Datasheet, PDF (344/602 Pages) Motorola, Inc – ENHANCED 32-BIT MICROPROCESSOR

Memory Management Unit
9.9.2 Task Memory Map Definition
The MC68030 provides several different means by which the supervisor can access the
user address spaces. The supervisor can access any user address, regardless of how the
virtual space is partitioned, with the MOVES (move space) instruction. Some systems
provide a complete 4-Gbyte virtual memory map for each task. Indeed, an operating system
that runs other operating systems in a virtual machine environment must provide a complete
map to accurately emulate the full addressing range for the subordinate operating system.
With the large address space of the MC68030, each individual user task or all user tasks
can share the address space with the operating system. One method of performing this
function is implemented in the example operating system in the next section. Sharing the
address space provides direct access to user data items by the operating system. Another
advantage of this mapping method is that tasks can easily share code. Common routines
such as file I/O handlers and arithmetic conversion packages can be written re-entrantly and
be restricted to read-only access from all user tasks in the system.
The simplest example of a shared virtual address space system is one in which each user
and supervisor process is given a unique virtual address range within the single 4-Gbyte
virtual address space. In other words, the system has only one linear virtual address space;
all processes run somewhere in that space. Only one translation table tree is required for
the entire system, but each task can have individual tables if desired. With the common tree
approach, the operating system can access any item of any task without modifying the root
pointer. Otherwise, only the currently active task is immediately accessible, which often is
adequate. To switch tasks, the operating system only has to update the user program and
user data pointers in the highest level translation table indexed by the function code. This
gives each task access to its own data only. This scheme has the advantages of simple table
management and easy sharing of common items by giving them the same virtual address
for all tasks in the system. This scheme might be ideal for real-time systems that do not
require more complexity in memory management facilities.
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MC68030 USER’S MANUAL
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