Motorola M68000 Betriebsanweisung PDF herunterladen (Seite 189)

The 68010 175

the tenth word in the virtual page corresponds to the tenth word in the

physical page. Since the virtual address space is normally much larger

than the physical memory, it is necessary to temporarily store the pages

that won’t fit into physical memory. For example, we might have just

1 megabyte of physical memory, while the 68010 can address up to 16

megabytes. Normally a disk memory is used to store the pages that are

not in physical memory. This is sometimes called a backing store. The

backing store does not have to be as large as the addressing capability of

the CPU if the size of the virtual address space is reduced.

Now we can tie up the loose ends. If an instruction references an

address that is mapped to physical memory, the memory management

hardware makes that address immediately available and no bus error

is generated. If a reference is made to an address that is not currently

mapped, a bus error exception will occur. This is normally called a page

fault when paging is used. The operating system now gets control. It is

responsible for finding the proper page on the backing store and bringing

it into memory. Once it is in memory, the operating system can execute

the RTE instruction with the stack pointer pointing at the same place it

was when the bus error was initially processed. This will return control

to the instruction that generated the bus error, and it will now be able to

access the address.

This may sound simple, but an operating system has a lot of bookeep-

ing to do in order to get all the pages mapped correctly. Many operating

systems textbooks will provide more information on the techniques used

to implement paging. Figure 15 shows a possible mapping of virtual pages

to physical page frames. Notice that some of the pages are not mapped.

Any reference to an address in one of these pages will generate a bus

error and hence a page fault.

Virtual Machines

Like virtual memory, a virtual machine provides the illusion of a

bare-bones machine. In other words, it appears to the user that all the

functionality of the machine is available. The user must not be aware

that a virtual machine operating system is running underneath her own

operating system.

In order to implement a true virtual machine, not only does the CPU

have to appear as a bare CPU, but any input/output devices must also

be made available to each user. Virtual memory can provide the illusion

of a specific physical memory (all users must have access to the same

addressing range). Since input/output devices are memory-mapped, they

can be replicated or shared for each user and appear at the same physical

1 2 ... 184 185 186 187 188 189 190 191 192 193 194 ... 255 256

Keine Kommentare

Motorola M68000 Betriebsanweisung Seite 189