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Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf
Showing 226-227 out of 1137
Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 226
SEC. 3.3
VIRTUAL MEMORY
195
memory, the hardware performs the necessary mapping on the fly. When the pro-
gram references a part of its address space that is
not
in physical memory, the oper-
ating system is alerted to go get the missing piece and re-execute the instruction
that failed.
In a sense, virtual memory is a generalization of the base-and-limit-register
idea. The 8088 had separate base registers (but no limit registers) for text and data.
With virtual memory, instead of having separate relocation for just the text and
data segments, the entire address space can be mapped onto physical memory in
fairly small units.
We will show how virtual memory is implemented below.
Virtual memory works just fine in a multiprogramming system, with bits and
pieces of many programs in memory at once. While a program is waiting for
pieces of itself to be read in, the CPU can be given to another process.
3.3.1 Paging
Most virtual memory systems use a technique called
paging
, which we will
now describe. On any computer, programs reference a set of memory addresses.
When a program executes an instruction like
MOV REG,1000
it does so to copy the contents of memory address 1000 to
REG
(or vice versa, de-
pending on the computer).
Addresses can be generated using indexing, base regis-
ters, segment registers, and other ways.
CPU
package
CPU
The CPU sends virtual
addresses to the MMU
The MMU sends physical
addresses to the memory
Memory
management
unit
Memory
Disk
controller
Bus
Figure 3-8.
The position and function of the MMU.
Here the MMU is shown as
being a part of the CPU chip because it commonly is nowadays. However, logi-
cally it could be a separate chip and was years ago.
These program-generated addresses are called
virtual addresses
and form the
virtual address space
.
On computers without virtual memory, the virtual address
Page 227
196
MEMORY MANAGEMENT
CHAP. 3
is put directly onto the memory bus and causes the physical memory word with the
same address to be read or written. When virtual memory is used, the virtual ad-
dresses do not go directly to the memory bus. Instead, they go to an
MMU
(
Mem-
ory Management Unit
) that maps the virtual addresses onto the physical memory
addresses, as illustrated in Fig. 3-8.
A very simple example of how this mapping works is shown in Fig. 3-9. In
this example, we have a computer that generates 16-bit addresses, from 0 up to
64K
−
1. These are the virtual addresses. This computer, however, has only 32 KB
of physical memory. So although 64-KB programs can be written, they cannot be
loaded into memory in their entirety and run.
A complete copy of a program’s core
image, up to 64 KB, must be present on the disk, however, so that pieces can be
brought in as needed.
The virtual address space consists of fixed-size units called pages.
The corres-
ponding units in the physical memory are called
page frames
.
The pages and page
frames are generally the same size.
In this example they are 4 KB, but page sizes
from 512 bytes to a gigabyte have been used in real systems. With 64 KB of virtual
address space and 32 KB of physical memory, we get 16 virtual pages and 8 page
frames. Transfers between RAM and disk are always in whole pages.
Many proc-
essors support multiple page sizes that can be mixed and matched as the operating
system sees fit. For instance, the x86-64 architecture supports 4-KB, 2-MB, and
1-GB pages, so we could use 4-KB pages for user applications and a single 1-GB
page for the kernel. We will see later why it is sometimes better to use a single
large page, rather than a large number of small ones.
The notation in Fig. 3-9 is as follows. The range marked 0K–4K means that
the virtual or physical addresses in that page are 0 to 4095.
The range 4K–8K
refers to addresses 4096 to 8191, and so on.
Each page contains exactly 4096 ad-
dresses starting at a multiple of 4096 and ending one shy of a multiple of 4096.
When the program tries to access address 0, for example, using the instruction
MOV REG,0
virtual address 0 is sent to the MMU. The MMU sees that this virtual address falls
in page 0 (0 to 4095), which according to its mapping is page frame 2 (8192 to
12287). It thus transforms the address to 8192 and outputs address 8192 onto the
bus. The memory knows nothing at all about the MMU and just sees a request for
reading or writing address 8192, which it honors. Thus, the MMU has effectively
mapped all virtual addresses between 0 and 4095 onto physical addresses 8192 to
12287.
Similarly, the instruction
MOV REG,8192
is effectively transformed into
MOV REG,24576
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