The Memory System 221
Processor Introduced Main memory Virtual memory
80286 1982 16 MB 1 GB
80386DX 1985 4 GB 64 TB
80486DX 1989 4 GB 64 TB
Pentium 1993 4 GB 64 TB
Pentium Pro 1995 64 GB 64 TB
Pentium II 1997 64 GB 64 TB
Pentium III 1999 64 GB 64 TB
Pentium 4 2000 64 GB 64 TB
Itanium 2001 64 GB 64 TB
Itanium 2 2002 64 GB 64 TB
Table 7.3 Virtual memory available in Intel processors
From the discussions so far, it must be clear to the reader, by now, that the system memory, divided into
primary and secondary parts, needs adequate attention to be properly utilized. This technique is known
as memory management, which is composed of hardware and the software, generally within the operat-
ing system. Two major responsibilities of the memory manager are
R Protect the memory area from unwanted accidental access
R Utilize the available memory area in best possible way for all relevant software.
Other duties of the memory manager is to keep track of entire memory space, both free as well as
occupied, allot spaces as and when necessary to respective programs, generate addresses as per the system
requirements and oversee the smooth running of the system from memory usage point of view. As pages or
segments of program are to be frequently loaded from disc to RAM or RAM to disc, it oversees the direct
memory access (DMA) operation also, as explained below.
7.8.1 DMA
DMA is the technique by using which data may be directly communicated between memory and other
peripheral devices (generally discs) bypassing the processor (CPU). This is achieved by a combination of
hardware and software. During normal operation, all data communication would be conducted through the
processor. For example, if any port data to be stored within memory, the data would rst be received by
the processor from the port. The processor then stores the data within a memory location. The same route
(through the processor) is followed for sending out any data from memory to the outside world.
However, in certain special cases, where larger quantity of data is involved, this technique of com-
munication consumes more time, which may be reduced by directly sending the data to or from memory.
Examples maybe a le-reading from disc or storing data from main memory to a disc. In such cases, no
processing of data is necessary and the direct communication between the memory and concerned I/O
device saves time. To explain how DMA works, let us take the example of a le reading from disc. The
related hardware is presented through Figure 7.11 .
M07_GHOS1557_01_SE_C07.indd 221M07_GHOS1557_01_SE_C07.indd 221 4/29/11 5:12 PM4/29/11 5:12 PM

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