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Chapter 4. Physical and Virtual Memory

Very limited expansion capabilities (a change in CPU architecture would be required)

Expensive (more than one dollar/byte)

However, at the other end of the spectrum, off line backup storage is:

Very slow (access times may be measured in days, if the backup media must be shipped long

distances)

Very high capacity (10s   100s of gigabytes)

Essentially unlimited expansion capabilities (limited only by the floorspace needed to house the

backup media)

Very inexpensive (fractional cents/byte)

By using different technologies with different capabilities, it is possible to fine tune system design for

maximum performance at the lowest possible cost. The following sections explore each technology in

the spectrum.

4.2.1. CPU Registers

Every present day CPU design includes registers for a variety of purposes, from storing the address

of the currently executed instruction to more general purpose data storage and manipulation. CPU

registers run at the same speed as the rest of the CPU; otherwise, they would be a serious bottleneck

to overall system performance. The reason for this is that nearly all operations performed by the CPU

involve the registers in one way or another.

The number of CPU registers (and their uses) are strictly dependent on the architectural design of the

CPU itself. There is no way to change the number of CPU registers, short of migrating to a CPU with

a different architecture. For these reasons, the number of CPU registers can be considered a constant,

as they are unchangeable without great pain.

4.2.2. Cache Memory

The purpose of cache memory is to act as a buffer between the very limited, very high speed CPU

registers and the relatively slower and much larger main system memory   usually referred to as

RAM

1

. Cache memory has an operating speed similar to the CPU itself, so that when the CPU accesses

data in cache, the CPU is not kept waiting for the data.

Cache memory is configured such that, whenever data is to be read from RAM, the system hardware

first checks to see if the desired data is in cache. If the data is in cache, it is quickly retrieved, and

used by the CPU. However, if the data is not in cache, the data is read from RAM and, while being

transferred to the CPU, is also placed in cache (in case it will be needed again). From the perspective

of the CPU, all this is done transparently, so that the only difference between accessing data in cache

and accessing data in RAM is the amount of time it takes for the data to be returned.

In terms of storage capacity, cache is much smaller than RAM. Therefore, not every byte in RAM

can have its own location in cache. As such, it is necessary to split cache up into sections that can

be used to cache different areas of RAM, and to have a mechanism that allows each area of cache to

cache different areas of RAM at different times. However, given the sequential and localized nature

of storage access, a small amount of cache can effectively speed access to a large amount of RAM.

When writing data from the CPU, things get a bit more complicated. There are two different ap 

proaches that can be used. In both cases, the data is first written to cache. However, since the purpose

1. While "RAM" is an acronym for "Random Access Memory," and a term that could easily apply to any

storage technology that allowed the non sequential access of stored data, when system administrators talk about

RAM they invariably mean main system memory.