Computers today. Гурьянова В.И. - 14 стр.

not differentiate between different types of information, it is up to the software
to give significance to what the memory sees as nothing but a series of numbers.
      In almost all modern computers, each memory cell is set up to store bi-
nary numbers in groups of eight bits (called a byte). Each byte is able to repre-
sent 256 different numbers; either from 0 to 255 or –128 to +128. To store larger
numbers, several consecutive bytes may be used (typically, two, four or eight).
When negative numbers are required, they are usually stored in two’s comple-
ment notation. Other arrangements are possible, but are usually not seen outside
of specialized applications or historical contexts. A computer may store any kind
of information in memory as long as it can be somehow represented in numeri-
cal form. Modern computers have billions or even trillions of bytes of memory.
      The CPU contains a special set of memory cells called registers that can
be read and written too much more rapidly than the main memory area. There
are typically between two and one hundred registers depending on the type of
CPU. Registers are used for the most frequently needed data items to avoid hav-
ing to access main memory every time data is needed. Since data is constantly
being worked on, reducing the need to access main memory (which is often slow
compared to the ALU and control units) greatly increases the computer’s speed.
      Computer main memory comes in two principal varieties: random access
memory or RAM and read-only memory or ROM. RAM can be read and written to
anytime the CPU commands it, but ROM is pre-loaded with data and software that
never changes, so the CPU can only read from it. ROM is typically used to store the
computer’s initial start-up instructions. In general, the content of RAM is erased
when the power to the computer is turned off while ROM retains its data indefinitely.
In a PC, the ROM contains a specialized program called the BIOS that orchestrates
loading the computer’s operating system from the hard disk drive into RAM when-
ever the computer is turned on or reset. In embedded computers, which frequently do
not have disk drives, all of the software required to perform the task may be stored in
ROM. Software that is stored in ROM is often called firmware because it is notion-
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