Hardware Reference
In-Depth Information
Normally UTF-8 is used to encode only the 17 Unicode planes, even though
the scheme has far more than 1,114,112 code points. However, if anthropologists
discover new tribes in New Guinea or elsewhere whose languages are not currently
known (or if we make contact later with extraterrestrials), UTF-8 will be up to the
job of adding their alphabets or ideographs.
Computer systems are built up from three types of components: processors,
memories, and I/O devices. The task of a processor is to fetch instructions one at a
time from a memory, decode them, and execute them. The fetch-decode-execute
cycle can always be described as an algorithm and, in fact, is sometimes carried
out by a software interpreter running at a lower level. To gain speed, many com-
puters now have one or more pipelines or have a superscalar design with multiple
functional units that operate in parallel. A pipeline allows an instruction to be bro-
ken into steps and the steps for different instructions executed at the same time.
Multiple functional units is another way to gain parallelism without affecting the
instruction set or architecture visible to the programmer or compiler.
Systems with multiple processors are increasingly common. Parallel com-
puters include array processors, on which the same operation is performed on mul-
tiple data sets at the same time, multiprocessors, in which multiple CPUs share a
common memory, and multicomputers, in which multiple computers each have
their own memories but communicate by message passing.
Memories can be categorized as primary or secondary. The primary memory
is used to hold the program currently being executed. Its access time is short—a
few tens of nanoseconds at most—and independent of the address being accessed.
Caches reduce this access time even more. They are needed because processor
speeds are much greater than memory speeds, meaning that having to wait for
memory accesses all the time greatly slows down processor execution. Some
memories are equipped with error-correcting codes to enhance reliability.
Secondary memories, in contrast, have access times that are much longer
(milliseconds or more) and dependent on the location of the data being read or
written. Tapes, flash memory, magnetic disks, and optical disks are the most com-
mon secondary memories. Magnetic disks come in many varieties, including IDE
disks, SCSI disks, and RAIDs. Optical disks include CD-ROMs, CD-Rs, DVDs,
and Blu-rays.
I/O devices are used to transfer information into and out of the computer.
They are connected to the processor and memory by one or more buses. Examples
are terminals, mice, game controllers, printers, and modems. Most I/O devices use
the ASCII character code, although Unicode is also used and UTF-8 is gaining ac-
ceptance as the computer industry becomes more Web-centric.
