Lesson Three TEXT Compoents of a Computer System Now that you have some
 idea of what microcomputers can do, we can begin the discussion of how 
thdy do it. The idea of a general-purpose machine to solve mathematical 
problems was first proposed over 150 years go by a farsighted individua
l named Charles Babbage. The device Babbage proposed to build consisted
 of four components that uncannily reseble a modern computer: The store
 to hold data and the results of calculations: Babbage designed the sto
ree to contain 1000 numbers of 50 digits each. The mill to perform the 
mathematical operations. Gears and levers to transfer data back and for
th between the store and the mill. An input/output unit to read externa
l data into the store and display results of calcuations produced by th
e mill. Babbage's machine (named the Analytical Engine and subsequently
 modified into a more ambitious Difference Engine) would, had it been c
ompleted, it would have been the first general-purpose computer. Unfort
unately, the technology of the day was incapable of producing the mecha
nical comonets to have lost 17, 000 pounds of his own money (and an equ
al amount of government funding). His critics wrote that "the governmen
t received nothing for its money and should at least have gotten a clev
er toy." As with so many other geniuses who were ahead of their time, B
abbage was proved correct nearly a century after his death, since all m
odern computers are desingned along the lines he laid out. A computer a
ccepts data from an external source (input), stores it temporarily in m
emory while calculating an answer (processing), and pressents the resul
ts (output). Thus the computer itself is only one element of the total 
picture, as can be seen from Figure 1, which depicts the essential hard
ware compoents of computer system. These components and their relations
 hip to one another are found in all computer systems, from the latest 
technological marvel. Consider: Central processing unit (CPU) performs 
the actual calculations and consists of control unit and arithmetic-lo
gic unit. The components of the CPU are more fully described in the nex
t section. Main memory also known as primary memory or random-access me
mory (RAM) temporarily store any program executed by the computer as we
ll as the data on which the program operates. Main memories are designe
d for high-speed, short-term (temporary) access to data as opposed to a
uxiliary storage, which provides permanent (albeit slower) access. Auxi
liary storage (also called secondary storage or external storage) provi
des a place where data can be permanently stored, and from where it can
 be transferred to and from main memory. Floppy disks with storage capa
cities of up to 1.44 million characters and hard (fixed) disks with cap
acities of 10 to 100 million characters are examples of auxiliary stora
ge used with personal computers. Input devices accept data from an exte
rnal source and convert to electric signals which are sent to the CPU. 
The keyboard, mouse, and joystick are common input devices used with mi
crocomputers. Output devices accept electric signals from the CPU and c
onvert them to a form suitable for output. Printers and monitors are th
e most common type of output devices used with microcomputers. Pringers
 provide permanent record (hard copy) of computer output.