CONTROL  UNIT The   control   unit   maintains   order   within   the computer   system   and   directs   the   flow   of   traffic (operations) and data. The flow of control is indicated by  the  dotted  arrows  on  figure  1-1.  The  control  unit selects   one   program   statement   at   a   time   from   the program  storage  area,  interprets  the  statement,  and sends   the   appropriate   electronic   impulses   to   the arithmetic-logic unit and storage section to cause them to carry out the instruction. The   control   unit   does   not   perform   the   actual processing  operations  on  the  data.  Specifically,  the control unit manages the operations of the CPU, be it a single-chip  microprocessor  or  a  fill-size  mainframe. Like a traffic director, it decides when to start and stop (control  and  timing),  what  to  do  (program  instructions), where  to  keep  information  (memory),  and  with  what devices to communicate (I/O). It controls the flow of all data  entering  and  leaving  the  computer.  It  accomplishes this   by   communicating   or   interfacing   with   the arithmetic-logic   unit,   memory,   and   I/O   areas.   It provides the computer with the ability to function under program  control. Depending  on  the  design  of  the computer,  the  CPU  can  also  have  the  capability  to function  under  manual  control  through  man/machine interfacing. The control unit consists of several basic logically defined  areas. These  logically  defined  areas  work closely with each other.  Timing in a computer regulates the  flow  of  signals  that  control  the  operation  of  the computer. The instruction and control portion makes up  the  decision-making  and  memory-type  functions. Addressing  is  the  process  of  locating  the  operand (specific   information)   for   a   given   operation.   An interrupt  is a break in the normal flow of operation of a computer  (e.g.,  CTRL  +  ALT  +  DEL).   Control memory  is   a   random-access   memory   (RAM) consisting  of  addressable  storage  registers. Cache memory  is  a  small,  high-speed  RAM  buffer  located between the CPU and main memory; it can increase the speed of the PC. Read-only memory (ROM) are chips with  a  set  of  software  instructions  supplied  by  the manufacturer built into them that enables the computer to perform its I/O operations. The control unit is also capable of shutting down the computer  when  the  power  supply  detects  abnormal conditions. ARITHMETIC-LOGIC  UNIT The  arithmetic-logic  unit  (ALU)  performs  all arithmetic    operations    (addition,    subtraction, multiplication,   and   division)   and   logic   operations. Logic  operations  test  various  conditions  encountered during processing and allow for different actions to be taken based on the results. The data required to perform the arithmetic and logical functions are inputs from the designated CPU registers and operands. The  ALU  relies  on  basic  items  to  perform  its operations.  These  include  number  systems,  data  routing circuits   (adders/subtracters),   timing,   instructions, operands,   and   registers. Figure   1-2   shows   a representative   block   diagram   of   an   ALU   of   a microcomputer. PRIMARY  STORAGE  (MAIN  MEMORY) The  primary  storage  section  (also  called  internal storage, main storage, main memory,  or just memory) serves  four  purposes: . To hold data transferred from an I/O device to the input storage area,  where it remains until the computer is ready to process it. This is indicated by the solid arrow on figure 1-1. .  To  hold  both  the  data  being  processed  and  the intermediate   results   of   the   arithmetic-logic operations.  This  is  a  working  storage  area within   the   storage   section.   It   is   sometimes referred to as a scratch pad memory. .  To  hold  the  processing  results  in  an   output storage area for transfer to an I/O device. Figure  1-2.—Representative  block  diagram  of  an  ALU. 1-3