In carrying out the addition of two figures, the control unit works as follows: 
1. To establish the initial condition; all units are placed in an initial state (CLEAR) 
2. To release the keyboard for the input 
3. To transfer to the central processing unit (CPU) the figures encoded by pressing the keys, 
until one of the operational keys (=,+) is pressed 
4. To wait for the second figure and to store this in the CPU 
5. To carry out the operation (i.e. to add the first and second figures) 
6. To display the result until a further CLEAR signal appears 
However, this outline of the principle on which a pocket calculator is constructed has two important 
limitations: 
1. Only mathematical functions can be executed 
2. The control unit can cope only with a fixed sequence of operations 
The universal processing of binary data is made possible by the use of a microcomputer. This also 
consists of a central processing unit and a control unit, which together form the microprocessor (MP) 
as such. However, in order to make possible a dialogue between man and machine, the microcomputer (MC) 
has a different type of periphery. 
The main difference between pocket calculator and microcomputer thus consists in the construction of 
the input/output circuits. Whilst pocket calculators generally only have a keyboard and a display, | 
microcomputers have entry and exit conductors which provide the link between computer, man and machine, 
when control and computing tasks are being executed. 
The microcompressor is the central component of the microcomputer. Most microprocessors now available 
contain both the central processing unit and the control unit for the complete microcomputer system. 
In such a system, data channels are necessary. These are generally described as buses; 
- address bus consists of conductors and determines the number of addressable storage lines. 
The data flow is always directed from the microprocessor to storage. 
- data bus consists of m conductors and determines the word size of the data and the type 
designation of the microcomputer. 
- control bus consists of several conductors for exchanging commands and to signalize internal 
operational states. 
For control and the simpler of arithmetical functions, single-chip MC systems are particularly well 
suited. These combine the various components of a microcomputer described above in a single integrated | 
circuit (Fig 2). | 
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
| I | 
PROGRAM STORAGE DATA STORAGE I 
1024 BYTES 64 BYTES 
CONTROL 8-BIT 
UNIT CPU 
8-BIT 27 INPUT/OUTPUT 
TIMER/COUNTER CONDUCTORS K— 
  
  
  
Fig 2: Block Diagram ofthe Single-Chip Microcomputer (Intel 8048) most widely used today. 
In addition to the CPU and program storage, this integrated circuit also has 27 input/output 
conductors and an 8-bit timer/counter. 
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