The Main Memory Unit:
It is used for storing programs and data. The memory locations of memory unit is uniquely specified by the memory address of the location. M(X) is used to indicate the location of the memory unit M with address X.
The data transfer between memory unit and CPU takes place with the help of data register DR. When CPU wants to read some information from memory unit, the information first brings to DR, and after that it goes to appropriate position.Similarly, data to be stored to memory must put into DR first, and then it is stored to appropriate location in the memory unit.
The address of the memory location that is used during memory read and memory write operations are stored in the memory register AR.
The information fetched from the memory is a operand of an instruction, then it is moved from DR to data processing unit (either to AC or MQ). If it is an operand, then it is moved to program control unit (either to IR or IBR).
Two additional registers for the temporary storage of operands and results are included in data processing units: the accumulator AC and the multiplier-quotient register MQ.
Two instructions are fetch simultaneously from M and transferred to the program control unit. The instruction that is not to be executed immediately is placed in the instruction buffer register IBR. The opcode of the other instruction is placed in the instruction register IR where it is decoded.
In the decoding phase, the control circuits generate the required control signals to perform the specified operation in the instruction.
The program counter PC is used to store the address of the next instruction to be fetched from memory.
Input Output Device :
Input devies are used to put the information into computer. With the help of input devices we can store information in memory so that CPU can use it. Program or data is read into main memory from input device or secondary storage under the control of CPU input instruction.
Output devices are used to output the information from computer. If some results are evaluated by computer and it is stored in computer, then with the help of output devices, we can present it to the user. Output data from the main memory go to output device under the control of CPU output instruction.