How the registers inside CPUwork
In the first von Neumann computer
Each “instruction” written by the human engineer
Was made of forty zeroes and ones.
When the instruction (copy of it) was transmitted
From the main memory to the CPU,
On the forty lane superhighway
It was first received in the “Data Register”
Which had forty flip flop chambers.
And which was a most important register.
But each instruction in that ancient computer
Was actually a combination of two instructions
Of twenty bits each and so the Data Register
Broke the big instruction into two.
It sent the second half to another register
For temporary safekeeping
And processed the first twenty bits forthwith.
The Data register again breaks the first half
Into eight bit and twelve bit parts
The first eight bits indicated the mathematical circuit.
To CPU accordingly switches on that specified circuit
(One of the add/subtract/multiply/divide circuits
Contained in the registers of the ALU).
The twelve bit part of the first half
Indicated the 010101code of “memory house ”
The code is sent to the “memory access register”
A command is sent by CPU to the concerned memory house.
As per command from the CPU
A copy of the 010101 code of the number
Contained in that memory house
Goes to the ALU’s mathematical registers.
The 010101 code of the answer coming from ALU
Again had to travel to the Data Register
And from there to a specified memory house
In the main memory board
Reserved for the answer as per CPU’s order.
Then only the second part of the instruction.
Is taken for processing and it is processed in a similar way.
The first eight bits denote the mathematical circuit .
The remaining twelve bits show the 010101 code
Of the memory house containing the number ‘
The “instructions” bring only the 010101 codes
Of only the “memory houses”
And not of the actual numbers
Contained in those memory houses.
This we should understand in the clearest way.
The “memory access register” helps the CPU
In identifying the memory house as per its code
And helps CPU issue the electric pulse required
To bring that number to the Data Register.
(In the form of its twelve bit 010101 code) .
Another important register in CPU called “Index Register”
Intimates the 010101 code of the memory house
Containing the next instruction
And sends it to the Memory Access Register
This register in turn informs the CPU
To send electric pulse to that memory house
To make it send a copy of the instruction
To the versatile Data Register!
You may feel that all this process is long and tedious
But it takes just a billionth of second for the computer
To process all the forty bit instructions of entire program..
The Main Memory, the CPU and the ALU
All working in close coordination
Can process a million instructions in a second
With out any human intervention
That is the great advantage of the “von Neumann machine “
THE OPERATING SYSTEM
The CPU sends “question” pulses to all the parts
And wants only “yes/no answer” pulses.
As per the reply pulses given by the parts
The CPU switches the big circuits.
All these question/answer pulses both ways
Form a coded language and are recorded
In a permanent section of main memory
As a permanent reference book.
We may say in a way that it has developed
Into the modern “operating system” or “OS”
We can also view the operating system in another way.
The big doctors can not work in hospital
Without help of compounders and nurses.
The big professors can not work in a university
Without a big group of assistants and clerks.
The big actorscan not act in a studio
Without help of the several helping staff
Similarly the big “Processors”
Inside any big moderncomputer
Can not do the data processing work
Without a powerful “operating system”
In modern “desk top computers”
The big dictionary-like operating system
Is recorded in “hard disc” by the manufacturer.
When computer is switched on
The main memory comes alive
And a copy of the OS flows on to it from hard disc
And remains live and active
Till the machine is switched off..
Dr von Neumann clearly defined
The complicated switching duties of CPU .
The CPU is responsible
For ensuring correct functioning of all circuits
And so checks all circuits periodically.
It has also a fixed written list of “microinstructions”
Relating to each standard instruction.
It has to do all switching based only on this list.
The OS which is a part of the CPU
Fixes priorities for use of CPU and ALU
By the other computer parts
(Like the main memory and the input and output devices)..
Such routine duties overburden the main CPU.
So a section of the CPU is developed as the OS
As separate commander to attend to such duties.
Only when the automatic
And self-loading MSDOS
(“Microsoft Disc Operating System”)
Popularized by the legendary Bill Gates
Became available to the common man,
The small “von Neumann machines”
Which we call as the “desk tops computers”
Became popular worldwide and were sold in millions.
They are the common “computers”
Which has become now as common
As the TV, radio and the telephone..