But how can a lifeless part CPU do the work
Dr von Neumann devised an entire world
Of almost magical computer architecture and working plan.
For this purpose he kept many special purpose registers
Inside each processor like alu and CPU.
One register could divide
Forty bit long 010101 strings into smaller specified pieces
And send to smaller registers specially made for them.
One register could decode the binary code
Relating to electric circuits
So that CPU could issue necessary electric signals.
The DR register made of forty chambers
Is actually two registers of twenty chambers each
Each twenty bit register is divided
Into still smaller registers
Of eight bit and twelve bit parts
Thus the DR takes in the forty bit instruction
Coming from the main memory board
In the special forty bit super highway
The DR divides the forty bit instruction
Into two twenty bit parts.
First group of twenty bits are retained
And the second group of twenty bits
Automatically flows to another register called IBR.
The IBR is a temporary store (it is a “buffer”.)
The first group of twenty bits is again cut into two parts
First eight bits is a code to switch on
The special electric circuit in ALU
To “add/subtract/multiply/ divide”
The remaining twelve bits is the address
Of the particular memory house
In which thenumber
To be added/subtracted
Or to be multiplied or divided
Is resting for a little billionth of a second.
Remember here one of the foremost rules
Of the von Neumann machine
The registers mention
Only the name of the memory houses
Where numbers are resting
They do not contain the number!
The eight bit code of the mathematical circuit
Is sent to the CPU the chief switching expert.
The CPU sends electric signals
Which immediately switch on that circuit in the ALU
The twelve bit part is the address (in 010101code)
Of the memory house in the main memory board
In which the number on which
The “add/subtract/multiply/ divide”
Is to be done and which is resting
In the memory house.
Only this address of the memory house is sent to
The register called IR (which works inside cpu)
The very important duty of IR
Is to link these “house names” like X, Y, Z
Given by the human engineer with the “house numbers”
Allotted by the manufacturer of the computer
To the “memory houses” of the Main Memory.
(In modern computers the main memory board
Is a mega city with millions of memory houses,
Built from millions of submicroscopic transistors
In a shirt button size silicon chip.
But in the ancient computers of 1940s
There were perhaps only a few hundred memory houses
On the magnetic main memory board!)
If the register IR supplies that” house number”.
Another register called Memory Address Register (MAR)
Directly points the direct electrical path
To that particular memory house, you may say.
To that memory house pointed out by MAR
The CPU sends the prescribed standard electric signals.
Then automatically and instantly
The 010101 binary code
Of that number contained in that memory house
Travels through one of the computer’s super highways
Into the register DR (already familiar to us.)
DR in turn forwards that binary code of the number
To the“accumulator” register inside the ALU.
(About the “accumulator” register
Which exists inside the alu we already know.
It is like a water drum into which
Water using a standard measuring jar
Is poured and poured and level goes on increasing.
Or water is removed with the same measuring jar
And level decreases accordingly.
The accumulator drum always shows the current level).
Into “accumulator register” of ALU
Numbers are “poured in” or “poured out”
And the register shows current answer worked out in ALU.
The wonder is that all this action is over
In a billionth of a second
And answers start flowing out in to other
Identified memory houses on main memory board
Awaiting orders of CPU.
Are fully implemented
And final answer is finally worked out