Question:

Why AX is called accumulator? What is the rule of AX register in CPU operation?

Solution:

Accumulator means collection of things. AX is called accumulator because in AX data and intermediate arithmetic and logic results are stored temporarily. In CPU, for some arithmetic operation, such as multiplication, division, subtraction, addition. AX holds one of the operands which will be processed by  ALU and it also holds the results of operation done by the ALU.

In the EU(Execution Unit) AX is a 16 bit register(Higher Order byte of the AX is named AH and lower order byte of the AX register is AL)

So, the role of AX in CPU is very significant because without the accumulator, it would be a problem to write the result of each calculation, addition, multiplication, subtraction, division etc. to main memory. Perhaps, only be read write back again for the use in the next instruction, which is a very slower process.

So, AX register makes arithmetic operation faster and so it is very important in CPU.

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Question:

"Segment register can locate any memory as the base address of the respective segment among the whole memory"- Justify the statement.

Solution:

8086 can address 2^20 = 1 Mega Byte of memory and 8086 works with 4 segments; Code Segment(CS), Data Segment(DS), Stack Segment(SS), Extra Segment(ES) with this 1 Mega byte of memory. Each of this segment are of 64K Byte and can be positioned anywhere within the 1MB memory space.
These segment registers hold the upper 16 bit of the starting address for last segment and BIU automatically inserts zero for the lowest 4 bits of the 20 bits starting address.

For example, 

if the code segment register is 348AH, then BIU(Bus Interface Unit) inserts zero for the lowest 4 bits to produce 20 bits starting address of code segment in memory which will be 348A0H.
Similarly if 204CH is the data segment register, 204C0H will be the base address of data segment in memory.

In this way, every segments base is defined. So, we cans say that "Segment register can locate any memory cell as the base address of the  respective segment among the whole memory".

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Question:

Why CPU actually works on Binary digit?

Solution:

The reason of CPU's working in binary digit:
A binary unit or a bit is the smallest unit if data which is represented by 0 or 1. Computer processors or CPU are made of billions of transistors. A transistor is a tiny switch that is activated by the electronic signal it receives.
The digits 1 and 0 used in binary reflect the ON and OFF states of a transistor means upper and lower level of electric signals. We know Computer programs are sets of instruction. Each instruction is translated into machine code -simple binary codes that activates the CPU. Computer code is written and this is converted by a translator into binary instructions so that the CPU can execute the instruction.

So, we can say that why CPU actually works on binary digits.

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Question:

We know that 8086 is a 16 bit microprocessor, But it's memory mapping is 20 bits. How is it possible?

Solution:

8086 is a 16 bit microprocessor that means its arithmetic logic unit can execute 16 bit multiple instruction at a time and it's internal registers most of the instructions are 16 bits.

But 8086 has 20 bit address bus means it can address any one of 2^20 = 1 MB memory locations. The technique is given below:
The BIU (Bus Interface Unit) of the CPU consists of four segment register. Upper 16 bits of the starting address for that segment.

In BIU, pointer registers also called offset consists the distance from the base address to the next instruction byte to be fetched.

To make from 16 bit to 20 bit memory address the BIU automatically inserts zeroes for the lowest 4 bits of the segment base address, then the IP is added to the code segment(CS). Then the 16 bit offset is added to the segment. Thus 20 bit memory mapping is possible.

For example,

If the CS register contains 348AH, the starting address for the code segment is 348A0H (Here 0 is appended)

If IP is 4214H, then the 20 bit physical address of memory is

                         CS  = 348A0H
                         IP   =   4214H
       -------------------------------------------
      Physical address = 38AB4H


This is why 8086 memory mapping is 20 bits.

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Question:

What do you mean by a n bit microprocessor?

Solution:

The n-bit microprocessor means the ALU(Arithmetic Logic Unit). This processor can work with n-bit multiple instruction at a time, regardless of the number of addres or data bus lines and it's internal registersand most of its instructions are designed to work with n-bit binary words.

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