Introduction

There are 8-bit register such as AH, AL, BH, BL etc.
Then there are 16-bit register such as AX, BX, CX etc.

Accumulator

There is a central register in a processor which is used to perform Mathematical and Logical operations, which is called accumulator.
The word-size of a processor is equal to the memory width of the accumulator, which is normally 32-bits.

Pointer, Index, Base Registers

These register are not used to store data but to store the address of the data.
Take an example where a for-loop is used to iterate over an array.
We store the index of array in the register which behaves as a navigation point.
Therefore, we generally use these register when the memory addresses are unknown before the run-time-environment has been triggered.

Flag Registers or Program Status Registers

These register are also 32-bit or 16-bit in size.
However, unlike the accumulator, the whole group of bits in meaningless as a unit.
Rather, its value lies inside individual bits.

Program Counter or Instruction Pointer

Our program consists of instructions which need to be executed in sequence, one after the other.
This register stores the address of the next instruction to be executed.
Our instructions like “add“ need to be translated into numbers for the computer to understand.
This number is called an opcode.
But remembering these opcode is difficult.
Therefore, we map symbols to them, called instructions-mnemonics through a program called the assembler.

Instruction Groups

The instructions-mnemonics depend on the manufacturer.
Looking at the group of these instructions-mnemonics alone tells a lot about the underlying architecture.
These groups are called instruction-set.

Data Movement Instructions

mov ax, bx
lda  1234

Arithmetic and Logic Instructions

add ax, 1234
add bx, 0534
add bx, [1200]

The brackets [] indicate a memory location.

Program Control Instructions

cmp ax, 0  
jne 1234

Sometimes we want to manipulate the basic behavior of the instruction-pointer to jump to some other part of code execution.
In the above code block, we are comparing if ax is equal to 0, if it is, then we jump to memory location 1234.

Special Instructions

There are some special ones as well like:

cli
sli

To simply put, cli stops the processor from listening to external things (could be done to make the #processor perform the important task at hand).
sli command restores the normal functionality of the processor.