General Purpose registers

The x86 general purpose registers are not really 
as general purpose as their name implies. 

That is because these general purpose registers 
have some highly specialized tasks that can often 
only be done by using only one or two specific registers. 

(In other architectures, any general purpose register 
can be used for any purpose.)

The x86 general purpose registers further subdivide 
into registers specializing in data and others 
specializing in addressing.

A lot of operations can be done either inside a register 
or directly inside RAM without requiring the data to be 
loaded into a register first. The 1970s heritage of 
this architecture shows through by this behaviour.

Note: with the advent of the 64-bit extensions to x86 
in AMD64, this odd behaviour has now been cleaned up 
in 64-bit mode. General purpose registers are now truly 
general purpose and they can be used interchangeably.

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8-bit and 16-bit register subsets


8-bit and 16-bit subsets of these registers are 
also accessible. (For backward compatibility) 

For example, the lower 16-bits of the 32-bit EAX registers 
can be accessed by calling it the AX register. 

Some of the 16-bit registers can be further subdivided into 
8-bit subsets too; 

   for example, 

      the upper 8-bit half of AX is called AH, and
      the lower half is called AL. 

Similarly, EBX is subdivided into BX (16-bit), 
which in turn is divided into BH and BL (8-bit).

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General data registers


All of the four following registers may be used as 
general purpose registers. 

However each has some specialized purpose as well. 

Each of these registers also have 16-bit or 8-bit subset names.

   EAX 
       Dedicated accumulator which is used for all major calculations.
       Many arithmetic operations will use EAX/AX as source and
       also store result in this register.
 
   ECX 
       The universal loop counter which has a special interpretation 
       for loops. 
       (You can put a value in this register and a special
        "decrement and branch" will decrement the counter and
	branch (loop) when not zero...)

   EDX 
       The data register, which is an extension to the accumulator, 
       stores data relevant to the operation applied to 
       the accumulator.
 
   EBX 
       The Base Register - used like M68000 address register to
       store the base of an array.

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General address registers

Used only for address pointing. They have 16-bit subset names, 
but no 8-bit subsets.

   ESP 
       Stack pointer. Is used to hold the top address of the stack.
 
   EBP 
       Base pointer. Is used to hold the address of 
       the current ***stack frame***. 
       It is also sometimes used as free storage. 

   ESI 
       Source index. 
       Commonly used for string operations. 
       It has a one-byte opcode for loading data from memory 
       to the accumulator. 

   EDI 
       Destination index. 
       Commonly used for string operations. 
       Has a one-byte STOS instruction to write data out of 
       the accumulator.
 
	You can use ESI and DSI and the count register to copy string 
	in a very tight loop !

   EIP 
       Instruction pointer. 
       Holds the current instruction address.