Assembly - Registers

Processor operations mostly involve processing data. This data can be stored in memory and accessed from thereon. However, reading data from and storing data into memory slows down the processor, as it involves complicated processes of sending the data request across the control bus, and into the memory storage unit and getting the data through the same channel.

To speed up the processor operations, the processor includes some internal memory storage locations, called registers.

The registers stores data elements for processing without having to access the memory. A limited number of registers are built into the processor chip.

Processor Registers

There are ten 32-bit and six 16-bit processor registers in IA-32 architecture. The registers are grouped into three categories:

• General registers
• Control registers
• Segment registers

The general registers are further divided into the following groups:

• Data registers
• Pointer registers
• Index registers

Data Registers

Four 32-bit data registers are used for arithmetic, logical and other operations. These 32-bit registers can be used in three ways:

1. As complete 32-bit data registers: EAX, EBX, ECX, EDX.
2. Lower halves of the 32-bit registers can be used as four 16-bit data registers: AX, BX, CX and DX.
3. Lower and higher halves of the above-mentioned four 16-bit registers can be used as eight 8-bit data registers: AH, AL, BH, BL, CH, CL, DH, and DL.
   
4. Some of these data registers has specific used in arithmetical operations.
   AX is the primary accumulator; it is used in input/output and most arithmetic instructions. For example, in multiplication operation, one operand is stored in EAX, or AX or AL register according to the size of the operand.
   BX is known as the base register as it could be used in indexed addressing.
   CX is known as the count register as the ECX, CX registers store the loop count in iterative operations.
   DX is known as the data register. It is also used in input/output operations. It is also used with AX register along with DX for multiply and divide operations involving large values. 
   
   
   
   
   Pointer Registers
   The pointer registers are 32-bit EIP, ESP and EBP registers and corresponding 16-bit right portions - IP, SP and BP. There are three categories of pointer registers:
   • Instruction Pointer (IP) - the 16-bit IP register stores the offset address of the next instruction to be executed. IP in association with the CS register (as CS:IP) gives the complete address of the current instruction in the code segment.
   • Stack Pointer (SP) - the 16-bit SP register provides the offset value within the program stack. SP in association with the SS register (SS:SP) refers to be current position of data or address within the program stack.
   • Base Pointer (BP) - the 16-bit BP register mainly helps in referencing the parameter variables passed to a subroutine. The address in SS register is combined with the offset in BP to get the location of the parameter. BP can also be combined with DI and SI as base register for special addressing. 
     
     
     
   • Index Registers
     The 32-bit index registers ESI and EDI and their 16-bit rightmost portions SI and DI are used for indexed addressing and sometimes used in addition and subtraction. There are two sets of index pointers:
     • Source Index (SI) - it is used as source index for string operations
     • Destination Index (DI) - it is used as destination index for string operations.