Types of registers based on the data length
64-bit registers - These do not exist in the i586 instruction set. They exist only in the 64-bit processors. Their names start with the R character.
32-bit registers - Their names start with the E character.
16-bit registers
8-bit registers
Categories of registers
- General registers
- Control registers
- Segment registers
Categories of general registers
- Data registers
- Pointer registers
- Index registers
Data registers
There are four data 32-bit data registers: EAX, EBX, ECX, EDX
- Lower halves of the 32-bit registers can be used as four 16-bit data registers: AX, BX, CX and DX.
- 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.
Regarding the 64-bit processors: There are also four data 64-bit data registers: RAX, RBX, RCX, RDX
Pointer registers
There are three data 32-bit pointer registers: EIP, ESP, EBP
Their 16-bit equivalent are IP, SP, BP
Instruction Pointer (EIP)
The 32-bit **EIP** register holds the address of the next instruction to be executed. When combined with the **CS** register (as **CS:EIP**), it provides the full address of the current instruction within the code segment.
- The **CS:EIP** pair gives the logical address pointing to the instruction that the processor will execute next.
Stack Pointer (ESP)
The 32-bit **ESP** register holds the offset within the stack. When used with the **SS** register (**SS:ESP**), it refers to the current location in the stack.
- The **SS:ESP** pair provides the complete address within the stack segment.
Base Pointer (EBP)
The 32-bit **EBP** register helps in referencing the parameters passed to a subroutine. The address from the **SS** register is combined with the offset in **EBP** to locate the function parameters. **EBP** can also be used with **ESI** and **EDI** for special addressing.
- The **SS:EBP** pair points to the base of the current stack frame, facilitating access to function arguments and local variables.
Index registers
There are two data 32-bit pointer registers: ESI, EDI
Their 16-bit equivalent are SI, DI
Source Index (SI)
It is used as source index for string operations.
Destination Index (DI)
It is used as destination index for string operations.