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)
EIP register stores the offset address of the next instruction to be executed. The **IP** in association with the **CS** register (as **CS:IP**) gives the complete address of the current instruction in the code segment.
- **CS:IP** is a logical address that points to the current instruction in memory.
- Example:
* If **CS = 0x1000** and **IP = 0x0040**, the logical address of the instruction would be **0x1000:0x0040**, which is the address the CPU will fetch the next instruction from.
Stack Pointer (SP)
The 16-bit **SP** register holds the offset value within the program stack. **SP** in association with the **SS** register (**SS:SP**) refers to the current position of data or address within the program stack.
- **SS:SP** provides the complete address within the stack.
- Example:
* If **SS = 0x2000** and **SP = 0x0100**, the full address of the stack data is **0x2000:0x0100**.
Base Pointer (BP)
The 16-bit **BP** register helps in referencing the parameter variables passed to a subroutine. The address in the **SS** register is combined with the offset in **BP** to locate the parameters. **BP** can also be combined with **DI** and **SI** for special addressing.
- **SS:BP** points to the base of the current stack frame.
- Example:
* In a function call, **BP** could point to the start of the parameters passed to the function, allowing easy access to them. * For example, **BP + 2** might refer to the first argument of a function, while **BP - 4** could refer to a local variable.
Example Code
Here is an example of using the **IP**, **SP**, and **BP** registers:
push ax ; Pushes AX onto the stack mov bp, sp ; BP now points to the top of the stack (same as SP at this point) sub sp, 2 ; Adjust SP for local variables (reserve space) mov [bp-2], 0x1234 ; Store 0x1234 at [BP-2], which is a local variable