CPU Register - General Purpose Register (GPR)

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1 - About

The General purpose registers are available for storing:

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3 - Mode

3.1 - 32-bit

Register Name Use
EAX Accumulator Accumulator for operands and results data, Used in arithmetic operations
EBX Base Pointer to data in the DS segment
ECX Counter Counter for string, loop and shift/rotate operations
EDX Data I/O pointer, used in arithmetic operations and I/O operations.
Stream Operations
ESI Source Index Pointer to data in the segment pointed to by the DS register; source pointer for string operations, Used as a pointer to a source in stream operations
EDI Destination Index Pointer to data (or destination) in the segment pointed to by the ES register; destination pointer for string operations, Used as a pointer to a destination in stream operations
Stack
ESP Stack pointer Stack pointer (in the SS segment), Pointer to the top of the stack. As a general rule it should not be used for another purpose.
EBP Base Pointer Base Pointer to data on the stack (in the SS segment), point to the base of the stack
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3.2 - 16-bit

The following bit can be referenced by name:

  • The lower 16 bits of the 32-bit general-purpose registers that map directly to the register set found in the 8086 and Intel 286 processors (.X)
  • Each of the lower two bytes of the registers (.H for high and .L for low)

16-bit register high bytes name low bytes 32-bit register
AX AH AL EAX
BX BH BL EBX
CX CH CL ECX
DX DH DL EDX
BP EBP
SI ESI
DI EDI
SP ESP
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3.3 - 64-bit

The Registers only available in 64-bit mode are R8-R15 and XMM8-XMM15.

Addressable General Purpose Registers:

operand size Register Type Available Registers (With Rex) Available Registers (Without REX)
8-bit Byte Registers AL, BL, CL, DL, DIL, SIL, BPL, SPL, R8L - R15L AL, BL, CL, DL, AH, BH, CH, DH
16-bit Word Registers AX, BX, CX, DX, DI, SI, BP, SP, R8W - R15W AX, BX, CX, DX, DI, SI, BP, SP
32-bit Doubleword Registers EAX, EBX, ECX, EDX, EDI, ESI, EBP, ESP, R8D - R15D EAX, EBX, ECX, EDX, EDI, ESI, EBP, ESP
64-bit Quadword Registers RAX, RBX, RCX, RDX, RDI, RSI, RBP, RSP, R8-R15 NA

REX prefixes are used to generate 64-bit operand sizes or to reference registers R8-R15.

4 - Documentation / Reference

  • Vol1 - 3.4 - Basic Program execution registers - https://software.intel.com/en-us/articles/intel-sdm
  • The special uses of general-purpose registers by instructions are described in Chapter 5, “Instruction Set Summary,” in this volume. See also: Chapter 3, Chapter 4 and Chapter 5 of Intel® 64 and IA-32 Architectures Software Developer’s Manual, Volumes 2A, 2B & 2C
computer/cpu/register/general.txt · Last modified: 2019/01/07 09:22 by gerardnico