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Intel Pentium CPU Instruction Set Reference

Basic Architecture Overview

General purpose registers

EAXAccumulator. Accumulator for operands and results data.
EBXBase. Pointer to data in the DS segment.
ECXCount. Counter for string and loop operations.
EDXData. I/O pointer.
ESISource Index. Pointer to data in the segment pointed to by the DS register; source pointer for string operations.
EDIDestination Index. Pointer to data (or destination) in the segment pointed to by the ES register; destination pointer for string operations.
EBPBase Pointer. Pointer to data on the stack (in the SS segment).
EAX (32-bit)
 AX (16-bit)
AH (8-bit)AL (8-bit)

EBX (32-bit)
 BX (16-bit)
BH (8-bit)BL (8-bit)

ECX (32-bit)
 CX (16-bit)
CH (8-bit)CL (8-bit)

EDX (32-bit)
 DX (16-bit)
DH (8-bit)DL (8-bit)

EBP (32-bit)
 BP (16-bit)

ESI (32-bit)
 SI (16-bit)

EDI (32-bit)
 DI (16-bit)

ESP (32-bit)
 SP (16-bit)

Segment registers

CSCode Segment.
DSData Segment.
SSStack Segment.
ESExtra Data Segment.
FSExtra Data Segment.
GSExtra Data Segment.

Special purpose registers

EIPInstruction Pointer. Pointer to the next instruction in a code segment. Automatically updated.
ESPStack pointer. Pointer to the top of the stack (in the SS segment). Used with POP, PUSH, CALL, etc.
EIP (32-bit)
 IP (16-bit)

ESP (32-bit)
 SP (16-bit)

EFLAGS register

EFLAGSFlags. The state of various conditions in the cpu. The purpose of individual bits of the EFLAGS register is described below.

15141312 11100908 07060504 03020100
  Nested Task I/O privilege level Overflow Direction Interrupt Trap Sign Zero   Auxiliary Carry   Parity   Carry
31302928 27262524 23222120 19181716
             Identification Virtual Interrupt Pending Virtual Interrupt Flag Alignment Check Virtual Mode Resume