Vol. 1 3-11

BASIC EXECUTION ENVIRONMENT

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General-purpose registers. These eight registers are available for storing operands and pointers.

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Segment registers. These registers hold up to six segment selectors.

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EFLAGS (program status and control) register. The EFLAGS register report on the status of the program 
being executed and allows limited (application-program level) control of the processor. 

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EIP (instruction pointer) register. The EIP register contains a 32-bit pointer to the next instruction to be 
executed. 

3.4.1 General-Purpose 

Registers

The 32-bit general-purpose registers EAX, EBX, ECX, EDX, ESI, EDI, EBP, and ESP are provided for holding the 
following items:

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Operands for logical and arithmetic operations

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Operands for address calculations

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Memory pointers

Although all of these registers are available for general storage of operands, results, and pointers, caution should 
be used when referencing the ESP register. The ESP register holds the stack pointer and as a general rule should 
not be used for another purpose. 
Many instructions assign specific registers to hold operands. For example, string instructions use the contents of 
the ECX, ESI, and EDI registers as operands. When using a segmented memory model, some instructions assume 
that pointers in certain registers are relative to specific segments. For instance, some instructions assume that a 
pointer in the EBX register points to a memory location in the DS segment. 

Figure 3-4.  General System and Application Programming Registers

0

31

EAX
EBX
ECX
EDX
ESI
EDI
EBP
ESP

Segment Registers

CS
DS
SS
ES
FS
GS

0

15

0

31

EFLAGS

EIP

31

0

General-Purpose Registers

Program Status and Control Register

Instruction Pointer