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Processor Programming (Continued)

3.3.1.2

Segment Registers

The 16-bit segment registers, part of the main memory

addressing mechanism, are described in Section 3.5 “Off-

set, Segment, and Paging Mechanisms” on page 64. The six

segment registers are:

Code Segment

Data Segment

Stack Segment

Extra Segment

Additional Data Segment

The segment registers are used to select segments in main

memory. A segment acts as private memory for different

elements of a program such as code space, data space

and stack space.

There are two segment mechanisms, one for real and vir-

tual 8086 operating modes and one for protected mode.

Initialization and transition to protected mode is described

in Section 3.9.4 “Initialization and Transition to Protected

Mode” on page 93. The segment mechanisms are

described in Section 3.5.2 “Segment Mechanisms” on

page 66.

Theactivesegment register is selected accordingtothe

rules listed in Table 3-3 and the type of instruction being

currently processed. In general, the DS register selector is

used for data references. Stack references use the SS reg-

ister, and instruction fetches use the CS register. While

some selections may be overridden, instruction fetches,

stack operations, and the destination write operation of

string operations cannot be overridden. Special segment-

override instruction prefixes allow the use of alternate seg-

ment registers. These segment registers include the ES,

FS, and GS registers.

3.3.1.3

Instruction Pointer Register

The Instruction Pointer (EIP) register contains the offset

into the current code segment of the next instruction to be

executed. The register is normally incremented by the

length of the current instruction with each instruction exe-

cution unless it is implicitly modified through an interrupt,

exception, or an instruction that changes the sequential

execution flow (for example JMP and CALL).

Table 3-3 illustrates the code segment selection rules.

Table 3-3. Segment Register Selection Rules

Type of Memory Reference

Implied (Default)

Segment

Segment-Override

Prefix

Code Fetch

None

Destination of PUSH, PUSHF, INT, CALL, PUSHA instructions

None

Source of POP, POPA, POPF, IRET, RET instructions

None

Destination of STOS, MOVS, REP STOS, REP MOVS instructions

None

Other data references with effective address using base registers of:

EAX, EBX, ECX, EDX, ESI, EDI, EBP, ESP

CS,ES, FS,GS, SS

CS, DS, ES, FS, GS

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