Intel 64 and IA-32 Architectures Software Developers Manual Volume 2B, Instruction Set Reference, N-Z

B-6 Vol. 2B

INSTRUCTION FORMATS AND ENCODINGS

B.1.4.4 Sign-Extend (s) Bit

The sign-extend (s) bit occurs in instructions with immediate data fields that are

being extended from 8 bits to 16 or 32 bits. See Table B-7.

B.1.4.5 Segment Register (sreg) Field

When an instruction operates on a segment register, the reg field in the ModR/M byte

is called the sreg field and is used to specify the segment register. Table B-8 shows

the encoding of the sreg field. This field is sometimes a 2-bit field (sreg2) and other

times a 3-bit field (sreg3).

NOTES:

1. Do not use reserved encodings.

Table B-7. Encoding of Sign-Extend (s) Bit

Effect on 8-Bit

Immediate Data

Effect on 16- or 32-Bit

Immediate Data

0None None

1 Sign-extend to fill 16-bit or 32-bit destination None

Table B-8. Encoding of the Segment Register (sreg) Field

2-Bit sreg2 Field

Segment Register

Selected 3-Bit sreg3 Field

Segment Register

Selected

00 ES 000 ES

01 CS 001 CS

10 SS 010 SS

11 DS 011 DS

100 FS

101 GS

110 Reserved

111 Reserved