B-4
INSTRUCTION FORMATS AND ENCODINGS
B.1.4.Segment Register Field (sreg)
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-6 shows the encoding of the
sreg field. This field is sometimes a 2-bit field (sreg2) and other times a 3-bit field (sreg3).
* Do not use reserved encodings.
B.1.5.Special-Purpose Register (eee) Field
When the control or debug registers are referenced in an instruction they are encoded in the eee
field, which is located in bits 5, 4, and 3 of the ModR/M byte. Table B-7 shows the encoding of
the eee field.
* Do not use reserved encodings.
Table B-6. 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*
Table B-7. Encoding of Special-Purpose Register (eee) Field
eee
Control Register
Debug Register
000
CR0
DR0
001
Reserved*
DR1
010
CR2
DR2
011
CR3
DR3
100
CR4
Reserved*
101
Reserved*
Reserved*
110
Reserved*
DR6
111
Reserved*
DR7
