Intel 80386 manual Dptm typ , data gm,nt

80386

attributes include the 32-bit base linear address of the segment, the 20-bit length and granularity of the segment, the protection level, read, write or execute privileges, the default size of the operands (16-bit or 32-bit), and the type of segment. All of the attribute information about a segment is contained in 12 bits in the segment descriptor. Figure 4-5 shows the gen- eral format of a descriptor. All segments on the 80386 have three attribute fields in common: the P bit, the DPL bit, and the S bit. The Present P bit is 1 if the segment is loaded in physical memory, if P= 0 then any attempt to access this segment causes a not present exception (exception 11). The Descrip- tor Privilege Level DPL is a two-bit field which speci- fies the protection level 0-3 associated with a seg- ment.

The 80386 has two main categories of segments system segments and non-system segments (for code and data). The segment S bit in the segment descriptor determines if a given segment is a system segment or a code or data segment. If the S bit is 1 then the segment is either a code or data segment, if it is 0 then the segment is a system segment.

4.3.4.2386 CODE, DATA DESCRIPTORS (S= 1)

Figure 4-6 shows the general format of a code and data descriptor and Table 4-1 illustrates how the bits in the Access Rights Byte are interpreted.

31				o
SEGMENT BASE 15	... 0		SEGMENT LIMIT 15 ... 0		o
		LIMIT	ACCESS	BASE
BASE 31 ... 24 G	0 0 0		RIGHTS		+4
		19 ... 16		23 ... 16
			BYTE
DIB 1 ~ Default Instructions Attnbutes are 32·81ts			G Granulanty Bit	1 - Segment length IS page granular
o~ Default Instruction Attributes are 16·8its				o~ Segment length is byte granular

Bit must be zero (0) for compatibility with future processors

Figure 4-6. Segment Descriptors

Table 4-1. Access Rights Byte Definition for Code and Data Descriptions

Bit

NameFunction

Position

7	Present (P)	P =	1	Segment is mapped into physical memory.
		P =	0	No mapping to physical memory exits, base and limit are
				not used.
6-5	Descriptor Privilege			Segment privilege attribute used in privilege tests.
	Level (DPL)
4	Segment Descrip-	S =	1	Code or Data (includes stacks) segment descriptor
	tor (S)	S=O		System Segment Descriptor or Gate Descriptor

	3	Executable (E)	E	0		D'''';ptm typ' ;, data "gm,nt	rData
	2	Expansion Direc-	ED~=		0 Expand up segment, offsets must be ,,; limit.
		tion (ED)	ED =		1 Expand down segment, offsets must be > limit.		Segment
	1	Writeable (W)	W =	0		Data segment may not be written into.	(S =	1,
Type			W =	1		Data segment may be written into.	E = 0)
Field	3	Executable (E)	E =	1		Descriptor type is code segment:
Definition
	2	Conforming (C)	C =	1		Code segment may only be executed	Code
						when CPL :2: DPL and CPL	Segment
						remains unchanged.	(S =	1,
	1	Readable (R)	R=O			Code segment may not be read.	E =	1)
			R = 1 Code segment may be read.				r
	0	Accessed (A)	A=O			Segment has not been accessed.
			A=1		Segment selector has been loaded into segment register
						or used by selector test instructions.
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