Intel 80386 manual Differences Between 386 and 286 Descriptors, Segment Base 15, Selector Fields

intJ80386

Task gates are used to switch tasks. Task gates may only refer to a task state segment (see section

4.4.6Task Switching) therefore only the destination selector portion of a task gate descriptor is used, and the destination offset is ignored.

Exception 13 is generated when a destination selec- tor does not refer to a correct descriptor type, i.e., a code segment for an interrupt, trap or call gate, a TSS for a task gate.

The access byte format is the same for all gate de- scriptors. P= 1 indicates that the gate contents are valid. P = 0 indicates the contents are not valid and causes exception 11 if referenced. DPL is the de- scriptor privilege level and specifies when this de- scriptor may be used by a task (see section 4.4 Protection). The S field, bit 4 of the access rights byte, must be 0 to indicate a system control descrip- tor. The type field specifies the descriptor type as indicated in Figure 4-8.

4.3.4.7DIFFERENCES BETWEEN 386 AND 286 DESCRIPTORS

In order to provide operating system compatibility between the 80286 and 80386, the 386 supports all of the 80286 segment descriptors. Figure 4-9 shows the general format of an 80286 system segment de- scriptor. The only differences between 286 and 386 descriptor formats are that the values of the type fields, and the limit and base address fields have been expanded for the 386. The 80286 system seg- ment descriptors contained a 24-bit base address and 16-bit limit, while the 386 system segment de- scriptors have a 32-bit base address, a 20-bit limit field, and a granularity bit.

By supporting 80286 system segments the 80386 is able to execute 286 application programs on a 80386 operating system. This is possible because the processor automatically understands which de-

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SEGMENT BASE 15 ... 0

Intel Reserved

Set to 0

BASE	Base Address of the segment
LIMIT	The length of the segment
P	Present Bit 1 =Present 0 = Not Present

scriptors are 286-style descriptors and which de- scriptors are 386-style descriptors. In particular, if the upper word of a descriptor is zero, then that de- scriptor is a 286-style descriptor.

The only other differences between 286-style de- scriptors and 386 descriptors is the interpretation of the word count field of call gates and the B bit. The word count field specifies the number of 16-bit quan- tities to copy for 286 call gates and 32-bit quantities for 386 call gates. The B bit controls the size of PUSHes when using a call gate; if B = 0 PUSHes are 16 bits, if B = 1 PUSHes are 32 bits.

4.3.4.8 SELECTOR FIELDS

A selector in Protected Mode has three fields: Local or Global Descriptor Table Indicator (TI), Descriptor Entry Index (Index), and Requestor (the selector's) Privilege Level (RPL) as shown in Figure 4-10. The TI bits select one of two memory-based tables of descriptors (the Global Descriptor Table or the Local Descriptor Table). The Index selects one of 8K de- scriptors in the appropriate descriptor table. The RPL bits allow high speed testing of the selector's privilege attributes.

4.3.4.9 SEGMENT DESCRIPTOR CACHE

In addition to the selector value, every segment reg- ister has a segment descriptor cache register asso- ciated with it. Whenever a segment register'scon- tents are changed, the 8-byte descriptor associated with that selector is automatically loaded (cached) on the chip. Once loaded, all references to that seg- ment use the cached descriptor information instead of reaccessing the descriptor. The contents of the descriptor cache are not visible to the programmer. Since descriptor caches only change when a seg- ment register is changed, programs which modify the descriptor tables must reload the appropriate segment registers after changing a descriptor'sval- ue.

				o
SEGMENT LIMIT 15 ... 0				o
P \ DPL	lsi	TYPE	\	23 ... 16 +4
				BASE
DPL		..
	DeSCriptor PnVllege Level 0-3

SSystem Descriptor 0 = System 1 = User

TYPE Type of Segment

Figure 4-9. 286 Code and Data Segment Descriptors

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