376 datasheet(25/95 Pages) INTEL

25 / 95 page

376 EMBEDDED PROCESSOR

vides both isolation and protection for a task’s seg-

ments while still allowing global data to be shared

among tasks

Unlike the 6-byte GDT or IDT registers which contain

a base address and limit the visible portion of the

LDT register contains only a 16-bit selector This se-

lector refers to a Local Descriptor Table descriptor in

the GDT (see Figure 21)

INTERRUPT DESCRIPTOR TABLE

The third table needed for 80376 systems is the In-

terrupt Descriptor Table The IDT contains the de-

scriptors which point to the location of up to 256

interrupt service routines The IDT may contain only

task gates interrupt gates and trap gates The IDT

should be at least 256 bytes in size in order to hold

the descriptors for the 32 Intel Reserved Interrupts

Every interrupt used by a system must have an entry

in the IDT The IDT entries are referenced by INT

instructions external interrupt vectors and excep-

tions

DESCRIPTORS

The object to which the segment selector points to

is called a descriptor Descriptors are eight-byte

quantities which contain attributes about a given

region of linear address space These attributes in-

clude the 32-bit logical base address of the seg-

ment the 20-bit length and granularity of the seg-

ment the protection level read write or execute

privileges and the type of segment All of the attri-

bute information about a segment is contained in 12

bits in the segment descriptor Figure 33 shows the

general format of a descriptor All segments on the

the 80376 have three attribute fields in common the

Present bit (P) the Descriptor Privilege Level bits

(DPL) and the Segment bit (S) Pe1 if the segment

is loaded in physical memory if P e 0 then any

attempt to access the segment causes a not present

exception (exception 11) The DPL is a two-bit field

which specifies the protection level 0 – 3 associated

with a segment

The 80376 has two main categories of segments

system segments and non-system segments (for

code and data) The segment bit S determines if a

given segment is a system segment a code seg-

ment or a 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

Note that although the 80376 is limited to a

16-Mbyte Physical address space (224) its base ad-

dress allows a segment to be placed anywhere in a

4-Gbyte linear address space When writing code for

the 80376 users should keep code portability to an

80386 processor (or other processors with a larger

physical address space) in mind A segment base

address can be placed anywhere in this 4-Gbyte lin-

ear address space but a physical address will be

BYTE

ADDRESS

SEGMENT BASE 150

SEGMENT LIMIT 150

BASE

LIMIT

BASE

3124

G10 V

1916

DPL

TYPE

2316

BASE

Base Address of the segment

LIMIT

The length of the segment

Present Bit

1 e Present

0 e Not Present

DPL

Descriptor Privilege Level 0 – 3

Segment Descriptor

0 e System Descriptor

1 e Code or Data Descriptor

TYPE

Type of Segment

Accessed Bit

Granularity Bit

1 e Segment length is 4 Kbyte Granular

0 e Segment length is byte granular

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

AVL

Available field for user or OS

Figure 33 Segment Descriptors

SEGMENT BASE 150

SEGMENT LIMIT 150

BASE

LIMIT

ACCESS

BASE

3124

G10 V

1916

RIGHTS

2316

BYTE

Granularity Bit

1 e Segment length is 4 Kbyte granular

0 e Segment length is byte granular

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

AVL Available field for user or OS

Figure 34 Code and Data Descriptors

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