Intel 80287, 80286 manual Virtual-to-Physical Address Translation

MEMORY MANAGEMENT AND VIRTUAL ADDRESSING

VIRTUAL ADDRESS
I SELECTOR	I		I	TARGET
		OFFSET		SEGMENT
TI		0-
			PHYSICAL	DATUM
DESCRIPTOR			ADDRESS
I TABLE		I

SEGMENT

BASE

SEGMENT

DESCRIPTOR

----

INDEX

G3010B

Figure 6-6. Virtual-to-Physical Address Translation

As shown previously in figure 6-3, an 8-byte segment descriptor encodes the following information about a particular segment:

Size. This 16-bit field, comprising bytes 0 and 1 of a segment descriptor, specifies an unsigned integer as the size, in bytes (from 1 byte to 64K bytes), of the segment.

Unlike segments in the 8086 (or the 80286 in Real Address Mode)-which are never explicitly limited to less than a full 64K bytes-Protected Mode segments are always assigned a specific size value. In conjunction with the protection features described in Chapter 7, this assigned size allows the enforcement of a very desirable and natural rule: inadvertent accesses to locations beyond a segment's actual boundaries are prohibited.

Base. This 24-bit field, comprising bytes 2 through 4 of a segment descriptor, specifies the physi- cal base address of the segment; it thus defines the actual location of the segment within the 16- megabyte real memory space. The base may be any byte address within the 16-megabyte real memory space.

Access. This 8-bit field comprises byte 5 of a segment descriptor. This access byte specifies a variety of additional information about a segment, particularly in regard to the protection features of the 80286. Fer example, cede seg!TI.ents are distinguished from d~t~ ~~ement~; ~nd certain special access restrictions (such as Execute-Only or Read-Only) may be defined for segments of each type. Access byte values of OOH or 80H will alway~ denote "invalid."

Figure 6-7 shows the access byte format for both code and data segment descriptors. Detailed discus- sion of the protection related fields within an access byte (Conforming, Execute-Only, Descriptor Privi- lege Level, Expand Down, and Write-Permitted), and their use in implementing protection policies, is deferred to Chapter 7. The two fields Accessed and Present are used for virtual memory implementations.

6-8