Intel 80287, 80286 manual Real Address Mode

REAL ADDRESS MODE

Whenever the 80286 accesses memory in Real Address Mode, it generates a 20-bit physical address from a segment selector and offset value. The segment selector value is left-shifted four bit positions to form the segment base address. The offset is extended with 4 high order zeroes and added to the base to form the physical address (see figure 5-1).

Therefore, every segment is required to start at a byte address that is evenly divisible by 16; thus, each segment is positioned at a 20-bit physical address whose least significant four bits are zeroes. This arrangement allows the 80286 to interpret a segment selector as the high-order 16 bits of a 20-bit segment base address.

No limit or access checks are performed by the 80286 in the Real Address Mode. All segments are readable, writable, executable, and have a limit of OFFFFH (65,535 bytes). To save physical memory, you can use unused portions of a segment as another segment by overlapping the two (see figure 5-2). The Intel 8086 software development tools support this feature via the segment override and group operators. However, programs that access segment B from segment A become incompatible in the protected virtual address mode.

16 BIT SEGMENT SELECTOR

__________________	__________________
15	o

G30108

Figure 5-1a. Forming the Segment Base Address

SEGMENT BASE

+

OFFSET

19 15 o

PHYSICAL ADDRESS

19 o

G30108

Figure 5-1b. Forming the 20-bit Physical Address in the Real Address Mode

5-2