User's Manual

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
~ __________________ ~A, __________________ ~
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