User's Manual

80286 BASE ARCHITECTURE
Most memory references do not require the instruction to specify a full 32-bit pointer address. Operands
that are located within one of the currently addressable segments, as determined by the four segment
registers (see section 2.3.2,
"Segment Registers"), can be referenced very efficiently simply by means
of the 16-bit offset. This form of address
is called by short address. The choice of segment (CS, DS,
ES,
or SS) is either implicit within the instruction itself or explicitly specified by means of a segment
override prefix (see below).
See figure 2-11 for a diagram of the addressing process.
2.4.3.1 SEGMENT SELECTION
All instructions that address operands in memory must specify the segment and the offset. For speed
and compact instruction encoding, segment selectors are usually stored in the high speed segment
registers. An instruction need specify only the desired segment register and an offset in order to address
a memory operand.
Most instructions need not explicitly specify which segment register
is used. The correct segment regis-
ter is automatically chosen according to the rules of table 2-1 and table 2-2. These rules follow the way
programs are written (see figure 2-12) as independent modules that require areas for code and data, a
stack, and access to external data areas.
There
is a close connection between the type of memory reference and the segment in which that
operand resides (see the next section for a discussion of how memory addressing mode calculations are
performed).
As a rule, a memory reference implies the current data segment (Le., the implicit segment
selector
is in DS) unless the BP register is involved in the address specification, in which case the
current stack segment
is implied (i.e, SS contains the selector).
POINTER
I SEGMENT OFFSET
31 16 15 0
-----r
~--~~------~
OPERAND
SELECTED SELECTED
SEGMENT
1
MEMORY
1
G3010B
Figure 2-11. Two-Component Address
2-18