User's Manual

THE 80286 INSTRUCTION SET
LOS/ LES-Load Doubleword Pointer
Opcode
C5 If
C4 If
FLAGS MODIFIED
None
Instruction
LDS rw,ed
LES rw,ed
FLAGS UNDEFINED
None
OPERATION
Clocks
7,pm=21
7,pm=21
Description
Load
EA doubleword into DS and word register
Load
EA doubleword into ES and word register
The four-byte pointer at the memory location indicated by the second operand is loaded into a segment
register and a word register. The first word of the pointer (the offset)
is loaded into the register indicated
by the first operand. The last word of the pointer (the selector)
is loaded into the segment register (DS
or ES) given by the instruction opcode.
When the segment register
is loaded, its associated cache is also loaded. The data for the cache is
obtained from the descriptor table entry for the selector given.
A null selector (values
0000-0003) can be loaded into DS or ES without a protection exception. Any
memory reference using such a segment register value will cause a
#GP(O) exception but will not result
in a memory reference. The saved segment register value
will be null.
Following is a list of checks and actions taken when loading the DS or ES registers:
If selector is non-null then:
Selector index must be within its descriptor table limits else
#GP (selector)
Examine descriptor
AR byte:
Data segment
or readable non-conforming code segment
Descriptor DPL
~ CPL else #GP (selector)
Descriptor DPL
~ selector RPL else #GP(selector)
Readable conforming code segment
No DPL, RPL,
or CPL checks
Else
#GP (selector)
Segment must be present else
#NP (selector)
Load registers from operand
Load segment register descriptor cache
If selector is null then:
Load registers from operand
Mark segment register cache as invalid
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