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BASIC INSTRUCTION SET
Example: LDS SI, STRING_X. Loads
DS
with the word identifying the segment pointed to by
STRING-X,
and loads the offset of
STRING-X
into SI. Specifying SI as the destination
operand
is
a convenient
way
to
prepare for a string operation on a source string that
is
not
in the current data segment.
LES
(Load Pointer Using ES) operates identically
to
LDS except that ES receives the offset word
rather than DS.
Example: LES DI, DESTINATION-X. Loads ES with the word identifying the segment pointed to
by DESTINATION_X, and loads the offset of DESTINATION-X into DI. This instruc-
tion provides a convenient
way
to
select a destination for a string operation if the desired
location
is
not in the current extra segment.
3.9
FLAG
CONTROL INSTRUCTIONS
The flag control instructions provide a method of changing the state of bits in the flag register.
3.9.1 Carry Flag Control Instructions
The carry flag instructions are useful in conjunction with rotate-with-carry instructions RCL and RCR.
They can initialize the carry flag, CF,
to
a known state before execution of a rotate that moves the
carry bit into one end of the rotated operand.
STC
(Set Carry Flag) sets the carry flag (CF)
to
1.
Example: STC
CLC (Clear Carry Flag) zeros the carry flag (CF).
Example: CLC
CMC
(Complement Carry Flag) reverses the current status of the carry flag (CF).
Example: CMC
3.9.2
Direction Flag Control Instructions
The direction flag control instructions are specifically included to set or clear the direction flag, DF,
which controls the left-to-right or right-to-left direction of string processing.
IF
DF=O, the processor
automatically increments the string memory pointers, SI and DI, after each execution of a string primi-
tive.
If
DF=
1,
the processor decrements these pointer values. The initial state of DF
is
O.
CLD (Clear Direction Flag) zeros DF, causing the string instructions
to
auto-increment SI
and/or
DI.
CLD
does
not affect any other flags.
Example: CLD
STD
(Set Direction Flag) sets DF to
1,
causing the string instructions to auto-decrement SI
and/or
DI. STD
does
not affect any other flags.
Example: STD
3-25
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