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INTRODUCTION
instruction within the current code segment
for
an
intrasegment transfer, or to a different
code segment with
an
intersegment transfer.
The transfer
is
made unconditionally any
time the instruction
is
executed. An intra-
segment transfer
is
always made relative to
the current value
of
the instruction pointer.
Program segments which only use intraseg-
ment transfers are, therefore, relocatable in
memory. The conditional transfer instruc-
tions
mayor
may not transfer control,
depending on the state
of
the
CPU
flags
at
the time the instruction
is
executed.
The
18
instructions (Fig.
1-20),
each test a
different combination
of
flags for a condi-
tion.
If
the condition
is
true, control
is
transferred to the target address specified for
the instruction. If the condition
is
false, then
control passes to the instruction that follows
the conditional jump.
CONDITIONAL TRANSFERS
JA/JNBE
Jump
if above/ not below nor equal
JAE/JNB
Jump
if above
or
equal/not
below
JB/JNAE
Jump
if below
/not
above nor equal
JBE/JNA
Jump
if below
or
equal/not
above
JC Jump if carry
JE/JZ
Jump
if
equal/zero
JG/JNLE Jump if
greater/not
less nor equal
JGE/JNL
Jump if greater or
equal/not
less
JLlJNGE
Jump
if
less/not
greater nor equal
JLE/JNG Jump if less or
equal/not
greater
JNC Jump
if
not carry
JNE/JNZ
Jump
if not equal/ not zero
JNO
Jump
if not overflow
JNP/JPO
Jump
if not
parity/parity
odd
JNS
Jump if not sign
JO
Jump if overflow
JP/JPE Jump if
parity/parity
even
JS
Jump
if sign
NOTE:
The iteration control instructions regulate the
repetition of software loops. These instruc-
tions use the CX register as a counter. The
LOOPNE
instruction for instance decre-
ments a count, checks to
see
if the count
is
zero, and branches back to the beginning of
the program loop. The equivalent function
would require multiple instructions in an
older 8-bit instruction set, such
as
the 8080's.
The interrupt instructions allow interrupt
servic~
routines
to
be activated by both pro-
grams and external hardware devices. The
effect of software initiated interrupts is sim-
ilar to hardware initiated interrupts.
The processor control instructions (Fig.
1-21)
allow programs to control various
CPU
func-
tions to update flags and to synchronize the
8088
with external events. Finally, the
NOP
instruction causes the
8088
CPU
to do
nothing.
UNCONDITIONAL TRANSFERS
CALL Call procedure
RET
Return from procedure
JMP
Jump
ITERATION CONTROLS
LOOP
Loop
LOOPE/ LOOPZ Loop if
equal/zero
LOOPNE/ LOOPNZ
Loop if not
equal/not
zero
JCXZ Jump if register
CX
= 0
INTERRUPTS
INT
Interrupt
INTO Interrupt if overflow
IRET
Interrupt return
"Above" and "below" refer to the relationship
of
two unsigned values. "Greater" and "less" refer to the
relationship
of
two signed values.
Figure 1-20. Program Transfer Instructions
1-12