Microprocessors
Chapter 7
Program Control Instructions
prepared by
Dr. Mohamed A. Shohla
Chapter Overview
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The Jump Group
Unconditional Jump (JMP)
Conditional Jumps and Conditional Sets
Loop
Procedures
CALL
RET
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Microprocessors Course
6-2
The Jump Group
• The main program control instruction, jump (JMP), allows the
programmer to skip sections of a program and branch to any part of
the memory for the next instruction.
• A conditional jump instruction allows the programmer to make
decisions based upon numerical tests.
• The results of numerical tests are held in the flag bits, which are
then tested by conditional jump instructions.
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Microprocessors Course
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Unconditional Jump (JMP)
• Three types of unconditional jump instructions are available to the
microprocessor: short jump, near jump, and far jump.
• The short jump is a two-byte instruction that allows jumps or
branches to memory locations within +127 and -128 bytes from
the address following the jump.
• The three-byte near jump allows a branch or jump within ±32K
bytes from the instruction in the current code segment.
• Finally, the five-byte far jump allows a jump to any memory
location within the real memory system. The short and near jumps
are often called intrasegment jumps, and the far jumps are often
called intersegment jumps.
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Faculty of Electronic Engineering – Dept. of Computer Science & Eng.
Microprocessors Course
6-4
Short Jump:
• Short jumps are called relative jumps because they can be
moved, along with their related software, to any location in the
current code segment without a change.
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‫جام عة الم نوف ية‬
A short jump to four
memory locations
beyond the address
of the next
instruction.
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Microprocessors Course
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Near Jump:
• The near jump is similar to the short jump, except that the distance
is farther.
• A near jump passes control to an instruction in the current code
segment located within ±32K bytes from the near jump instruction.
The distance is ±2G in the 80386 and above when operated in
protected mode.
A near jump that adds the
displacement (0002H) to
contents of IP.
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Microprocessors Course
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Far Jump:
• A far jump instruction obtains a new segment and offset address
to accomplish the jump.
•A far jump instruction
replaces the contents of
both CS and IP with
four bytes following the
opcode.
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Faculty of Electronic Engineering – Dept. of Computer Science & Eng.
Microprocessors Course
6-7
Conditional Jumps and Conditional Sets
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Conditional jumps are always short jumps in the 8087-80286
microprocessors. This limits the range of the jump to within +127 bytes
and -128 bytes from the location following the conditional jump.
In the 80386/80486 conditional jumps are either short or near jumps.
Instruction
JA
JAE
JB
JBE
JC
JE or JZ
JG
JGE
JL
JLE
JNC
JNE or JNZ
JNO
JNS
JNP/JPO
JO
JP/JPE
JS
JCXZ
JECXZ
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Condition Tested
C = 0 and Z = 0
C=0
C=1
C = 1 or Z = 1
C=1
Z=1
Z = 0 and S = 0
S=0
S≠0
Z = 1 or S ≠ 0
C=0
Z=0
0=0
S=0
P=0
0=1
P= 1
S=1
CX = 0
ECX = 0
Comment
Jump above
Jump above or equal to
Jump below
Jump below or equal to
Jump carry set
Jump equal to or jump zero
Jump greater than
Jump greater than or equal to
Jump less than
Jump less than or equal to
Jump carry cleared
Jump not equal to or jump not zero
Jump no overflow
Jump no sign
Jump no parity/jump parity odd
Jump on overflow
Jump on parity/jump parity even
Jump on sign
Jump if CX = 0
Jump if ECX = 0 (80386/80486 only)
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Microprocessors Course
6-8
•A procedure that searches a table of 100
bytes for a 0AH
SCAN PROC NEAR
MOV SI, OFFSET TABLE
MOV CX,100
MOV AL,OAH
CLD
REPNE SCASB
JCXZ NOT_FOUND
RET
SCAN ENDP
‫جام عة الم نوف ية‬
;address TABLE
;load count
;load search data
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Microprocessors Course
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Loop
• The LOOP instruction is a combination of a decrement CX and a
conditional jump.
• LOOP decrements CX and if CX <> 0, it jumps to the address
indicated by the label.
• If CX becomes a 0, the next sequential instruction executes.
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Microprocessors Course
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•A procedure that adds word in BLOCK1
and BLOCK2
ADDS PROC NEAR
MOV CX,100
MOV SI,OFFSET BLOCK1
MOV DI,OFFSET BLOCK2
AGAIN:
LODSW
ADD AX,ES:[DI]
STOSW
LOOP AGAIN
ADDS ENDP
‫جام عة الم نوف ية‬
;load count
;address BLOCK1
;address BLOCK2
;get BLOCK1 data
;add BLOCK2 data
;store in BLOCK2
;repeat 100 times RET
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Microprocessors Course
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Procedures
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The procedure or subroutine is an important part of any computer
system’s architecture.
A procedure is a group of instructions that usually performs one task.
A procedure is a reusable section of the software that is stored in
memory once, but used as often as necessary.
This saves memory space and makes it easier to develop software.
The only disadvantage of a procedure is that it takes the computer a
small amount of time to link to the procedure and return from it.
Example
SUMS
SUMS
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PROC
ADD
ADD
ADD
RET
ENDP
NEAR
AX, BX
AX, CX
AX, DX
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Microprocessors Course
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CALL
The CALL instruction transfers the flow of the program to the procedure.
The CALL instruction differs from the jump instruction because a CALL
saves a return address on the stack.
The return address returns control to the instruction that follows the
CALL in a program when a RET instruction executes.
The effect of a near
call instruction on
the stack and the
SP, SS, and IP
registers. Notice
how the old IP is
stored on the
stack.
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Microprocessors Course
6 - 13
•
•
RET
The return instruction (RET) removes either a 17-bit number (near
return) from the stack and places it into IP or a 32-bit number (far return)
and places it into IP and CS.
The near and far return instructions are both defined in the procedure's
PROC directive. This automatically selects the proper return instruction.
The effect of a
near RET
instruction on the
stack and the SP,
SS, and IP
registers.
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Microprocessors Course
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Chapter 7
Questions and Answers
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Faculty of Electronic Engineering – Dept. of Computer Science & Eng.
Microprocessors Course
6 - 15
Which type of JMP instruction (short, near,
or far) assembles for the following:
(a) if the distance is 0210H bytes
(b) if the distance is 0020H bytes
(c) if the distance is 10000H bytes
(a) near
(b) short
(c) far
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Faculty of Electronic Engineering – Dept. of Computer Science & Eng.
Microprocessors Course
6 - 16
Explain what the JMP AX instruction
accomplishes. Also identify it as a near
or a far jump instruction.
•
‫جام عة الم نوف ية‬
The JMP AX instruction is a near jump to the
offset address loaded in AX.
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Microprocessors Course
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Contrast the operation of a JMP [DI]
with a JMP FAR PTR [DI].
The JMP [DI] instruction is a near jump that obtain the jump
address from the data segment memory location
addressed by DI. The other jump is a far jump.
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Faculty of Electronic Engineering – Dept. of Computer Science & Eng.
Microprocessors Course
6 - 18
(Develop a short sequence of instructions
that stores a 00H into 150H bytes of
memory, beginning at extra segment
memory location DATA. You must use the
LOOP instruction to help perform this task.
MOV
CLD
MOV
MOV
DI, OFFEST DATA
CX, 150H
AL, 0
AGAIN:
STOSB
LOOP AGAIN
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Faculty of Electronic Engineering – Dept. of Computer Science & Eng.
Microprocessors Course
6 - 19
Develop a sequence of instructions that searches through a
block of 100H bytes of memory. This program must count all
the unsigned numbers that are above 42H and all that are
below 42H. Byte-sized data segment memory location UP must
contain the count of numbers above 42H, and data segment
location DOWN must contain the count of numbers below 42H.
UP
DB
DOWN DB
MOV
MOV
MOV
AGAIN:CMP
JA
INC
JMP
L1:
INC
L2:
DEC
LOOP
‫جام عة الم نوف ية‬
0
0
DI, OFFSET BLOCK
CX, 100
AL, 42H
AL, [DI]
L1
DOWN
L2
UP
DI
AGAIN
;address data
;load counter
;put 66 IN AL
;search
Faculty of Electronic Engineering – Dept. of Computer Science & Eng.
Microprocessors Course
6 - 20
Develop a sequence of instructions that shows how
data in one block of memory (BLOCK1) adds to data
in a second block (BLOCK2) using LOOP to control
how many numbers add. Store the result over top of
data in BLOCK2
;procedure that adds word in BLOCK1 and BLOCK2
ADDS
PROC NEAR
MOV
CX,100
;load count
MOV
SI, OFFSET BLOCK1
MOV
DI, OFFSET BLOCK2
AGAIN: LODSW
;get BLOCK1 data
ADD
AX,ES:[DI]
;add BLOCK2 data
STOSW
;store in BLOCK2
LOOP AGAIN
;repeat 100 times
RET
ADDS ENDP
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Faculty of Electronic Engineering – Dept. of Computer Science & Eng.
Microprocessors Course
6 - 21
Write a program to fill 1000H-byte block of
memory in the extra segment beginning at
address BLOCK with the data byte 20H
(ASCII space).
MOV
MOV
MOV
CLD
REP
‫جام عة الم نوف ية‬
CX,1000H
DI, OFFSET BLOCK
AL, 20H
;load count
;address BLOCK
STOSB
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