1. Jump Condition

2. Topic Control Flow Structure Control Flow Structures
Conditional Jump
Unconditional Jump
Control Flow Structures
IF-THEN
IF-THEN-ELSE
CASE
Branches with Compound Conditions

3. Labels r needed in cases where one instruction refers to another.
An Example of Jump
Display the entire IBM character set
The function number
.MODEL SMALL
.CODE
.STARTUP
MOV AH, 2 ; display char function
MOV CX, 256 ; no. of chars to display
MOV DL, 0 ; DL has ASCII code for null char
PRINT_LOOP:
INT 21H ; display a char
INC DL ; increment ASCII code
DEC CX ; decrement counter
JNZ PRINT_LOOP: ; keep going if CX not 0
.EXIT
END
Calls system routine/functions
Statement label
Labels r needed in cases where one instruction refers to another.
Section 6-1 of Assembly Language Programming Book

4. Conditional Jumps JNZ Syntax: Jxxx destination_label
True or False [no gray area – like our minds!]
JNZ is an example of conditional jump instruction
Checks the Z flag. If Z = 0 then jump to the location
Three categories of conditional jumps
Signed jumps, for signed interpretation
Unsigned jumps, for unsigned interpretation
Single-flag jumps, operates on settings of individual flags

5. How to decide? Implement?
CPU looks at the FLAGS register
If jump conditions r TRUE – the CPU adjusts the IP  to point to the destination_label,
so that the instruction at this label will be done next.
If FALSE – no change in IP

6. 1. Signed Conditional Jumps
Opcodes
Description
Condition for jumps
JG/JNLE
Jump if Greater than
Jump if Not Less than or Equal to
ZF = 0 and SF = OF
JGE/JNL
Jump if Greater than or Equal to
Jump if Not Less than
SF = OF
JL/JNGE
Jump if Less than
Jump if Not Greater than or Equal to
SF <> OF
JLE/JNG
Jump if less than or equal
Jump if not greater than
ZF = 1 or SF <> OF

7. 2. Unsigned Conditional Jumps
Symbol
Description
Condition for jumps
JA/JNBE
Jump if above
Jump if not below or equal
CF = 0 and ZF = 0
JAE/JNB
Jump if above or equal
Jump if not below
CF = 0
JB/JNAE
Jump if below
Jump if not above or equal
CF = 1
JBE/JNA
Jump if below or equal
Jump if not above
CF = 1 or ZF = 1

8. 3. Single-Flag Jumps Symbol Description Condition for jumps JE/JZ
Jump if equal
Jump if equal to zero
ZF = 1
JNE/JNZ
Jump if not equal
Jump if not zero
ZF = 0 JC
Jump if carry
CF = 1
JNC
Jump if no carry
CF = 0 JO
Jump if overflow
OF = 1
JNO
Jump if no overflow
OF = 0 JS
Jump if sign negative
SF = 1
JNS
Jump if nonnegative sign
SF = 0
JP/JPE
Jump if parity even
PF = 1
JNP/JPO
Jump if parity odd
PF = 0

9. Range of a Conditional Jump
ref
Range of a Conditional Jump
The destination label must precede the Jump instruction by no more than 126 bytes
Or, follow by no more than 127 bytes
LABEL:
; statement
JNZ LABEL
126 bytes
JZ LABEL
; statements
LABEL:
; statement
127 bytes

11. CMP Instruction
The jump condition is often provided by the CMP (compare) instruction
CMP destination, source
 dest[contents] – source[contents]
It is like SUB, except that destination is not changed
Destination may not be a constant
The result is not stored but the flags are affected
CMP AX, 10
JG BELOW
CMP AX, BX
JG BELOW ;JG – jump if >
If AX = 7FFFh, and BX = 0001h, the result is 7FFFh h = 7FFEh.
ZF = SF = OF = 0, JG is satisfied, so control transfers to label BELOW

12. Signed vs. Unsigned Jumps
Each signed jump corresponds to an analogous unsigned jump
e.g., signed JG (if >) corresponds to unsigned JA (if above)
Use depends on the interpretation
The jumps operate on different flags
Symbol
Description
Condition for jumps
JG/JNLE
Jump if greater than
Jump if not less than or equal to
ZF = 0 and SF = OF
JA/JNBE
Jump if above
Jump if not below or equal
CF = 0 and ZF = 0
Wrong jumps  wrong results! [same as life]

13. Until 4/11
Wrong jumps  wrong results! [same as life]

14. Signed vs. Unsigned Jumps cont.
For signed interpretation, let us take
AX = 7FFFh, BX = 8000h and we execute
Even though 7FFFh > 8000h in a signed sense, the program does not jump to BELOW_LABEL  why?
Because 7FFFh < 8000h in an unsigned sense
JA, which is the unsigned jump
CMP AX, BX
JA BELOW_LABEL

15. Signed vs. Unsigned Jumps cont. working with CHAR
With standard ASCII character set [character code 0-31 for control chars; for printable characters] – either signed/unsigned jumps may be used.
Why?
Because the sign bit of a byte containing a character code is always  zero [0].
BUT, unsigned jumps should be used when comparing extended ASCII characters [code 80h ~ FFh]

16. Extended ASCII codes (character code 128-255)
There are several different variations of the 8-bit ASCII table. E.g., ISO , also called ISO Latin-1. Codes contain the Microsoft® Windows Latin-1 extended characters.

17. The JMP Instruction JMP destination/target_label
ref
The JMP Instruction
JMP (jump) instruction causes an unconditional jump
Syntax is:
JMP can be used to get around the range restriction [126/127 byte]
Flags – no change
JMP destination/target_label
TOP:
; the loop body contains so many instructions
; that label TOP is out of range for JNZ. Solution is-
DEC CX
JNZ BOTTOM
JMP EXIT
BOTTOM:
JMP TOP
EXIT:
MOV AX, BX
TOP:
; body of the loop, say 2 instructions
DEC CX ; decrement counter
JNZ TOP ; keep looping if CX > 0
MOV AX, BX
Section 6-3: Assembly Language Programming

18. TOP: EXIT: When CX=0 - It will not Jump to BOTTOM
It will go to next instr. JMP EXIT
JMP TOP  is unconditional – just Jump!
TOP:
; the loop body contains so many instructions
; that label TOP is out of range for JNZ Solution is-
DEC CX
JNZ BOTTOM
JMP EXIT
BOTTOM:
JMP TOP
EXIT:
MOV AX, BX

19. Lets Jump to another lecture?

20. References Ch 6, Assembly Language Programming – by Charls Marut
Some materials are from Dr. Sazzad, NSU