Assembly Language for Intel 8086 - PowerPoint Presentation

Slide1

Assembly Language for Intel 8086Jump Condition

Ch

6, Assembly Language Programming – by

Charls

Marut

Some materials are from Dr.

Sazzad

, NSUSlide2

TopicControl Flow StructureConditional Jump

Unconditional Jump

Control Flow Structures

Branches with Compound ConditionsSlide3

An Example of JumpDisplay the entire IBM character set

.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 charPRINT_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

Section 6-1 of Assembly Language Programming Book

The function number

Statement label

Labels r needed in cases where one instruction refers to another.Slide4

Conditional JumpsJNZ 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 jumpsSigned jumps, for signed interpretationUnsigned jumps, for unsigned interpretationSingle-flag jumps, operates on settings of individual flagsSlide5

How to decide? Implement?CPU looks at the FLAGS registerIf jump conditions r TRUE – the CPU adjusts the IP [instruction pointer 3.2.5] to point to the

destination_label

,

so that the instruction at this label will be done next.

If FALSE – no change in IPSlide6

1. Signed Conditional Jumps

Opcodes

Description

Condition for

jumps

JG/JNLE

Jump if Greater thanJump if Not Less than or Equal toZF = 0 and SF = OFJGE/JNLJump if Greater than or Equal toJump if Not Less thanSF = OFJL/JNGEJump if Less thanJump if Not Greater than or Equal toSF <> OFJLE/JNGJump if less than or equal

Jump if not greater thanZF = 1 or SF <> OFSlide7

2. Unsigned Conditional Jumps

Symbol

Description

Condition for

jumps

JA/JNBE

Jump if aboveJump if not below or equalCF = 0 and ZF = 0JAE/JNBJump if above or equalJump if not belowCF = 0JB/JNAEJump if belowJump if not above or equalCF = 1JBE/JNAJump if below or equalJump if not aboveCF = 1 or ZF = 1Slide8

3. Single-Flag Jumps

Symbol

Description

Condition for

jumps

JE/JZ

Jump if equalJump if equal to zeroZF = 1JNE/JNZJump if not equalJump if not zeroZF = 0JCJump if carryCF = 1JNCJump if no carryCF = 0JOJump if overflowOF = 1JNO

Jump if no overflowOF = 0

JSJump if sign negative

SF = 1JNS

Jump if nonnegative sign

SF = 0JP/JPE

Jump if parity evenPF = 1

JNP/JPO

Jump if parity odd

PF

= 0Slide9

Range of a Conditional JumpThe destination label must precede the Jump

instruction by no more than 126 bytes

Or, follow by no more than 127 bytes

LABEL

:

; statement

; statement JNZ LABEL126 bytesJZ LABEL ; statements ; statementsLABEL: ; statement ; statement

127 bytes

refSlide10

CMP InstructionThe 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 changedDestination may not be a constantThe result is not stored but the flags are affectedCMP AX, BXJG BELOW ;JG – jump if >CMP AX, 10JG BELOWIf AX = 7FFFh, and BX = 0001h, the result is 7FFFh - 0001h = 7FFEh. ZF = SF = OF = 0, JG is satisfied, so control transfers to label BELOWSlide11

Signed vs. Unsigned JumpsEach 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 flagsSymbolDescriptionCondition for jumpsJG/JNLEJump if greater thanJump if not less than or equal toZF = 0 and SF = OFJA/JNBEJump if aboveJump if not below or equalCF = 0 and ZF = 0Wrong jumps  wrong results! [same as life]Slide12

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 senseJA, which is the unsigned jumpCMP AX, BXJA BELOW_LABELSlide13

With standard ASCII character set [character code 0-31 for control chars; 32-127 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] Signed vs. Unsigned Jumps cont.working with CHARSlide14

Extended ASCII codes (character code 128-255)There are several different variations of the 8-bit ASCII table. E.g., ISO 8859-1, also called ISO Latin-1. Codes 129-159 contain the Microsoft® Windows Latin-1 extended characters.

http://www.ascii-code.com/Slide15

The JMP InstructionJMP (jump) instruction causes an

unconditional

jump

Syntax is:

JMP

can be used to get around the range restriction [126/127 byte]Flags – no changeJMP destination/target_labelTOP:; body of the loop, say 2 instructionsDEC CX ; decrement counterJNZ TOP ; keep looping if CX > 0MOV AX, BXTOP:; the loop body contains so many instructions ; that label TOP is out of range for JNZ. Solution is- DEC CX JNZ BOTTOM JMP EXITBOTTOM: JMP TOPEXIT: MOV AX, BX

Section 6-3: Assembly Language Programming

refSlide16

TOP:; the loop body contains so many instructions

; that label TOP is out of range for JNZ.

Solution is-

DEC CX

JNZ

BOTTOM

JMP EXITBOTTOM: JMP TOPEXIT: MOV AX, BXWhen CX=0 - It will not Jump to BOTTOM It will go to next instr. JMP EXIT JMP TOP  is unconditional – just Jump!