Chapter 5 Braching and Looping
Introduction
? Master jump instructions
? Master loop instructions
? Master the method of implementing loop structures and selective
structures
? Master the method of programming arrays
Program structure
Contents
1. Jump instructions
2. Loop instructions
3. Implementing loop structures
4. Application : Arrays programming
1. Jump instructions
Jump instructions
1.1 Unconditional Jumps
Target address－>CS:EIP
? Statement lable : address of other assembly language statement
? Similar to GOTO
1.1.1 Jump direction
? Backward reference
? Transfer control to a point that precedes the jmp statement itself.
? Example 5.1
? Forward reference
? Transfer control to a point that is behind the jmp statement itself.
? Jmp quit
Example :5.0
JMP quit
……………………..
quit: INVOKE ExitProcess , 0
Example:5.1
output explain ; initial instructions
mov sum ,0 ; sum := 0
mov ebx ,0 ; count := 0
forever:
output prompt ; prompt for number
input number,16 ; read ASCII characters
atod number ; convert to integer
add sum,eax ; add number to sum
inc ebx ; add 1 to count
dtoa value,ebx ; convert count to ASCII
output countLabel ; display label for count
output value ; display count
dtoa value ,sum ; convert sum to ASCII
output sumLabel ; display label for sum
output value ; display sum
mov eax,sum ; get sum
cdq ; extend sum to 64 bits
idiv ebx ; sum / count
dtoa value,eax ; convert average to ASCII
output avgLabel ; display label for average
output value ; output average
output nextPrompt ; skip down
; start next prompt
jmp forever ; repeat
1.1.2 About JMP instructions
? Intersegment jump : Change CS register
? Intersegment jump : Not change CS register
1.1.3 Relative jump/direct jump
Contains the sign offset of the target from the JMP statement itself .
? Positive for a forward reference
? Negative for a backward reference
? Target addr = offset +addr of next instruction
? Target addr label is contained in jump instruction
? Example : Jmp forever
1.1.4 Offset size
1.1.5 Indirect jump
Use a 32-bit address for the target.
Address is stored in a register or in a memory double word.
Example:
Jmp edx ;edx->EIP
Target dword 98098912h
Jmp Target ;[target]->EIP
Jmp DWORD PTR [ebx] ;[ebx]->EIP
1.2 Conditional Jumps
J- target Statement ;target address->EIP
? Identifies the condition under which the jump is to be executed.
? If the condition holds, then the jump takes place;
? Otherwise, the next instruction is executed.
? Target Statement must be relative addressing
Conditional jump instructions do not modify the flags : They only react to
previously set flag values .
Conditional jump instructions
Example: 5.2
Pseudo code
Add value to balance ;
If balance < 0
Then
….{design for negative balance }
Elseif balance = 0
Then
…..{design for zero balance}
Else
…..{design for positive balance}
End if ; Code fragment
Add balance , eax
Ins elseifZero
…..
Jmp endBalanceCheck
ElseifZero : jnz elsePos
….
Jmp endBanlanceCheck
elsePos : ……
endBanlanceCheck :
1.2.1 Set or unset flag
1.2.2 Compare instruction
CMP operand 1 , operand 2
Calculating operand 1 minus operand2, like a SUB instruction
Set flags but do not change operand 1
Addressing mode is the same as SUB
Example: 5.3
If Structure --first
Code fragment
cmp ebx , 10; value <10
jnl elseLarge
inc smallCount
jmp endValueCheck
elaseLarge:
inc lartgeCout endValueCheck: Pseudo code
if value<10
then
add 1 to smallCount;
else
add 1 to largetCount;
endif
If Structure –second
Pseudo code
if (total>=100) or (count=10)
then
add value to total;
endif Code fragment
cmp total , 100; total <=100?
jge addValue
cmp cx, 10; count=100?
jne endAddCheck
addValue: mov ebx, value; copy value
add total , ebx ; add value to total
endAddCheck:
If Structure –third
Pseudo code
if(count>0)and (ch=backspace)
then
subtract 1 from count;
endif Code fragment
cmp cx , 0 ; count > 0 ?
jng endCheckCh
cmp al , backspace ; ch a backspace ?
jne endCheckCh
dec count ; subtract 1 from count
endCheckCh:
2. Loop instructions
2.1 Loop statement label
Statement label is the label of a statement that is a short offset from
the loop instruction.
ECX -1->ECX
If ECX =0, then execute the statement following the loop
instruction
If ECX !=0, then a jump to the instruction at statement label takes
place
Example:5.4
Pseudo code
for count : = 20 downto 1 loop
….{ body of loop }
end for Code fragment
mov ecx , 20 ; number of iterations
forCount : …. ; body of loop
loop forCount ; repeat body 20 times
mov ecx , number ;number of iterations
cmp ecx , 0
je/jecxz endFor ; skip loop if number = 0
forIndes : …. ; body of loop
loop forIndes ; repeat body number times
Example:5.5
Pseudo code
for index : = 1 to 50 loop
….{ body of loop }
end for
Code fragment
mov ecx , 50 ; number of iterations
mov ebx , 1 ; index : = 1
forCount : …… ; body of loop
inc ebx
loop forCount ; repeat body 20 times
2.2 Conditional loop
Loopz / loope
If ECX ! = 0 and ZF = 1
then loop again .
Loopnz / loopne
If ECX ! = 0 and ZF = 0
then loop again.
2.3 For loop structure
Pseudo code
for year : = downto 1 until balance = 0 loop
…..{ body of loop }
end for Code fragment
mov ecx , 10 ; maximum number of iterations
forYear : …..
; body of loop
cmp ebx , 0 ; balance = 0 ?
loopne forYear ; repeat body 20 times
2.4 Other instructions
Lea destination , source
? Destination will normally be a 32-bit register ;
? Source is any reference to memory ;
? The address of the source is loaded into the register ;
? Mov destination , OFFSET source ;
Jecxz targetstatement
Jump if ecx = 0 ;
3. Implementing loop structures
Loop structures include While, Until and For.
? Use jump instructions
? Use loop instructions
While
while continuation condition loop
....{body of loop}
end while ;
Until
until termination condition loop
....{body of loop}
end until ;
For
for index :=
initialValue to finalValue loop
....{body of loop}
end for ;
3.1 While loop structure
Example 5.6
Pseudo code
while (sum<1000) loop
…{body of loop}
end while; Code fragment
whileSum:
cmp sum , 1000 ; sum<1000?
jnl endWhileSum ;exit loop if not
…
jmp whileSum ; go check condition again
endWhileSum:
Example 5.7
Pseudo code
X : = 0 ; twoToX : = 1 ;
while
twoToX<=number
multiply twoToX by 2 ;
add 1 to x ;
end while ;
subtract 1 from x Code fragment
mov cx , 0
mov eax , 1
whileLe : cmp eax , number
jnle endWhileLE
Body : add eax , eax
inc cx
jmp whileLE
endWhileLE :
dec cx
Example 5.8
Pseudo code
while (sum<1000) and ( count <=24 ) loop
…{ body of loop }
end while;
Code fragment
whileSum:
cmp sum , 1000 ; sum<1000?
jnl endWhileSum ;exit loop if not
cmp cx , 24 ;count <=24
;body of loop
jmp whileSum ;go check condition again
endWhileSum:
3.2 For loop structure
For loop structure
for index:=initialValue to finalValue loop
…. ..{body of loop}
end for; While loop structure
Index:=initialValue
while index <=finalValue loop
…..{body of loop}