80 x86课程英文版的课件chapter 5.ppt

1、Chapter 5 Branching and Looping,Contents: Jump instructions target addressCS:EIP Statementlable : address of other assembly language statement Similar to GOTO,JMP quit . . quit: INVOKE ExitProcess, 0,Example:5.1,output explain ; initial instructions mov sum,0 ; sum := 0 mov ebx,0 ; count := 0 foreve

2、r: output prompt ; prompt for number input number,16 ; read ASCII characters atod number ; convert to integer,Example:5.1,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,Example:

3、5.1,dtoa value,sum ; convert sum to ASCII output sumLabel ; display label for sum output value ; display sum,Example:5.1,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 ; outpu

4、t average,Example:5.1,output nextPrompt ; skip down ； start next prompt jmp forever ; repeat,Jump direction,Backward reference（向后跳转） Transfer control to a point that precedes the jmp statement itself. Example5.1 Forward reference（向前跳转） Transfer control to a point that is behind the jmp statement its

5、elf. Jmp quit,About JMP instructions,JMP instructions will change the value in the EIP register; Two kinds of JMP instructions Intersegment jump （段间转移） Change CS register Intrasegment jump（段内转移） Not change CS register,Relative jump/direct jump相对跳转/直接转移,Contains the sign displacement of the target fr

6、om the JMP statement itself. Positive for a forward reference Negative for a backward reference Target addressdisplacementaddr. of next instruction Target address label is contained in jump instruction. Example: Jmp forever,Displacement size,short relative jump（短转移） Displacement is a single byte nea

7、r relative format（近转移） 32-bit displacement,Indirect jump（间接转移）,Use a 32-bit address for the target. Address is stored in a register or in a memory doubleword Example:,jmp edx ;edx-EIP Target dword 98098912h jmp Target ; target-EIP jmp DWORD PTR ebx ; ebx-EIP,5.2 Conditional JUMP （条件转移）,J- targetStat

8、ement ;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. targetStatement must be relative addressing,Conditional jump instructions do not modify the flags; they only re

9、act to previously set flag values.,Example,add value to balance; if balance0 then design for negative banlance elseif balance=0 then design for zro balance Else design for positive balance End if;,add balance , eax jns elseIfZero jmp endBalanceCheck elseIfZero: jnz elsePos jmp endBanlanceCheck elseP

10、os: endBanlanceCheck:,Note the begin and end of selective structure.,Set or reset flag,Set flag（标志位置位） Give the value 1 to a flag Reset/clear flag（标志位复位） Give the value 0 to a flag Compare instructions are the most common way to establish flag values.,compare instruction,CMP operand1 , operand2 Calc

11、ulating operand1 minus operand2, like a SUB instruction Set flags but do not change operand1 Addressing mode is the same as SUB.,Example,if eax100 then jump to Bigger;,cmp eax, 100 ja Bigger ;(1) jg Bigger ;(2),Jump or not? EAX=00000000H (2) NOT JUMP EAX=80000000H JUMP NOT JUMP,IF structure,if value

12、10 then add 1 to smallCount; else add 1 to largetCount; endif,cmp ebx , 10; value 10 jnl elseLarge inc smallCount jmp endValueCheck elaseLarge: inc lartgeCout endValueCheck:,IF structure(2),if (total=100) or (count=10) then add value to total; endif,cmp total , 100; total =100? jge addValue cmp cx,

13、10; count=100? jne endAddCheck addValue: mov ebx, value; copy value add total , ebx ; add value to total endAddCheck:,IF structure(3),if (count0) and (ch=backspace) then subtract 1 from count; endif,cmp cx , 0 ; count0? jng endCheckCh cmp al , backspace ; ch a backspace? jne endCheckCh dec count ; s

14、ubtract 1 from count endCheckCh:,5.3 Implementing Loop Structures,Loop structures include while, until and for loops. Use jump instructions Use loop instructions,while continuation condition loop .body of loop end while;,for index:=initialValue to finalValue loop .body of loop end for;,until termina

15、tion condition loop .body of loop end until;,while loop structure,while (sum1000) loop body of loop end while;,whileSum: cmp sum , 1000 ; sum1000? jnl endWhileSum ;exit loop if not jmp whileSum ; go check condition again endWhileSum:,while loop structure(2),X:=0; twoToX:=1; while twoToX=number multi

16、ply twoToX by 2; add 1 to x; end while; subtract 1 from x,mov cx, 0 mov eax, 1 whileLE: cmp eax, number jnle endWhileLE Body: add eax, eax inc cx jmp whileLE endWhileLE: dec cx,while loop structure(3),while (sum1000) and (count=24) loop body of loop end while;,whileSum: cmp sum , 1000 ; sum1000? jnl

17、 endWhileSum ;exit loop if not cmp cx , 24 ; count=24 jnle endWhileSum ; exit if not ;body of loop jmp whileSum ; go check condition again endWhileSum:,while loop structure(4),while (sum1000) or (flag=1) loop body of loop end while;,whileSum: cmp eax , 1000 ; sum1000? jl body ;execute bbody if so cm

18、p dh , 1 ; flag=1? jne endWhileSum ; exit if not Body： ;body of loop jmp whileSum ; go check condition again endWhileSum:,while loop structure(5),sum:=0 while (number keyed in is not negative) loop add number to sum; end while;,mov ebx , 0 whileNotNeg: output prompt input number ,10 atod number js e

19、ndwhile add ebx , eax jmp whileSum endWhile:,For loop structure,Index:=initialValue while index =finalValue loop .body of loop add 1 to index; end while;,for index:=initialValue to finalValue loop .body of loop end for;,for loop structure,Prompt for tally of numbers; Input tally; Sun:=0 For count:=1

20、 to tally loop Prompt for number; Input number; Add number to sum; End for ;,output prompt1 input value , 20 atoi value mov tally , ax mov edx, 0 ; sum:=0 mov bx, 1; count:=1,forCount: cmp bx, tally jnle endfor; exit if not output prompt2 input value, 20 atod value add edx, eax inc bx jmp forCount ;

21、repeat endFor:,until loop structure,Count :=0; Until (sum1000) or (count =100) loop . body of loop Add 1 to count; End until;,mov cx,0;count :=0 Until: ;body of loop inccx;add 1 to count cmp sum, 1000; sum1000? jg endUntil; exit if sum 1000 cmp cx , 100; count=100? jne until ; continue if count not

22、=100 endUntil:,Loop instructions,Loop statementLabel statementLabel is the label of a statement that is a short displacement from the loop instruction. ECX -1-ECX if ECX =0, then execute the statement following the looop instruction if ECX !=0, then a jump to the instruction at statementLabel takes

23、place,For loop structure,for count:=20 downto 1 loop body of loop end for,mov ecx,20;number of iterations 循环次数 forCount: . . ;body of loop loop forCount ; repeat body 20 times,For loop structure,mov ecx,number;number of iterations 循环次数 cmp ecx , 0 je/jecxz endFor; skip loop if number=0 forIndex: . ;

24、body of loop loop forIndex ; repeat body number times,How many times would repeat if ECX=0?,232=4294967296,For loop structure,for counter := 50 downto 1 loop .body of loop end for;,mov ecx , 50; number of iterations forCount: ; body of loop dec ecx;decrement loop counter jecxz endfor; exit if counte

25、r =0 jmp forCounter ; otherwise repeat body,mov ecx , 50; number of iterations forCount: .; body of loop loop forCount; repeat body 20 times,For loop structure,for index:=1 to 50 loop body of loop end for,mov ecx,50;number of iterations 循环次数 mov ebx , 1 ; index:=1 forCount: . ;body of loop inc ebx l

26、oop forCount ; repeat body 20 times,Conditional loop,loopz/loope if ECX!=0 and ZF=1 then loop again loopnz/loopne if ECX!=0 and ZF=0 then loop again,For loop structure,for year:=10 downto 1 until balance=0 loop body of loop end for,mov ecx,10;maximum number of iterations forYear: . ;body of loop cmp

27、 ebx , 0 ;balance=0? loopne forYear ; repeat body 20 times,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

28、ecx=0,Game program,untilDone: output prompt1 ; ask player 1 for target input stringIn, 20 ; get number atod stringIn ; convert to integer mov target,eax ; store target output clear ; clear screen mov cx, 0 ; zero count,Game program(2),untilMatch: inc cx ; increment count of guesses output prompt2 ;

29、ask player 2 for guess input stringIn, 20 ; get number atod stringIn ; convert to integer cmp eax, target ; compare guess and target jne ifLess ; guess = target ?,Game program(3),equal: output gotItOutput ; display you got it jmp endCompare ifLess: jnl isGreater ; guess target ? output lowOutput ; d

30、isplay too low jmp endCompare isGreater: output highOutput ; display too high,Game program(4),endCompare: cmp eax, target ; compare guess and target jne untilMatch ; ask again if guess not = target itoa countOut, cx ; convert count to ASCII output countLabel ; display label, count and prompt input s

31、tringIn, 20 ; get response cmp stringIn, n ; response = n ? je endUntilDone ; exit if so cmp stringIn, N ; response = N ? jne untilDone ; repeat if not endUntilDone:,Program using array,; input a collection of numbers ; report their average and the numbers which are ;above average output directions

32、; display directions mov nbrElts,0 ; nbrElts := 0 lea ebx,nbrArray ; get address of nbrArray,Program using array,whilePos: output prompt ; prompt for number input number,20 ; get number atod number ; convert to integer jng endWhile ; exit if not positive mov ebx,eax ; store number in array inc nbrElts ; add 1 to nbrElts add ebx,4 ; get address of next item of array jmp whilePos