Conditional Processing Computer Organization Assembly Language Programming Dr

Conditional Processing Computer Organization & Assembly Language Programming Dr Adnan Gutub aagutub ‘at’ uqu. edu. sa [Adapted from slides of Dr. Kip Irvine: Assembly Language for Intel-Based Computers] Most Slides contents have been arranged by Dr Muhamed Mudawar & Dr Aiman El-Maleh from Computer Engineering Dept. at KFUPM

Presentation Outline v Boolean and Comparison Instructions v Conditional Jumps v Conditional Loop Instructions v Translating Conditional Structures v Indirect Jump and Table-Driven Selection v Application: Sorting an Integer Array /552 Conditional Processing Computer Organization & Assembly Language Programmingslide

AND Instruction v Bitwise AND between each pair of matching bits AND destination, source v Following operand combinations are allowed AND AND AND reg, mem, reg mem imm reg imm v AND instruction is often used to clear selected bits /553 AND Operands can be 8, 16, or 32 bits and they must be of the same size AND cleared 00111011 00001111 00001011 unchanged Conditional Processing Computer Organization & Assembly Language Programmingslide

Converting Characters to Uppercase v AND instruction can convert characters to uppercase 'a' = 0 1 1 0 0 1 'b' = 0 1 1 0 0 0 1 0 'A' = 0 1 0 0 0 1 'B' = 0 1 0 v Solution: Use the AND instruction to clear bit 5 L 1: mov ecx, LENGTHOF mystring mov esi, OFFSET mystring and BYTE PTR [esi], 11011111 b ; clear bit 5 inc esi loop L 1 /554 Conditional Processing Computer Organization & Assembly Language Programmingslide

OR Instruction v Bitwise OR operation between each pair of matching bits OR destination, source v Following operand combinations are allowed OR OR OR reg, mem, reg mem imm reg imm v OR instruction is often used to set selected bits /555 OR Operands can be 8, 16, or 32 bits and they must be of the same size OR set 00111011 11110000 11111011 unchanged Conditional Processing Computer Organization & Assembly Language Programmingslide

Converting Characters to Lowercase v OR instruction can convert characters to lowercase 'A' = 0 1 0 0 0 1 'B' = 0 1 0 'a' = 0 1 1 0 0 1 'b' = 0 1 1 0 0 0 1 0 v Solution: Use the OR instruction to set bit 5 L 1: mov ecx, LENGTHOF mystring mov esi, OFFSET mystring or BYTE PTR [esi], 20 h inc esi ; set bit 5 loop L 1 /556 Conditional Processing Computer Organization & Assembly Language Programmingslide

Converting Binary Digits to ASCII v OR instruction can convert a binary digit to ASCII 0 =0000 1 =00000001 '0' = 0 0 1 1 0 0 '1' = 0 0 1 1 0 0 0 1 v Solution: Use the OR instruction to set bits 4 and 5 or al, 30 h ; Convert binary digit 0 to 9 to ASCII v What if we want to convert an ASCII digit to binary? v Solution: Use the AND instruction to clear bits 4 to 7 and al, 0 Fh /557 ; Convert ASCII '0' to '9' to binary Conditional Processing Computer Organization & Assembly Language Programmingslide

XOR Instruction v Bitwise XOR between each pair of matching bits XOR destination, source v Following operand combinations are allowed XOR XOR XOR reg, mem, reg mem imm reg imm v XOR instruction is often used to invert selected bits /558 XOR Operands can be 8, 16, or 32 bits and they must be of the same size XOR inverted 00111011 11110000 11001011 unchanged Conditional Processing Computer Organization & Assembly Language Programmingslide

Affected Status Flags The six status flags are affected 1. Carry Flag: Cleared by AND, OR, and XOR 2. Overflow Flag: Cleared by AND, OR, and XOR 3. Sign Flag: Copy of the sign bit in result 4. Zero Flag: Set when result is zero 5. Parity Flag: Set when parity in least-significant byte is even 6. Auxiliary Flag: Undefined by AND, OR, and XOR /559 Conditional Processing Computer Organization & Assembly Language Programmingslide

String Encryption Program v Tasks: ² Input a message (string) from the user ² Encrypt the message ² Display the encrypted message ² Decrypt the message ² Display the decrypted message v Sample Output Enter the plain text: Attack at dawn. Cipher text: «¢¢Äîä-Ä¢-ïÄÿü-Gs Decrypted: Attack at dawn. /5510 Conditional Processing Computer Organization & Assembly Language Programmingslide

Encrypting a String KEY = 239 ; Can be any byte value BUFMAX = 128. data buffer BYTE BUFMAX+1 DUP(0) buf. Size DWORD BUFMAX The following loop uses the XOR instruction to transform every character in a string into a new value mov ecx, buf. Size mov esi, 0 ; loop counter ; index 0 in buffer L 1: xor buffer[esi], KEY ; translate a byte inc esi ; point to next byte loop L 1 /5511 Conditional Processing Computer Organization & Assembly Language Programmingslide

TEST Instruction v Bitwise AND operation between each pair of bits TEST destination, source v The flags are affected similar to the AND Instruction v However, TEST does NOT modify the destination operand v TEST instruction can check several bits at once ² Example: Test whether bit 0 or bit 3 is set in AL ² Solution: test al, 00001001 b ; test bits 0 & 3 ² We only need to check the zero flag ; If zero flag => both bits 0 and 3 are clear ; If Not zero /5512 => either bit 0 or 3 is set Conditional Processing Computer Organization & Assembly Language Programmingslide

NOT Instruction v Inverts all the bits in a destination operand NOT destination v Result is called the 1's complement v Destination can be a register or memory NOT reg NOT mem v None of the Flags is affected by the NOT instruction /5513 Conditional Processing Computer Organization & Assembly Language Programmingslide

CMP Instruction v CMP (Compare) instruction performs a subtraction Syntax: CMP destination, source Computes: destination – source v Destination operand is NOT modified v All six flags: OF, CF, SF, ZF, AF, and PF are affected v CMP uses the same operand combinations as SUB ² Operands can be 8, 16, or 32 bits and must be of the same size v Examples: assume EAX = 5, EBX = 10, and ECX = 5 cmp eax, ebx cmp eax, ecx /5514 ; OF=0, CF=1, SF=1, ZF=0 ; OF=0, CF=0, SF=0, ZF=1 Conditional Processing Computer Organization & Assembly Language Programmingslide

Unsigned Comparison v CMP can perform unsigned and signed comparisons ² The destination and source operands can be unsigned or signed v For unsigned comparison, we examine ZF and CF flags Unsigned Comparison ZF CF unsigned destination < unsigned source 1 unsigned destination > unsigned source 0 destination = source 1 0 To check for equality, it is enough to check ZF flag v CMP does a subtraction and CF is the borrow flag CF = 1 if and only if unsigned destination < unsigned source v Assume AL = 5 and BL = -1 = FFh cmp al, bl ; Sets carry flag CF = 1 /5515 Conditional Processing Computer Organization & Assembly Language Programmingslide

Signed Comparison v For signed comparison, we examine SF, OF, and ZF Signed Comparison Flags signed destination < signed source SF ≠ OF signed destination > signed source SF = OF, ZF = 0 destination = source ZF = 1 v Recall for subtraction, the overflow flag is set when … ² Operands have different signs and result sign ≠ destination sign v CMP AL, BL (consider the four cases shown below) /5516 Case 1 AL = 80 BL = 50 OF = 0 SF = 0 AL > BL Case 2 AL = -80 BL = -50 OF = 0 SF = 1 AL < BL Case 3 AL = 80 BL = -50 OF = 1 SF = 1 AL > BL Case 4 AL = -80 BL = 50 OF = 1 SF = 0 AL < BL Conditional Processing Computer Organization & Assembly Language Programmingslide

Next. . . v Boolean and Comparison Instructions v Conditional Jumps v Conditional Loop Instructions v Translating Conditional Structures v Indirect Jump and Table-Driven Selection v Application: Sorting an Integer Array /5517 Conditional Processing Computer Organization & Assembly Language Programmingslide

Conditional Structures v No high-level control structures in assembly language v Comparisons and conditional jumps are used to … ² Implement conditional structures such as IF statements ² Implement conditional loops v Types of Conditional Jump Instructions ² Jumps based on specific flags ² Jumps based on equality ² Jumps based on the value of CX or ECX ² Jumps based on unsigned comparisons ² Jumps based on signed comparisons /5518 Conditional Processing Computer Organization & Assembly Language Programmingslide

Jumps Based on Specific Flags v Conditional Jump Instruction has the following syntax: Jcond destination ; cond is the jump condition v Destination Label v Prior to 386 Jump must be within – 128 to +127 bytes from current location v IA-32 32 -bit offset permits jump anywhere in memory /5519 Conditional Processing Computer Organization & Assembly Language Programmingslide

Jumps Based on Equality v JE is equivalent to JZ v. JNE is equivalent to JNZ jecxz L 2 ; exit loop v JECXZ Checked once at the beginning L 1: . . . Terminate a loop if ECX is zero loop L 1 ; loop body L 2: /5520 Conditional Processing Computer Organization & Assembly Language Programmingslide

Examples of Jump on Zero v Task: Check whether integer value in EAX is even Solution: TEST whether the least significant bit is 0 If zero, then EAX is even, otherwise it is odd test eax, 1 jz Even. Val ; test bit 0 of eax ; jump if Zero flag is set v Task: Jump to label L 1 if bits 0, 1, and 3 in AL are all set Solution: and al, 00001011 b cmp al, 00001011 b je L 1 /5521 ; clear bits except 0, 1, 3 ; check bits 0, 1, 3 ; all set? jump to L 1 Conditional Processing Computer Organization & Assembly Language Programmingslide

Jumps Based on Unsigned Comparison Task: Jump to a label if unsigned EAX is less than EBX Solution: /5522 cmp eax, ebx jb Is. Below JB condition CF = 1 Conditional Processing Computer Organization & Assembly Language Programmingslide

Jumps Based on Signed Comparisons Task: Jump to a label if signed EAX is less than EBX Solution: /5523 cmp eax, ebx jl Is. Less JL condition OF ≠ SF Conditional Processing Computer Organization & Assembly Language Programmingslide

Compare and Jump Examples Jump to L 1 if unsigned EAX is greater than Var 1 Solution: cmp eax, Var 1 ja L 1 JA condition CF = 0, ZF = 0 Jump to L 1 if signed EAX is greater than Var 1 Solution: cmp eax, Var 1 jg L 1 JG condition OF = SF, ZF = 0 Jump to L 1 if signed EAX is greater than or equal to Var 1 Solution: /5524 cmp eax, Var 1 jge L 1 JGE condition OF = SF Conditional Processing Computer Organization & Assembly Language Programmingslide

Computing the Max and Min v Compute the Max of unsigned EAX and EBX Solution: mov cmp jae mov done: Max, eax Max, ebx done Max, ebx ; assume Max = eax ; Max = ebx v Compute the Min of signed EAX and EBX Solution: /5525 mov cmp jle mov done: Min, eax Min, ebx done Min, ebx ; assume Min = eax ; Min = ebx Conditional Processing Computer Organization & Assembly Language Programmingslide

Application: Sequential Search ; Receives: esi = array address ; ecx = array size ; eax = search value ; Returns: esi = address of found element search PROC USES ecx jecxz notfound L 1: cmp [esi], eax ; je found ; add esi, 4 ; loop L 1 notfound: mov esi, 0 ; found: ret ; search ENDP /5526 array element = search value? yes? found element no? point to next array element if not found then esi = 0 if found, esi = element address Conditional Processing Computer Organization & Assembly Language Programmingslide

BT Instruction v BT = Bit Test Instruction v Syntax: BT r/m 16, r 16 BT r/m 32, r 32 BT r/m 16, imm 8 BT r/m 32, imm 8 v Copies bit n from an operand into the Carry flag v Example: jump to label L 1 if bit 9 is set in AX register bt AX, 9 jc L 1 /5527 ; CF = bit 9 ; jump if Carry to L 1 Conditional Processing Computer Organization & Assembly Language Programmingslide

Next. . . v Boolean and Comparison Instructions v Conditional Jumps v Conditional Loop Instructions v Translating Conditional Structures v Indirect Jump and Table-Driven Selection v Application: Sorting an Integer Array /5528 Conditional Processing Computer Organization & Assembly Language Programmingslide

LOOPZ and LOOPE v Syntax: LOOPE destination LOOPZ destination v Logic: ² ECX = ECX – 1 ² if ECX > 0 and ZF=1, jump to destination v Useful when scanning an array for the first element that does not match a given value. /5529 Conditional Processing Computer Organization & Assembly Language Programmingslide

LOOPNZ and LOOPNE v Syntax: LOOPNZ destination LOOPNE destination v Logic: ² ECX – 1; ² if ECX > 0 and ZF=0, jump to destination v Useful when scanning an array for the first element that matches a given value. /5530 Conditional Processing Computer Organization & Assembly Language Programmingslide

LOOPZ Example The following code finds the first negative value in an array. data array SWORD 17, 10, 30, 4, -5, 8. code mov esi, OFFSET array – 2 ; start before first mov ecx, LENGTHOF array ; loop counter L 1: add esi, 2 ; point to next element test WORD PTR [esi], 8000 h ; test sign bit loopz L 1 ; ZF = 1 if value >= 0 jnz found ; found negative value notfound: . . . ; ESI points to last array element found: . . . ; ESI points to first negative value /5531 Conditional Processing Computer Organization & Assembly Language Programmingslide

Your Turn. . . Locate the first zero value in an array If none is found, let ESI be initialized to 0. data array SWORD -3, 7, 20, -50, 10, 0, 4. code mov esi, OFFSET array – 2 ; start before first mov ecx, LENGTHOF array ; loop counter L 1: add esi, 2 ; point to next element cmp WORD PTR [esi], 0 ; check for zero loopne L 1 ; continue if not zero JE Found XOR ESI, ESI Found: /5532 Conditional Processing Computer Organization & Assembly Language Programmingslide

Next. . . v Boolean and Comparison Instructions v Conditional Jumps v Conditional Loop Instructions v Translating Conditional Structures v Indirect Jump and Table-Driven Selection v Application: Sorting an Integer Array /5533 Conditional Processing Computer Organization & Assembly Language Programmingslide

Block-Structured IF Statements v IF statement in high-level languages (such as C or Java) ² Boolean expression (evaluates to true or false) ² List of statements performed when the expression is true ² Optional list of statements performed when expression is false v Task: Translate IF statements into assembly language v Example: if( var 1 == var 2 ) X = 1; else X = 2; /5534 mov eax, var 1 cmp eax, var 2 jne elsepart mov X, 1 jmp next elsepart: mov X, 2 next: Conditional Processing Computer Organization & Assembly Language Programmingslide

Your Turn. . . v Translate the IF statement to assembly language v All values are unsigned if( ebx <= ecx ) { eax = 5; edx = 6; } cmp ja mov next: ebx, ecx next eax, 5 edx, 6 There can be multiple correct solutions /5535 Conditional Processing Computer Organization & Assembly Language Programmingslide

Your Turn. . . v Implement the following IF in assembly language v All variables are 32 -bit signed integers if (var 1 var 3 = } else { var 3 = var 4 = } <= var 2) { 10; 6; 7; mov eax, var 1 cmp eax, var 2 jle ifpart mov var 3, 6 mov var 4, 7 jmp next ifpart: mov var 3, 10 next: There can be multiple correct solutions /5536 Conditional Processing Computer Organization & Assembly Language Programmingslide

Compound Expression with AND v HLLs use short-circuit evaluation for logical AND v If first expression is false, second expression is skipped if ((al > bl) && (bl > cl)) {X = 1; } ; One Possible Implementation. . . cmp al, bl ; first expression. . . ja L 1 ; unsigned comparison jmp next L 1: cmp bl, cl ; second expression. . . ja L 2 ; unsigned comparison jmp next L 2: mov X, 1 ; both are true next: /5537 Conditional Processing Computer Organization & Assembly Language Programmingslide

Better Implementation for AND if ((al > bl) && (bl > cl)) {X = 1; } The following implementation uses less code By reversing the relational operator, We allow the program to fall through to the second expression Number of instructions is reduced from 7 to 5 cmp jbe mov next: /5538 al, bl next bl, cl next X, 1 ; ; ; first expression. . . quit if false second expression. . . quit if false both are true Conditional Processing Computer Organization & Assembly Language Programmingslide

Your Turn. . . v Implement the following IF in assembly language v All values are unsigned if ((ebx <= ecx) && (ecx > edx)) { eax = 5; edx = 6; } /5539 cmp ja cmp jbe mov next: ebx, ecx next ecx, edx next eax, 5 edx, 6 Conditional Processing Computer Organization & Assembly Language Programmingslide

Application: Is. Digit Procedure Receives a character in AL Sets the Zero flag if the character is a decimal digit if (al >= '0' && al <= '9') {ZF = 1; } Is. Digit PROC cmp al, '0' jb L 1 cmp al, '9' ja L 1 test al, 0 L 1: ret Is. Digit ENDP /5540 ; ; ; AL < yes? AL > yes? ZF = '0' ? ZF=0, return '9' ? ZF=0, return 1 Conditional Processing Computer Organization & Assembly Language Programmingslide

Compound Expression with OR v HLLs use short-circuit evaluation for logical OR v If first expression is true, second expression is skipped if ((al > bl) || (bl > cl)) {X = 1; } v Use fall-through to keep the code as short as possible cmp ja cmp jbe L 1: mov next: /5541 al, bl L 1 bl, cl next X, 1 ; ; ; is AL > BL? yes, execute if part no: is BL > CL? no: skip if part set X to 1 Conditional Processing Computer Organization & Assembly Language Programmingslide

WHILE Loops A WHILE loop can be viewed as IF statement followed by The body of the loop, followed by Unconditional jump to the top of the loop while( eax < ebx) { eax = eax + 1; } This is a possible implementation: top: cmp jae inc jmp next: /5542 eax, ebx next eax top ; ; eax < ebx ? false? then exit loop body of loop repeat the loop Conditional Processing Computer Organization & Assembly Language Programmingslide

Your Turn. . . Implement the following loop, assuming unsigned integers while (ebx <= var 1) { ebx = ebx + 5; var 1 = var 1 - 1 } top: cmp ja add dec jmp next: /5543 ebx, var 1 next ebx, 5 var 1 top ; ebx <= var 1? ; false? exit loop ; execute body of loop ; repeat the loop Conditional Processing Computer Organization & Assembly Language Programmingslide

Yet Another Solution for While Check the loop condition at the end of the loop No need for JMP, loop body is reduced by 1 instruction while (ebx <= var 1) { ebx = ebx + 5; var 1 = var 1 - 1 } cmp ja top: add dec cmp jbe next: /5544 ebx, var 1 next ebx, 5 var 1 ebx, var 1 top ; ebx <= var 1? ; false? exit loop ; execute body of loop ; ebx <= var 1? ; true? repeat the loop Conditional Processing Computer Organization & Assembly Language Programmingslide

Next. . . v Boolean and Comparison Instructions v Conditional Jumps v Conditional Loop Instructions v Translating Conditional Structures v Indirect Jump and Table-Driven Selection v Application: Sorting an Integer Array /5545 Conditional Processing Computer Organization & Assembly Language Programmingslide

Indirect Jump v Direct Jump: Jump to a Labeled Destination ² Destination address is a constant § Address is encoded in the jump instruction § Address is an offset relative to EIP (Instruction Pointer) v Indirect jump ² Destination address is a variable or register § Address is stored in memory/register § Address is absolute v Syntax: JMP mem 32/reg 32 ² 32 -bit absolute address is stored in mem 32/reg 32 for FLAT memory v Indirect jump is used to implement switch statements /5546 Conditional Processing Computer Organization & Assembly Language Programmingslide

Switch Statement v Consider the following switch statement: Switch (ch) case '0': case '1': case '2': case '3': case '4': default : } { exit(); count++; break; count--; break; count += 5; break; count -= 5; break; count = 0; v How to translate above statement into assembly code? v We can use a sequence of compares and jumps v A better solution is to use the indirect jump /5547 Conditional Processing Computer Organization & Assembly Language Programmingslide

Implementing the Switch Statement case 0: exit case 1: inc count jmp exitswitch case 2: dec count jmp exitswitch case 3: add count, 5 jmp exitswitch case 4: sub count, 5 jmp exitswitch default: mov count, 0 exitswitch: /5548 There are many case labels. How to jump to the correct one? Answer: Define a jump table and use indirect jump to the correct label Conditional Processing Computer Organization & Assembly Language Programmingslide

Jump Table and Indirect Jump v Jump Table is an array of double words ² Contains the case labels of the switch statement ² Can be defined inside the same procedure of switch statement jumptable DWORD case 0, case 1, case 2, case 3, case 4 Assembler converts labels to addresses v Indirect jump uses jump table to jump to selected label movzx sub cmp ja jmp /5549 eax, ch eax, '0' eax, 4 default jumptable[eax*4] ; ; ; move ch to eax convert ch to a number eax > 4 ? default case Indirect jump Conditional Processing Computer Organization & Assembly Language Programmingslide

Next. . . v Boolean and Comparison Instructions v Conditional Jumps v Conditional Loop Instructions v Translating Conditional Structures v Indirect Jump and Table-Driven Selection v Application: Sorting an Integer Array /5550 Conditional Processing Computer Organization & Assembly Language Programmingslide

Bubble Sort v Consider sorting an array of 5 elements: 5 1 3 2 4 First Pass (4 comparisons) Compare 5 with 1 and swap: Compare 5 with 3 and swap: Compare 5 with 2 and swap: Compare 5 with 4 and swap: 5 1 1 1 5 3 3 3 5 2 2 2 5 4 4 4 5 (swap) largest Second Pass (3 comparisons) Compare 1 with 3 (No swap): Compare 3 with 2 and swap: Compare 3 with 4 (No swap): 1 3 2 4 5 1 2 3 4 5 (no swap) Third Pass (2 comparisons) Compare 1 with 2 (No swap): Compare 2 with 3 (No swap): 1 2 3 4 5 (no swap) No swapping during 3 rd pass array is now sorted /5551 Conditional Processing Computer Organization & Assembly Language Programmingslide

Bubble Sort Algorithm v Algorithm: Sort array of given size bubble. Sort(array, size) { comparisons = size do { comparisons--; sorted = true; // assume initially for (i = 0; i<comparisons; i++) { if (array[i] > array[i+1]) { swap(array[i], array[i+1]); sorted = false; } } } while (! sorted) } /5552 Conditional Processing Computer Organization & Assembly Language Programmingslide

Bubble Sort Procedure – Slide 1 of 2 ; -------------------------; bubble. Sort: Sorts a DWORD array in ascending order ; Uses the bubble sort algorithm ; Receives: ESI = Array Address ; ECX = Array Length ; Returns: Array is sorted in place ; -------------------------bubble. Sort PROC USES eax ecx edx outerloop: dec ECX ; ECX = comparisons jz sortdone ; if ECX == 0 then we are done mov EDX, 1 ; EDX = sorted = 1 (true) push ECX ; save ECX = comparisons push ESI ; save ESI = array address /5553 Conditional Processing Computer Organization & Assembly Language Programmingslide

Bubble Sort Procedure – Slide 2 of 2 innerloop: mov EAX, [ESI] cmp EAX, [ESI+4] jle increment xchg EAX, [ESI+4] mov [ESI], EAX mov EDX, 0 increment: add ESI, 4 loop innerloop pop ESI pop ECX cmp EDX, 1 jne outerloop sortdone: ret bubble. Sort ENDP /5554 ; compare [ESI] and [ESI+4] ; [ESI]<=[ESI+4]? don’t swap ; swap [ESI] and [ESI+4] ; EDX = sorted = 0 (false) ; ; ; point to next element end of inner loop restore ESI = array address restore ECX = comparisons sorted == 1? No? loop back ; return Conditional Processing Computer Organization & Assembly Language Programmingslide

Summary v Bitwise instructions (AND, OR, XOR, NOT, TEST) ² Manipulate individual bits in operands v CMP: compares operands using implied subtraction ² Sets condition flags for later conditional jumps and loops v Conditional Jumps & Loops ² Flag values: JZ, JNZ, JC, JNC, JO, JNO, JS, JNS, JP, JNP ² Equality: JE(JZ), JNE (JNZ), JCXZ, JECXZ ² Signed: JG (JNLE), JGE (JNL), JL (JNGE), JLE (JNG) ² Unsigned: JA (JNBE), JAE (JNB), JB (JNAE), JBE (JNA) ² LOOPZ (LOOPE), LOOPNZ (LOOPNE) v Indirect Jump and Jump Table /5555 Conditional Processing Computer Organization & Assembly Language Programmingslide