Conditional Processing

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Conditional Processing

Computer Organization and Assembly Languages Yung-Yu Chuang

2005/11/03

with slides by Kip Irvine

Announcements

• Midterm exam: Room 103, 10:00am-12:00am next Thursday, open book, chapters 1-5.

• Assignment #2 is online.

Assignment #2 CRC32 checksum

unsigned int crc32(const char* data, size_t length) {

// standard polynomial in CRC32

const unsigned int POLY = 0xEDB88320;

// standard initial value in CRC32 unsigned int reminder = 0xFFFFFFFF;

for(size_t i = 0; i < length; i++){

// must be zero extended

reminder ^= (unsigned char)data[i];

for(size_t bit = 0; bit < 8; bit++) if(reminder & 0x01)

reminder = (reminder >> 1) ^ POLY;

else

reminder >>= 1;

}

return reminder ^ 0xFFFFFFFF;

}

Boolean and comparison instructions

• CPU Status Flags

• AND Instruction

• OR Instruction

• XOR Instruction

• NOT Instruction

• Applications

• TEST Instruction

• CMP Instruction

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Status flags - review

• The Zero flag is set when the result of an operation equals zero.

• The Carry flag is set when an instruction generates a result that is too large (or too small) for the

destination operand.

• The Sign flag is set if the destination operand is negative, and it is clear if the destination operand is positive.

• The Overflow flag is set when an instruction generates an invalid signed result.

• Less important:

– The Parity flag is set when an instruction generates an even number of 1 bits in the low byte of the destination operand.

– The Auxiliary Carry flag is set when an operation produces a carry out from bit 3 to bit 4

NOT instruction

• Performs a bitwise Boolean NOT operation on a single destination operand

• Syntax: (no flag affected)

NOT destination

• Example:

mov al, 11110000b not al

NOT

0 0 1 1 1 0 1 1 1 1 0 0 0 1 0 0

NOT

inverted

AND instruction

• Performs a bitwise Boolean AND operation between each pair of matching bits in two operands

• Syntax: (O=0,C=0,SZP)

AND destination, source

• Example:

mov al, 00111011b and al, 00001111b 0 0 1 1 1 0 1 1 0 0 0 0 1 1 1 1 0 0 0 0 1 0 1 1

AND

unchanged cleared

AND

bit extraction

OR instruction

• Performs a bitwise Boolean OR operation between each pair of matching bits in two operands

• Syntax: (O=0,C=0,SZP)

OR destination, source

• Example:

mov dl, 00111011b or dl, 00001111b

OR

0 0 1 1 1 0 1 1 0 0 0 0 1 1 1 1 0 0 1 1 1 1 1 1

OR

set unchanged

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XOR instruction

• Performs a bitwise Boolean exclusive-OR operation between each pair of matching bits in two operands

• Syntax: (O=0,C=0,SZP)

XOR destination, source

• Example:

mov dl, 00111011b xor dl, 00001111b

XOR

0 0 1 1 1 0 1 1 0 0 0 0 1 1 1 1 0 0 1 1 0 1 0 0

XOR

inverted unchanged

XOR is a useful way to invert the bits in an operand and data encryption

Applications ^{(1 of 5)}

mov al,'a' ; AL = 01100001b

and al,11011111b ; AL = 01000001b

• Task: Convert the character in AL to upper case.

• Solution: Use the AND instruction to clear bit 5.

Applications (2 of 5)

mov al,6 ; AL = 00000110b

or al,00110000b ; AL = 00110110b

• Task: Convert a binary decimal byte into its equivalent ASCII decimal digit.

• Solution: Use the OR instruction to set bits 4 and 5.

The ASCII digit '6' = 00110110b

Applications (3 of 5)

mov ax,40h ; BIOS segment mov ds,ax

mov bx,17h ; keyboard flag byte or BYTE PTR [bx],01000000b ; CapsLock on

• Task: Turn on the keyboard CapsLock key

• Solution: Use the OR instruction to set bit 6 in the

keyboard flag byte at 0040:0017h in the BIOS data area.

This code only runs in Real-address mode, and it does not

work under Windows NT, 2000, or XP.

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Applications ^{(4 of 5)}

mov ax,wordVal

and ax,1 ; low bit set?

jz EvenValue ; jump if Zero flag set

• Task: Jump to a label if an integer is even.

• Solution: AND the lowest bit with a 1. If the result is Zero, the number was even.

Applications ^{(5 of 5)}

or al,al

jnz IsNotZero ; jump if not zero

• Task: Jump to a label if the value in AL is not zero.

• Solution: OR the byte with itself, then use the JNZ (jump if not zero) instruction.

ORing any number with itself does not change its value.

TEST instruction

• Performs a nondestructive AND operation between each pair of matching bits in two operands

• No operands are modified, but the flags are affected.

• Example: jump to a label if either bit 0 or bit 1 in AL is set.

test al,00000011b

jnz ValueFound

• Example: jump to a label if neither bit 0 nor bit 1 in AL is set.

test al,00000011b jz ValueNotFound

CMP instruction (1 of 3)

• Compares the destination operand to the source operand

– Nondestructive subtraction of source from destination (destination operand is not changed)

• Syntax: (OSZCAP)

CMP destination, source

• Example: destination == source

mov al,5

cmp al,5 ; Zero flag set

• Example: destination < source

mov al,4

cmp al,5 ; Carry flag set

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CMP instruction ^{(2 of 3)}

• Example: destination > source

mov al,6

cmp al,5 ; ZF = 0, CF = 0

(both the Zero and Carry flags are clear)

The comparisons shown so far were unsigned.

CMP instruction ^{(3 of 3)}

• Example: destination > source

mov al,5

cmp al,-2 ; Sign flag == Overflow flag

The comparisons shown here are performed with signed integers.

• Example: destination < source mov al,-1

cmp al,5 ; Sign flag != Overflow flag

Setting and clearing individual flags

and al, 0 ; set Zero or al, 1 ; clear Zero or al, 80h ; set Sign and al, 7Fh ; clear Sign

stc ; set Carry

clc ; clear Carry

mov al, 7Fh

inc al ; set Overflow or eax, 0 ; clear Overflow

Conditional jumps

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Conditional structures

• There are no high-level logic structures such as if-then-else, in the IA-32 instruction set. But, you can use combinations of comparisons and jumps to implement any logic structure.

• First, an operation such as CMP, AND or SUB is executed to modified the CPU flags. Second, a conditional jump instruction tests the flags and change the execution flow accordingly.

CMP AL, 0 JZ L1 :

L1:

J cond instruction

• A conditional jump instruction branches to a label when specific register or flag conditions are met

Jcond destination

• Four groups: (some are the same) 1. based on specific flag values

2. based on equality between operands

3. based on comparisons of unsigned operands 4. based on comparisons of signed operands

Jumps based on specific flags Jumps based on equality

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Jumps based on unsigned comparisons

＞≧＜≦

Jumps based on signed comparisons

Examples

mov Large,bx cmp ax,bx jna Next mov Large,ax Next:

• Compare unsigned AX to BX, and copy the larger of the two into a variable named Large

mov Small,ax cmp bx,ax jnl Next mov Small,bx Next:

• Compare signed AX to BX, and copy the smaller of the two into a variable named Small

Examples

.date

intArray DWORD 7,9,3,4,6,1 .code

...

mov ebx, OFFSET intArray mov ecx, LENGTHOF intArray L1: test DWORD PTR [ebx], 1

jz found add ebx, 4 loop L1 ...

• Find the first even number in an array of unsigned integers

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String encryption

encoder message

(plain text)

unintelligible string (cipher text) key

encoder message

(plain text)

key

Encrypting a string

KEY = 239 .data

buffer BYTE BUFMAX DUP(0) bufSize DWORD ?

.code

mov ecx,bufSize ; loop counter mov esi,0 ; index 0 in buffer L1:

xor buffer[esi],KEY ; translate a byte

inc esi ; point to next byte

loop L1

Message: Attack at dawn.

Cipher text: «¢¢Äîä-Ä¢-ïÄÿü-Gs Decrypted: Attack at dawn.

Conditional loops

LOOPZ and LOOPE

• Syntax:

LOOPE destination LOOPZ destination

• Logic:

– ECX ← ECX – 1

– if ECX > 0 and ZF=1, jump to destination

• The destination label must be between -128 and +127 bytes from the location of the following instruction

• Useful when scanning an array for the first

element that meets some condition.

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LOOPNZ and LOOPNE

• Syntax:

LOOPNZ destination LOOPNE destination

• Logic:

– ECX ← ECX – 1;

– if ECX > 0 and ZF=0, jump to destination

LOOPNZ example

.data

array SWORD -3,-6,-1,-10,10,30,40,4 sentinel SWORD 0

.code

mov esi,OFFSET array mov ecx,LENGTHOF array next:

test WORD PTR [esi],8000h ; test sign bit

pushfd ; push flags on stack

add esi,TYPE array

popfd ; pop flags from stack

loopnz next ; continue loop

jnz quit ; none found

sub esi,TYPE array ; ESI points to value quit:

The following code finds the first positive value in an array:

Your turn

.data

array SWORD 50 DUP(?) sentinel SWORD 0FFFFh .code

mov esi,OFFSET array mov ecx,LENGTHOF array

L1: cmp WORD PTR [esi],0 ; check for zero

quit:

Locate the first nonzero value in the array. If none is found, let ESI point to the sentinel value:

Solution

.data

array SWORD 50 DUP(?) sentinel SWORD 0FFFFh .code

mov esi,OFFSET array mov ecx,LENGTHOF array

L1: cmp WORD PTR [esi],0 ; check for zero

pushfd ; push flags on stack

add esi,TYPE array

popfd ; pop flags from stack

loope next ; continue loop

jz quit ; none found

sub esi,TYPE array ; ESI points to value quit:

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Conditional structures

Block-structured IF statements

Assembly language programmers can easily translate logical statements written in C++/Java into assembly language. For example:

mov eax,op1 cmp eax,op2 jne L1

mov X,1 jmp L2 L1: mov X,2 L2:

if( op1 == op2 ) X = 1;

else X = 2;

Example

Implement the following pseudocode in assembly language. All values are unsigned:

cmp ebx,ecx ja next mov eax,5 mov edx,6 next:

if( ebx <= ecx ) {

eax = 5;

edx = 6;

}

Example

Implement the following pseudocode in assembly language. All values are 32-bit signed integers:

mov eax,var1 cmp eax,var2 jle L1

mov var3,6 mov var4,7 jmp L2

L1: mov var3,10 L2:

if( var1 <= var2 ) var3 = 10;

else {

var3 = 6;

var4 = 7;

}

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Compound expression with AND

• When implementing the logical AND operator, consider that HLLs use short-circuit evaluation

• In the following example, if the first expression is false, the second expression is skipped:

if (al > bl) AND (bl > cl) X = 1;

Compound expression with AND

cmp al,bl ; first expression...

ja L1 jmp next L1:

cmp bl,cl ; second expression...

ja L2 jmp next

L2: ; both are true

mov X,1 ; set X to 1

next:

if (al > bl) AND (bl > cl) X = 1;

This is one possible implementation . . .

Compound expression with AND

cmp al,bl ; first expression...

jbe next ; quit if false

cmp bl,cl ; second expression...

jbe next ; quit if false

mov X,1 ; both are true

next:

if (al > bl) AND (bl > cl) X = 1;

But the following implementation uses 29% less code by reversing the first relational operator. We allow the program to

"fall through" to the second expression:

Your turn . . .

Implement the following pseudocode in assembly language. All values are unsigned:

cmp ebx,ecx ja next cmp ecx,edx jbe next mov eax,5 mov edx,6 next:

if( ebx <= ecx

&& ecx > edx ) {

eax = 5;

edx = 6;

}

(There are multiple correct solutions to this problem.)

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Compound Expression with OR

• In the following example, if the first expression is true, the second expression is skipped:

if (al > bl) OR (bl > cl) X = 1;

Compound Expression with OR

cmp al,bl ; is AL > BL?

ja L1 ; yes

cmp bl,cl ; no: is BL > CL?

jbe next ; no: skip next statement

L1: mov X,1 ; set X to 1

next:

if (al > bl) OR (bl > cl) X = 1;

We can use "fall-through" logic to keep the code as short as possible:

WHILE Loops

while( eax < ebx) eax = eax + 1;

A WHILE loop is really an IF statement followed by the body of the loop, followed by an unconditional jump to the top of the loop. Consider the following example:

_while:

cmp eax,ebx ; check loop condition jae _endwhile ; false? exit loop

inc eax ; body of loop

jmp _while ; repeat the loop _endwhile:

Your turn . . .

_while: cmp ebx,val1 ; check loop condition ja _endwhile ; false? exit loop add ebx,5 ; body of loop dec val1

jmp while ; repeat the loop _endwhile:

while( ebx <= val1) {

ebx = ebx + 5;

val1 = val1 - 1 }

Implement the following loop, using unsigned 32-bit integers:

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Example: IF statement nested in a loop

while(eax < ebx) {

eax++;

if (ebx==ecx) X=2;

else X=3;

}

_while: cmp eax, ebx jae _endwhile inc eax

cmp ebx, ecx jne _else mov X, 2 jmp _while _else: mov X, 3

jmp _while _endwhile:

Table-driven selection

• Table-driven selection uses a table lookup to replace a multiway selection structure

(switch-case statements in C)

• Create a table containing lookup values and the offsets of labels or procedures

• Use a loop to search the table

• Suited to a large number of comparisons

Table-driven selection

.data

CaseTable BYTE 'A' ; lookup value

DWORD Process_A ; address of procedure EntrySize = ($ - CaseTable)

BYTE 'B'

DWORD Process_B BYTE 'C'

DWORD Process_C BYTE 'D'

DWORD Process_D

NumberOfEntries = ($ - CaseTable) / EntrySize

Step 1: create a table containing lookup values and procedure offsets:

Table-driven selection

mov ebx,OFFSET CaseTable ; point EBX to the table

mov ecx,NumberOfEntries ; loop counter

L1:cmp al,[ebx] ; match found?

jne L2 ; no: continue

call NEAR PTR [ebx + 1] ; yes: call the procedure

jmp L3 ; and exit the loop

L2:add ebx,EntrySize ; point to next entry

loop L1 ; repeat until ECX = 0

L3:

Step 2: Use a loop to search the table. When a match is found, we call the procedure offset stored in the current table entry:

required for procedure pointers

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Application: finite-state machines

• A finite-state machine (FSM) is a graph structure that changes state based on some input. Also called a state- transition diagram.

• We use a graph to represent an FSM, with squares or circles called nodes, and lines with arrows between the circles called edges (or arcs).

• A FSM is a specific instance of a more general structure called a directed graph (or digraph).

• Three basic states, represented by nodes:

– Start state

– Terminal state(s) – Nonterminal state(s)

Finite-state machines

• Accepts any sequence of symbols that puts it into an accepting (final) state

• Can be used to recognize, or validate a sequence of characters that is governed by language rules (called a regular expression)

FSM Examples

• FSM that recognizes strings beginning with 'x', followed by letters 'a'..'y', ending with 'z':

start 'x'

'a'..'y'

'z '

A B

C

• FSM that recognizes signed integers:

start

digit

+,- digit

digit

A B

C

Your turn . . .

• Explain why the following FSM does not work as well for signed integers as the one shown on the previous slide:

start

digit

A +,- B

digit

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Implementing an FSM

StateA:

call Getnext ; read next char into AL cmp al,'+‘ ; leading + sign?

je StateB ; go to State B cmp al,'-‘ ; leading - sign?

je StateB ; go to State B

call IsDigit ; ZF = 1 if AL = digit jz StateC ; go to State C

call DisplayErrorMsg ; invalid input found jmp Quit

The following is code from State A in the Integer FSM:

start

digit

+,-

digit digit

A B

C

Isdigit

Isdigit PROC cmp al,’0’

jb L1

cmp al,’9’

ja L1 test ax,0 L1: ret

Isdigit ENDP

Your turn

StateB:

call Getnext ; read next char into AL call Isdigit ; ZF = 1 if AL is a digit jz StateC

call DisplayErrorMsg ; invalid input found jmp Quit

start

digit

+,- digit

digit

A B

C

Implementing an FSM

start

digit

+,- digit

digit

A B

C

StateC:

call Getnext ; read next char into AL jz Quit ; quit if Enter pressed call Isdigit ; ZF = 1 if AL is digit jz StateC

cmp AL,ENTER_KEY ; Enter key pressed?

je Quit ; yes: quit

call DisplayErrorMsg ; no: invalid input jmp Quit

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Finite-state machine example

• [sign]integer.[integer][exponent]

sign → {+|-}

exponent → E[{+|-}]integer

High-level directives

.IF eax > ebx mov edx,1 .ELSE

mov edx,2 .ENDIF

• .IF, .ELSE, .ELSEIF, and .ENDIF can be used to create block-structured IF statements.

• Examples:

• MASM generates "hidden" code for you, consisting of code labels, CMP and conditional jump instructions.

.IF eax > ebx && eax > ecx mov edx,1

.ELSE

mov edx,2 .ENDIF

Relational and logical operators MASM-generated Code

mov eax,6 cmp eax,val1 jbe @C0001 mov result,1

@C0001:

.data

val1 DWORD 5 result DWORD ? .code

mov eax,6 .IF eax > val1 mov result,1 .ENDIF

Generated code:

MASM automatically generates an unsigned jump (JBE).

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.REPEAT directive

; Display integers 1 – 10:

mov eax,0 .REPEAT

inc eax call WriteDec call Crlf .UNTIL eax == 10

Executes the loop body before testing the loop condition associated with the .UNTIL directive.

Example:

.WHILE directive

; Display integers 1 – 10:

mov eax,0 .WHILE eax < 10

inc eax call WriteDec call Crlf .ENDW

Conditional Processing