Assembly Branches Loops and Arithmatics

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Branch loops and Arithmetic I

Assembly Instructions

• Brief overview of some basic instructions:∗ Arithmetic instructions∗ Jump instructions∗ Loop instruction∗ Logical instructions∗ Shift instructions∗ Rotate instructions

• These instructions allow you to write reasonable assembly language programs

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Arithmetic Instructions I

INC and DEC instructions∗ Format:

inc destination dec destination

∗ Semantics:destination = destination +/- 1

» destination can be 8-, 16-, or 32-bit operand, in memory or register

No immediate operand

• Examplesinc BX dec value

Arithmetic Instructions (cont’d)

Add instructions∗ Format:

add destination,source

∗ Semantics:destination = destination + source

• Examplesadd BX, AX add value,35

∗ inc AX is better than add AX,1– inc takes less space– inc is faster than add

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Add instructions∗ Addition with carry∗ Format:

adc destination,source

∗ Semantics:destination = destination + source + CF

• Example: 32-bit additionadd AX, CX ; add lower 16 bitsadc BX, DX ; add upper 16 bits with carry

∗ 32-bit result in BX:AX


Subtract instructions∗ Format:

sub destination,source

∗ Semantics:destination = destination - source

• Examplessub BX, AX sub value,35

∗ dec AX is better than sub AX,1– dec takes less space– dec is faster than sub

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Subtract instructions∗ Subtract with borrow∗ Format:

sbb destination,source

∗ Semantics:destination = destination - source - CF

∗ Like the adc, sbb is useful in dealing with more than 32-bit numbers

• Negationneg destination

∗ Semantics:destination = 0 - destination


CMP instruction∗ Format:

cmp destination,source

∗ Semantics:destination - source

∗ destination and source are not altered∗ Useful to test relationship (>, =) between two operands∗ Used in conjunction with conditional jump instructions

for decision making purposes• Examples

cmp EBX,EAX cmp count,100

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Status Flags

Status Flags (cont’d)

• Status flags are updated to indicate certain properties of the result∗ Example: If the result is zero, zero flag is set

• Once a flag is set, it remains in that state until another instruction that affects the flags is executed

• Not all instructions affect all status flags∗ add and sub affect all six flags∗ inc and dec affect all but the carry flag∗ mov, push, and pop do not affect any flags

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• Example; initially, assume ZF = 0mov AL,55H ; ZF is still zerosub AL,55H ; result is 0

; ZF is set (ZF = 1)push BX ; ZF remains 1mov BX,AX ; ZF remains 1pop DX ; ZF remains 1mov CX,0 ; ZF remains 1inc CX ; result is 1

; ZF is cleared (ZF = 0)


• Zero Flag∗ Indicates zero result

– If the result is zero, ZF = 1– Otherwise, ZF = 0

∗ Zero can result in several ways (e.g. overflow)mov AL,0FH mov AX,0FFFFH mov AX,1add AL,0F1H inc AX dec AX

» All three examples result in zero result and set ZF

∗ Related instructionsjz jump if zero (jump if ZF = 1)jnz jump if not zero (jump if ZF = 0)

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• Uses of zero flag∗ Two main uses of zero flag

» Testing equality– Often used with cmp instruction

cmp char,’$’ ; ZF = 1 if char is $

cmp AX,BX

» Counting to a preset value– Initialize a register with the count value– Decrement it using dec instruction– Use jz/jnz to transfer control


• Consider the following code

sum := 0for (i = 1 to M)

for (j = 1 to N)sum := sum + 1

end forend for

• Assembly code

sub AX,AX ; AX := 0mov DX,M

outer_loop:mov CX,N

inner_loop:inc AXloop inner_loopdec DXjnz outer_loop

exit_loops:mov sum,AX

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• Two observations∗ loop instruction is equivalent to

dec DXjnz outer_loop

» This two instruction sequence is more efficient than the loopinstruction (takes less time to execute)

» loop instruction does not affect any flags!

∗ This two instruction sequence is better than initializing DX = 1 and executing

inc DXcmp DX,Mjle inner_loop


• Carry Flag∗ Records the fact that the result of an arithmetic

operation on unsigned numbers is out of range∗ The carry flag is set in the following examples

mov AL,0FH mov AX,12AEHadd AL,0F1H sub AX,12AFH

∗ Range of 8-, 16-, and 32-bit unsigned numberssize range

8 bits 0 to 255 (28 − 1)16 bits 0 to 65,535 (216 − 1)32 bits 0 to 4,294,967,295 (232−1)

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∗ Carry flag is not set by inc and dec instructions» The carry flag is not set in the following examples

mov AL,0FFH mov AX,0inc AL dec AX

∗ Related instructionsjc jump if carry (jump if CF = 1)jnc jump if no carry (jump if CF = 0)

∗ Carry flag can be manipulated directly usingstc set carry flag (set CF to 1)clc clear carry flag (clears CF to 0)cmc complement carry flag (inverts CF value)


• Uses of carry flag∗ To propagate carry/borrow in multiword

addition/subtraction1 ← carry from lower 32 bits

x = 3710 26A8 1257 9AE7Hy = 489B A321 FE60 4213H

7FAB C9CA 10B7 DCFAH

∗ To detect overflow/underflow condition» In the last example, carry out of leftmost bit indicates overflow

∗ To test a bit using the shift/rotate instructions» Bit shifted/rotated out is captured in the carry flag» We can use jc/jnc to test whether this bit is 1 or 0

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• Overflow flag∗ Indicates out-of-range result on signed numbers

– Signed number counterpart of the carry flag∗ The following code sets the overflow flag but not the

carry flagmov AL,72H ; 72H = 114Dadd AL,0EH ; 0EH = 14D

∗ Range of 8-, 16-, and 32-bit signed numberssize range

8 bits − 128 to +127 27 to (27 − 1)16 bits − 32,768 to +32,767 215 to (215 − 1)32 bits −2,147,483,648 to +2,147,483,647 231 to (231 − 1)


Unsigned interpretation

mov AL,72Hadd AL,0EHjc overflow

no_overflow:

(no overflow code here). . . .

overflow:

(overflow code here). . . .

Signed interpretation

mov AL,72Hadd AL,0EHjo overflow

no_overflow:

(no overflow code here). . . .

overflow:

(overflow code here). . . .

• Signed or unsigned: How does the system know?∗ The processor does not know the interpretation∗ It sets carry and overflow under each interpretation

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∗ Related instructionsjo jump if overflow (jump if OF = 1)jno jump if no overflow (jump if OF = 0)

• Uses of overflow flag∗ Main use

» To detect out-of-range result on signed numbers


• Sign flag∗ Indicates the sign of the result

– Useful only when dealing with signed numbers– Simply a copy of the most significant bit of the result

∗ Examplesmov AL,15 mov AL,15add AL,97 sub AL,97clears the sign flag as sets the sign flag asthe result is 112 the result is −82(or 0111000 in binary) (or 10101110 in binary)

∗ Related instructionsjs jump if sign (jump if SF = 1)jns jump if no sign (jump if SF = 0)

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• Consider the count down loop:for (i = M downto 0)

<loop body>end for

• If we don’t use the jns, we need cmp as shown below:

cmp CX,0jl for_loop

The count down loop can be implemented as

mov CX,Mfor_loop:

<loop body>dec CXjns for_loop

• Usage of sign flag∗ To test the sign of the result∗ Also useful to efficiently implement countdown loops


• Auxiliary flag∗ Indicates whether an operation produced a carry or

borrow in the low-order 4 bits (nibble) of 8-, 16-, or 32-bit operands (i.e. operand size doesn’t matter)

∗ Example1 ← carry from lower 4 bits

mov AL,43 43D = 0010 1011Badd AL,94 94D = 0101 1110B

137D = 1000 1001B

» As there is a carry from the lower nibble, auxiliary flag is set

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• Parity flag∗ Indicates even parity of the low 8 bits of the result

– PF is set if the lower 8 bits contain even number 1 bits– For 16- and 32-bit values, only the least significant 8 bits

are considered for computing parity value∗ Example

mov AL,53 53D = 0011 0101Badd AL,89 89D = 0101 1001B

142D = 1000 1110B» As the result has even number of 1 bits, parity flag is set

∗ Related instructionsjp jump on even parity (jump if PF = 1)jnp jump on odd parity (jump if PF = 0)

FLAGS with Signed and Unsigned Numbers

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Unsigned Integers

• When you use unsigned integers the bits in the FLAGS register (also called “flags”) are important:∗ ZF: The Zero Flag (set to 1 if result is 0)∗ CF: The Carry Flag

• Consider: cmp a, b (which computes a-b)∗ If a = b: ZF is set, CF is not set∗ If a < b: ZF is not set, CF is set (borrow)∗ If a > b: ZF is not set, CF is not set

• Therefore, by looking at ZF and CF you can determine the result of the comparison!

Signed Integers

• For signed integers you should care about three flags∗ ZF: zero flag ∗ OF: overflow flag (set to 1 if the result overflows or

underflows)∗ SF: sign flag (set to 1 if the result is negative)

• Consider: cmp a, b (which computes a-b)∗ If a = b: ZF is set, OF is not set, SF is not set∗ If a < b: ZF is not set, and SF ≠ OF∗ If a > b: ZF is not set, and SF = OF

• Therefore, by looking at ZF, SF, and OF you can determine the result of the comparison!

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Signed Integers: SF and OF???

• Why do we have this odd relationship between SF an OF?• Consider two signed integers a and b, and remember that

we compute (a-b)• If a < b

∗ If there is no underflow, then (a-b) is a negative number!∗ If there is underflow, then (a-b) is (erroneously) a positive number∗ Therefore, in both cases SF ≠ OF

• If a > b∗ If there is no overflow, the (correct) result is positive∗ If there is an overflow, the (incorrect) result is negative∗ Therefore, in both cases SF = OF

The FLAGS register

• Is it very important to remember that many instructions change the bits of the FLAGS register

• So you should “act” on flag values immediately, and not expect them to remain unchanged inside FLAGS∗ or you can save them by-hand for later use perhaps

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Summary

cmp a,b ZF CF OF SF

a=b 1 0

a<b 0 1unsigned a>b 0 0

a=b 1 0 0

signeda<b 0 v !v

a>b 0 v v

JUMP and Branches (Conditional Jumps)

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Unconditional Jump

∗ Format:jmp label

∗ Semantics:» Execution is transferred to the instruction identified by label

• Target can be specified in one of two ways∗ Directly

» In the instruction itself

∗ Indirectly» Through a register or memory

Unconditional Jump (cont’d)

Example• Two jump instructions

∗ Forward jumpjmp CX_init_done

∗ Backward jumpjmp repeat1

• Programmer specifies target by a label

• Assembler computes the offset using the symbol table

. . .mov CX,10jmp CX_init_done

init_CX_20:mov CX,20

CX_init_done:mov AX,CX

repeat1:dec CX

. . .jmp repeat1

. . .

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Unconditional Jump (cont’d)

• Address specified in the jump instruction is not the absolute address∗ Uses relative address

» Specifies relative byte displacement between the target instruction and the instruction following the jump instruction

» Displacement is w.r.t the instruction following jmp– Reason: IP points to this instruction after reading jump

∗ Execution of jmp involves adding the displacement value to current IP

∗ Displacement is a signed 16-bit number» Negative value for backward jumps» Positive value for forward jumps

Target Location

• Inter-segment jump∗ Target is in another segment

CS = target-segment (2 bytes)IP = target-offset (2 bytes)

» Called far jumps (needs five bytes to encode jmp)

• Intra-segment jumps∗ Target is in the same segment

IP = IP + relative-displacement (1 or 2 bytes)∗ Uses 1-byte displacement if target is within −128 to +127

» Called short jumps (needs two bytes to encode jmp)∗ If target is outside this range, uses 2-byte displacement

» Called near jumps (needs three bytes to encode jmp)

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Target Location (cont’d)

• In most cases, the assembler can figure out the type of jump ∗ For backward jumps, assembler can decide whether to

use the short jump form or not• For forward jumps, it needs a hint from the

programmer∗ Use SHORT prefix to the target label∗ If such a hint is not given

» Assembler reserves three bytes for jmp instruction» If short jump can be used, leaves one byte of nop (no operation)

– See the next example for details

Example. . .

8 0005 EB 0C jmp SHORT CX_init_done0013 - 0007 = 0C

9 0007 B9 000A mov CX,10 10 000A EB 07 90 jmp CX_init_done

nop 0013 - 000D = 0711 init_CX_20: 12 000D B9 0014 mov CX,20 13 0010 E9 00D0 jmp near_jump

00E3 - 0013 = D014 CX_init_done: 15 0013 8B C1 mov AX,CX

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Example (cont’d)16 repeat1: 17 0015 49 dec CX 18 0016 EB FD jmp repeat1

0015 - 0018 = -3 = FDH. . . 84 00DB EB 03 jmp SHORT short_jump

00E0 - 00DD = 3 85 00DD B9 FF00 mov CX, 0FF00H 86 short_jump: 87 00E0 BA 0020 mov DX, 20H 88 near_jump: 89 00E3 E9 FF27 jmp init_CX_20

000D - 00E6 = -217 = FF27H

Indirect Jumps

• Direct jump∗ Target address is encoded in the instruction itself

• Indirect jump∗ Introduces a level of indirection

» Address is specified either through memory of a general-purpose register

∗ Examplejmp CX

jumps to the address in CX

∗ Address is absolute» Not relative as in direct jumps

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Indirect Jumps (cont’d)Switch (ch) {

Case ’0’:count[0]++; break;




Default:count[3]++;

}

Indirect Jumps (cont’d)

Turbo C assembly code for the switch statement

_main PROC NEAR. . .

mov AL,chcbwsub AX,48 ; 48 = ASCII for 0mov BX,AXcmp BX,3ja defaultshl BX,1 ; BX = BX * 2jmp WORD PTR CS:jump_table[BX]

Indirect jump

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Indirect Jumps (cont’d)case_0: inc WORD PTR [BP-10]

jmp SHORT end_switch case_1: inc WORD PTR [BP-8]



jmp SHORT end_switchdefault: inc WORD PTR [BP-2]end_switch:

. . ._main ENDP

Indirect Jumps (cont’d)jump_table LABEL WORD

DW case_0DW case_1DW case_2DW case_3

. . .

• Indirect jump uses this table to jump to the appropriate case routine

• The indirect jump instruction uses segment override prefix to refer to the jump_table in the CODE segment

Jump table for the indirect jump

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Conditional Jumps

• Three types of conditional jumps∗ Jumps based on the value of a single flag

» Arithmetic flags such as zero, carry can be tested using these instructions

∗ Jumps based on unsigned comparisons» Operands of cmp instruction are treated as unsigned

numbers

∗ Jumps based on signed comparisons» Operands of cmp instruction are treated as signed numbers

Conditional Jumps (cont’d)

Format:j<cond> lab

– Execution is transferred to the instruction identified by label only if <cond> is met

• Example: Testing for carriage returnread_char:

. . . cmp AL,0DH ; 0DH = ASCII carriage returnje CR_receivedinc CLjmp read_char

. . .CR_received:

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Jumps Based on Single Flags

Testing for zerojz jump if zero jumps if ZF = 1je jump if equal jumps if ZF = 1

jnz jump if not zero jumps if ZF = 0jne jump if not equal jumps if ZF = 0

jcxz jump if CX = 0 jumps if CX = 0(Flags are not tested)

Jumps Based on Single Flags (cont’d)

Testing for carryjc jump if carry jumps if CF = 1jnc jump if no carry jumps if CF = 0

Testing for overflowjo jump if overflow jumps if OF = 1jno jump if no overflow jumps if OF = 0

Testing for signjs jump if negative jumps if SF = 1jns jump if not negative jumps if SF = 0

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Jumps Based on Single Flags (cont’d)

Testing for parityjp jump if parity jumps if PF = 1jpe jump if parity jumps if PF = 1

is even

jnp jump if not parity jumps if PF = 0jpo jump if parity jumps if PF = 0

is odd

Jumps Based on Unsigned Comparisons

Mnemonic Meaning Conditionje jump if equal ZF = 1jz jump if zero ZF = 1

jne jump if not equal ZF = 0jnz jump if not zero ZF = 0

ja jump if above CF = ZF = 0jnbe jump if not below CF = ZF = 0

or equal

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Jumps Based on Unsigned Comparisons

Mnemonic Meaning Conditionjae jump if above CF = 0

or equaljnb jump if not below CF = 0

jb jump if below CF = 1jnae jump if not above CF = 1

or equal

jbe jump if below CF=1 or ZF=1or equal

jna jump if not above CF=1 or ZF=1

Jumps Based on Signed Comparisons

Mnemonic Meaning Conditionje jump if equal ZF = 1jz jump if zero ZF = 1

jne jump if not equal ZF = 0jnz jump if not zero ZF = 0

jg jump if greater ZF=0 & SF=OFjnle jump if not less ZF=0 & SF=OF

or equal

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Jumps Based on Signed Comparisons (cont’d)

Mnemonic Meaning Conditionjge jump if greater SF = OF

or equaljnl jump if not less SF = OF

jl jump if less SF ≠ OFjnge jump if not greater SF ≠ OF

or equal

jle jump if less ZF=1 or SF ≠ OFor equal

jng jump if not greater ZF=1 or SF ≠ OF

A Note on Conditional Jumps

target:. . .

cmp AX,BXje targetmov CX,10

. . .

traget is out of range for a short jump

• Use this code to get around

target:

. . . cmp AX,BXjne skip1jmp target

skip1:mov CX,10

. . .

• All conditional jumps are encoded using 2 bytes∗ Treated as short jumps

• What if the target is outside this range?

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Block-Structured IF Statements

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

mov eax,op1cmp eax,op2jne L1body1jmp L2

L1: body2L2:

if( op1 == op2 )body1;

elsebody2;

Your turn . . .

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

cmp ebx,ecxja nextbody

next:

if( ebx <= ecx ){

body}

(There are multiple correct solutions to this problem.)

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Your turn . . .

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

mov eax,var1cmp eax,var2jle L1mov var3,6mov var4,7jmp L2

L1: mov var3,10L2:

if( var1 <= var2 )var3 = 10;

else{

var3 = 6;var4 = 7;

}


Compound Expression with AND (1 of 3)

• We can implement a Boolean expression that uses the local AND operator:

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

• The Assembler will execute the body if both conditions are satisfied.

if (al > bl) AND (bl > cl){

body}

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cmp al,bl ; first expression...ja L1jmp next

L1:cmp bl,cl ; second expression...ja L2jmp next

L2: ; both are truemov X,1 ; set X to 1

next:

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

This is one possible implementation . . .


cmp al,bl ; first expression...jbe next ; quit if falsecmp bl,cl ; second expression...jbe next ; quit if falsemov 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:

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Your turn . . .

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

cmp ebx,ecxja nextcmp ecx,edxjbe nextmov eax,5mov edx,6

next:

if( ebx <= ecx&& ecx > edx )

{eax = 5;edx = 6;

}


Compound Expression with OR (1 of 2)

• We can implement a Boolean expression that uses the local OR operator

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

• The Assembler will execute the body if at least one condition is satisfied.

if (al > bl) OR (bl > cl)body;

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Compound Expression with OR (2 of 2)

cmp al,bl ; is AL > BL?ja L1 ; yescmp bl,cl ; no: is BL > CL?jbe next ; no: skip next statement

L1: mov X,1 ; set X to 1next:

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

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

Loop Instructions

Unconditional loop instruction∗ Format:

loop target

∗ Semantics:» Decrements CX and jumps to target if CX ≠ 0

– CX should be loaded with a loop count value• Example: Executes loop body 50 times

mov CX,50repeat:

<loop body>loop repeat

...

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Loop Instructions (cont’d)

• The previous example is equivalent tomov CX,50

repeat:<loop body>dec CXjnz repeat

...∗ Surprisingly,

dec CXjnz repeat

executes faster than loop repeat

Loop Instructions (cont’d)

• Conditional loop instructions∗ loope/loopz

» Loop while equal/zeroCX = CX – 1If (CX = 0 and ZF = 1)

jump to target

∗ loopne/loopnz» Loop while not equal/not zero

CX = CX – 1If (CX = 0 and ZF = 0)

jump to target

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For Loops• Let’s translate the following loop:

sum = 0;for (i = 0; i <= 10; i++)

sum += i• Translation

mov eax, 0 ; eax is summov ebx, 0 ; ebx is i

loop_start:cmp ebx, 10 ; compare i and 10jg loop_end ; if (i > 10) goto end_loopadd eax, ebx ; sum += Iinc ebxjmp loop_start ; goto loop

loop_end:

For Loops

• Let’s translate the following loop:sum = 0;for (i = 0; i <= 10; i++)

sum += i• The x86 loop instruction requires that

∗ The loop index be stored in ecx∗ The loop index be decremented∗ The loop exists when the loop index is equal to zero

• Given this, we really have to think of this loop in reversesum = 0for (i = 10; i > 0; i--)

sum += i• This loop is equivalent to the previous one, but now it can be directly

translated to assembly using the loop instruction

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Using the loop Instruction

• Here is our “reversed” loopsum = 0for (i = 10; i > 0; i--)

sum += i• And the translation

mov eax, 0 ; eax is summov ecx, 10 ; ecx is i

loop_start:add eax, ecx ; sum += iloop loop_start ; if i > 0, go to loop_start

While Loops

• A generic while loopwhile (condition) {

body}

• Translated as:while:

; instructions to set flags (e.g., cmp...)jxx end_while ; branches if condition=false; body of loopjmp while

end_while

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Do While Loops

• A generic do while loopdo {

body} while (condition)

• Translated as:do:

; body of loop; instructions to set flags (e.g., cmp...)jxx do ; branches if condition=true

Logical And Shift Instructions

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Logical Instructions

∗ Format:and destination,sourceor destination,sourcexor destination,sourcenot destination

∗ Semantics:» Performs the standard bitwise logical operations

– result goes to destination ∗ test is a non-destructive and instruction

test destination,source

∗ Performs logical AND but the result is not stored in destination (like the CMP instruction)

Logical Instructions (cont’d)

Example:. . . and AL,01H ; test the least significant bitjz bit_is_zero

<bit 1 code>jmp skip1

bit_is_zero:<bit 0 code>

skip1:. . .

• test instruction is better in place of and

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Shift Instructions• Two types of shifts

» Logical» Arithmetic

∗ Logical shift instructionsShift left

shl destination,count shl destination,CL

Shift rightshr destination,count shr destination,CL

∗ Semantics:» Performs left/right shift of destination by the value in count or CL register

– CL register contents are not altered

Shift Instructions (cont’d)

Logical shift∗ Bit shifted out goes into the carry flag

» Zero bit is shifted in at the other end

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∗ count is an immediate valueshl AX,5

∗ Specification of count greater than 31 is not allowed» If a greater value is specified, only the least significant 5 bits

are used

∗ CL version is useful if shift count is known at run time» Ex: when the shift count value is passed as a parameter in a

procedure call» Only the CL register can be used

Shift count value should be loaded into CLmov CL,5shl AX,CL


Arithmetic shift∗ Two versions as in logical shift

sal/sar destination,countsal/sar destination,CL

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Double Shift Instructions

• Double shift instructions work on either 32- or 64-bit operands

• Format ∗ Takes three operands

shld dest,src,count ; left shiftshrd dest,src,count ; right shift

∗ dest can be in memory or register∗ src must be a register∗ count can be an immediate value or in CL as in other

shift instructions

Double Shift Instructions (cont’d)

∗ src is not modified by doubleshift instruction∗ Only dest is modified∗ Shifted out bit goes into the carry flag

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Rotate Instructions

∗ Two types of ROTATE instructions∗ Rotate without carry

» rol (ROtate Left)» ror (ROtate Right)

∗ Rotate with carry» rcl (Rotate through Carry Left)» rcr (Rotate through Carry Right)

∗ Format of ROTATE instructions is similar to the SHIFT instructions

» Supports two versions– Immediate count value– Count value in CL register

Rotate Instructions (cont’d)

∗ Bit shifted out goes into the carry flag as in SHIFT instructions

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∗ Bit shifted out goes into the carry flag as in SHIFT instructions


• Example: Shifting 64-bit numbers∗ Multiplies a 64-bit value in EDX:EAX by 16

» Rotate versionmov CX,4

shift_left:shl EAX,1rcl EDX,1loop shift_left

» Doubleshift versionshld EDX,EAX,4shl EAX,4

• Division can be done in a similar a way

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Illustrative Examples

• Five examples can be downloaded from the website∗ Conversion of ASCII to binary representation

(BINCHAR.ASM)∗ Conversion of ASCII to hexadecimal by character

manipulation (HEX1CHAR.ASM) ∗ Conversion of ASCII to hexadecimal using the XLAT

instruction (HEX2CHAR.ASM)∗ Conversion of lowercase letters to uppercase by

character manipulation (TOUPPER.ASM)∗ Sum of individual digits of a number

(ADDIGITS.ASM)Last slide

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