Basic Assembly Instructions - McMaster Universitynedialk/COURSES/3f03/basic-instr.pdf · Basic...

Basic Assembly Instructions

Ned Nedialkov

McMaster UniversityCanada

SE 3F03January 2013

Multiplication Division FLAGS register Branch Instructions If statements Loop instructions

Outline

Multiplication

Division

FLAGS register

Branch Instructions

If statements

Loop instructions

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Multiplication

I mul is for unsigned integersI imul is for signed integersI 255 × 255 = 65025 if unsigned

= −1 if signedI FFh = 1111|1111

I as unsigned is 255I as signed is 1|1111111 = -1

I Two’s complement representationI first bit 1 means -; 0 means +I flip all the bitsI add 1

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mul

I mul sourceI source can be register or memoryI the other operand is implicit

source other operand resultbyte AL AXword AX DX:AXdword EAX EDX:EAX

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imul

I imul sourceI source can be register or memoryI the other operand is implicit

I imul dest, source

I imul dest, source1, source2

See Table 2.2 for details

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Division

I div is for unsigned integersI idiv is for signed integersI both work the same wayI div source

I source can be register or memory

source division quotient remainderbyte AX/source AL AHword (DX:AX)/source AX DXdword (EDX:EAX)/source EAX EDX

Do not forget to initialize DX or EDX

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FLAGS register

I Contains various flagsI cmp a, b

I subtracts a − bI does not store the resultI sets flags

I For unsigned integersI ZF zero flagI CF carry flag

I For signed integersI ZF zero flagI OF overflow flag; 1 when the result overflowsI SF sign flag; 1 when the result is negative

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cmp

I Unsigned integers

cmp a,ba-b ZF CF=0 1 0>0 0 0<0 0 1

I Signed integers

cmp a,ba-b ZF SF,OF=0 1>0 0 SF=O, SF=OF<0 0 SF=1, SF!=OF

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Branch Instructions

Unconditional branchesI jmp labelI call label

I unconditional branchI like goto label

Conditional branches

I jxx label

I check flagsI if true, branch (transfer execution control) to label

I otherwise, continue from the next statement

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I jxx short labelI the jump is ±128 bytes from the current locationI advantage: the offset is 1 byte

I jxx near labelI the jump is to any location within a segmentI label is 32 bitI default, same as jxx label

I jxx word labelI 16-bit label

I jxx far labelI outside a segment

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Do cmp a,bThen

if signed unsigneda=b je jea!=b jne jnea<b jl , jnge jb, jnaea>b jg, jnle ja, jnbea>=b jge, jnl jae, jnb

For more instructions, see the text

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If statementsi f (condition) {/* then block */}else {/* else block */}

Can be translated as;; code that sets flags;; e.g. cmp a,bjxx else_block;; code in then blockjmp end_if

else_block:;; code in else block

endi_if:

jxx is a suitable branch instruction12/21


i f (condition) {/* then block */}

Can be translated as

;; code that sets flags;; e.g. cmp a,bjxx end_if;; code in then block

endi_if:

jxx is a suitable branch instruction

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Examples

sum=0;i=i-1;i f (i>0) sum++;

Can be translated into

;; assume i is in ecxmov eax, 0 ;sum=0dec ecx ;i=i-1j z end_ifinc eax ;sum++

end_if:

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i f (eax>=5)ebx=1

elseebx=2


cmp eax, 5jge then_blockmov ebx, 2jmp next

then_block:mov ebx, 1

next:

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...or into

cmp eax, 5jnz else_blockmov ebx, 1jmp end_ifjmp next

else_block:mov ebx, 2

next:

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Loop instructionsloop instruction

Example

sum = 0;for (i=10; i>0; i--)

sum += i;


mov eax, 0 ;sum=0mov ecx, 10 ;ecx=10, loop counter

loop_start:add eax, ecx ;sum+=iloop loop_start ;ecx--, goto loop_start

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Example

sum = 0;for (i=1; i<=10; i++)

sum += i;

Is the following a correct translation?

mov ebx, 1mov eax, 0 ;sum=0mov ecx, 10 ;ecx=10, loop counter

loop_start:add eax, ebx ;sum+=iinc ebxloop loop_start ;ecx--, goto loop_start

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loop

I loop start_loop same asI decrement ecx by 1I if ecx!=0 goto start_loop

I loope start_loopI loopz start_loop same as

I decrement ecx by 1I if ecx!=0 and ZF==1 goto start_loop

I loopne start_loopI loopnz start_loop same as

I decrement ecx by 1I if ecx!=0 and ZF==0 goto start_loop

ZF unchanged if ecx=0

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While loops

Example

while (condition) {/* body of the while loop */}


while:;; code that sets flagsjxx end_while ;branch if false;; code in the while bodyjmp while

end_while:

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Do-while loops

Example

do {/* body of the loop */} while (condition)


do:;; body of the loop;; code that sets flagsjxx do ;branch if true

end_while:

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Basic Assembly Instructions - McMaster Universitynedialk/COURSES/3f03/basic-instr.pdf · Basic...

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