C program translation.ppt

7/27/2019 C program translation.ppt

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Translating C programs into assembly

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Program TranslationC is closer to assembly than almost any other language

Arithmetic/logic/bitwise operations resemble machineoperations

The use of pointers corresponds to memory access atmachine level

No high-level language complex elements

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Program TranslationFor now, usepseudo assembly instructions

Will see full details of assembly

Use of registers:

Denote registers as r0, r1, r2,

Registers can hold byte, half-word (short), word

(integer) r0 holds zero

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Program TranslationMemory operations:

Load: Load a memory variable at a given memoryaddress to a register

Store: Store a register value into a memory variable ata give memory address

No other operations are allowed on memory variables

load r1, &a ; r1


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Program TranslationData operations:

Add+, sub -, mul*, div /, and&, or|,xor^, shift-left

Can use two registers

Can use one register, one constant

Format:

add r1, r2, r3add r1, r1, 100

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Program TranslationCompare instruction: cmp

Can compare two registers

Can compare one register, one constant Cannot use memory

cmp r1, r2

cmp r1, 0

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Program TranslationBranch instructions: non-sequential control flow.

(1) Branch condition-when (2) Branch target-where

Should follow a compare instruction beq: branch if equal

bne: branch if not equal

blt: branch if less than

ble: branch if less than or equal

cmp r1, 0

beqlabel

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Arithmetic ExpressionsExercise:

a = b;

a = 10;

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Arithmetic ExpressionsExercises:

c = a + b;

e = (a & b) | (c & d);

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Translating ifstatementC ifstatement:

if (cond_expr)then_statement

else

else_statement

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Translating ifstatementgoto version:

t = cond_expr;

if (!t) goto false;then_statement

goto done;

flase:

else_statementdone:

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Translating ifstatementExercise:

if (x > y)

max = x;else

max = y;

1. Writegoto version

2. Translate into pseudo-assembly

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Translating ifstatementExercise:

if (x > 0)

x = 0;1. Writegoto version


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Translating do-while LoopC do-while statement:

do

body_statementwhile (cond_expr);

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Translating do-while LoopGoto version:

loop:

body_statement

t = cond_expr;

if (t) goto loop;

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Translating while LoopExercise: Calculate the parity bit of n

m = n;

parity = 0;do {

parity ^= (m & 1);

m >>= 1;

} while (m != 0);

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Translating while LoopC while statement:

while (cond_expr)body_statement

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Translating while Loopgoto version 1:

loop:

t = cond_expr;

if (!t) goto done;

body_statement

goto loop;

done:

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Translating while LoopGoto version 2:

goto test;

loop:body_statement

test:

t = cond_expr;if (t) goto loop;

done:

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Translating while LoopExercise: Calculate the parity bit of n

m = n;

parity = 0;while (m != 0) {

parity ^= (m & 1);

m >>= 1;

}

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TranslatingforLoopC for statement:

for (init_expr; cond_expr; update_expr)

body_statement

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TranslatingforLoopTranslated to while statement:

init_expr;

while (cond_expr) {body_statement

update_expr;

}

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Translating for LoopGoto version:

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TranslatingforLoopExercise: Calculate the sum of X[N]

sum = 0;

for (i = 0; i < N; i ++) {sum += X[i];

}

1. Writegoto version


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Memory PointersHow to use pointers?

int *pInt = &m;

*pInt = m;

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C program translation.ppt

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