Arm VERSION 1.2 Manuel d'utilisateur télécharger pdf (Page 34)

Writing ARM and Thumb Assembly Language

2.5.3 Using conditional execution in ARM state

You can use conditional execution of ARM instructions to reduce the number of branch

instructions in your code. This improves code density.

Branch instructions are also expensive in processor cycles. On ARM processors without

branch prediction hardware, it typically takes three processor cycles to refill the

processor pipeline each time a branch is taken.

Some ARM processors, for example ARM10

™

and StrongARM

, have branch

prediction hardware. In systems using these processors, the pipeline only needs to be

flushed and refilled when there is a misprediction.

2.5.4 Example of the use of conditional execution

This example uses two implementations of Euclid’s Greatest Common Divisor (gcd)

algorithm. It demonstrates how you can use conditional execution to improve code

density and execution speed. The detailed analysis of execution speed only applies to

an ARM7

™

processor. The code density calculations apply to all ARM processors.

In C the algorithm can be expressed as:

int gcd(int a, int b)

{

while (a != b) do

{

if (a > b)

a = a - b;

else

b = b - a;

}

return a;

}

You can implement the gcd function with conditional execution of branches only, in the

following way:

gcd CMP r0, r1

BEQ end

BLT less

SUB r0, r0, r1

B gcd

less

SUB r1, r1, r0

B gcd

end

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Arm VERSION 1.2 Manuel d'utilisateur Page 34