(ISO C, §6.7.3) When you declare a volatile variable, the Carbide C compiler takes the following precautions to respect the value of the variable:
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The compiler stores commonly used variables in processor registers to produce faster object code. However, the compiler never stores the value of a volatile variable in a processor register.
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The compiler uses its common sub-expression optimization to compute the addresses of commonly used variables and the results of often-used expressions once at the beginning of a function to produce faster object code. However, every time an expression uses a volatile variable, the compiler computes both the address of the volatile variable and the results of the expression that uses it.
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Listing 1 shows an example of volatile variables.
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Listing 1. Volatile Variables
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void main(void)
- {
- int i[100];
- volatile int a, b; /* a and b are not cached in registers. */
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a = 5;
- b = 20;
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i[a + b] = 15; /* compiler calculates a + b */
- i[a + b] = 30; /* compiler recalculates a + b */
- }
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The compiler does not place the value of a, b, or a+b in registers. But it does recalculate a+b in both assignment statements.
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+Volatile Variables
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Volatile Variables
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(ISO C, §6.7.3) When you declare a volatile variable, the Carbide C compiler takes the following precautions to respect the value of the variable:
+
+
The compiler stores commonly used variables in processor registers to produce faster object code. However, the compiler never stores the value of a volatile variable in a processor register.
+
The compiler uses its common sub-expression optimization to compute the addresses of commonly used variables and the results of often-used expressions once at the beginning of a function to produce faster object code. However, every time an expression uses a volatile variable, the compiler computes both the address of the volatile variable and the results of the expression that uses it.
+
+
Listing 1 shows an example of volatile variables.
+
+
Listing 1. Volatile Variables
+
void main(void)
+ {
+ int i[100];
+ volatile int a, b; /* a and b are not cached in registers. */
+
a = 5;
+ b = 20;
+
i[a + b] = 15; /* compiler calculates a + b */
+ i[a + b] = 30; /* compiler recalculates a + b */
+ }
+
+
The compiler does not place the value of a, b, or a+b in registers. But it does recalculate a+b in both assignment statements.
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