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RENESAS TOOL NEWS on January 16, 2004: RSO-M3T-NC308WA_2-040116D
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A Note on Using C Compiler M3T-NC308WA
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Please take note of the following problem in using the M3T-NC308WA C-compiler
(with an assembler and integrated development environment) for the M32C/80 and M16C/80 series MCUs:
- On casting an integer-type value to a pointer-type one
- Versions Concerned
M3T-NC308WA V.2.00 Release 1 through V.5.10 Release 1
- Description
When an integer-type value representing an address outside of the address space is cast to a pointer-type one,
the comparison of it with another pointer-type value may not be performed correctly. The reason is as follows:
In the M3T-NC308WA, a pointer-type value in the far area is 32-bit wide.
However, it is assigned to a 24-bit address register in some versions of the products because the whole address space can be represented in 24 bits.
If an integer-type value indicating the outside of the address space is cast to a pointer-type one in the far area,
all the uppermost 8 bits of 32-bit value must be changed to zeros since the real size of the storage area that holds far pointers is a minimum of 24 bits.
We are sorry that in the above case the uppermost 8 bits are not changed to zeros in the product concerned.
Then, casting an integer-type value indicating the outside of the address space to a pointer-type value in the far area and making the comparison of it with another pointer-type value may not bring a correct result depending on whether the cast value to compare is assigned to a 24-bit address register or not.
Note that in pointer-type values pointing to variables or functions on memory,
their uppermost 8 bits definitely become zeros, so that no problems arise.
- Conditions
This problem occurs if the following four conditions are satisfied:
| (1) |
An integer-type variable or constant is cast to a pointer-type one.
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| (2) |
The comparison of the cast value in (1) with another pointer-type value is made.
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| (3) |
The original value in (1) is not within a range from 0x00000000 to 0x00ffffff.
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| (4) |
As a result of compilation, either of the two values in (2) is assigned to a 24-bit address register before or at the comparison.
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- Example
The following is an example under the condition that optimizing option -O is used in the M3T-NC308WA V.5.10 Release 1.
Since whether a pointer-type value is assigned to a 24-bit address register or not depends on the product's version,
this problem does not occur if your product's version is not involved.
-----------------------------------------------------------------------
void func(char *p)
{
if (p == 0) return; /* Equality testing with 0: no problem */
if (p == (int *)(-1)) { /* Condition (1): integer-type constant
(-1) is cast */
/* Condition (2): cast value is tested */
/* Condition (3): -1 is not within a range
from 0 to 0x00ffffff */
return;
}
*p = 0;
}
void caller(void)
{
func((int *)(-1)); /* Condition (1): integer-type constant
(-1) is cast */
}
-----------------------------------------------------------------------
- Workaround
When casting a value outside of a range from 0 to 0x00ffffff to a pointer-type value,
perform an AND operation between the value before casting and a constant of 0x00ffffff to mask the uppermost 8 bits.
The following is a case where a constant of type int is handled; this method is also applicable to a variable of type int.
-------------------------------------------------------------------------
void func(char *p)
{
if (p == 0) return;
if (p == (int *)(-1 & 0x00ffffff)) { /* Uppermost 8 bits masked */
return;
}
*p = 0;
}
void caller(void)
{
func((int *)(-1 & 0x00ffffff)); /* Uppermost 8 bits masked */
}
-------------------------------------------------------------------------
- Schedule of Fixing the Problem
We plan to fix this problem in our next release of the product.
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