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MAEC TOOL NEWS:
MAECT-M3T-NC79WA-020301D
A Note on C Compilers
M3T-NC79WA and M3T-NC77WA
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Please take note of the following problem in using C compilers (with an assembler and integrated development environment) M3T-NC79WA and M3T-NC77WA.
- On switch-case statements
- Products and Versions Concerned
| For the 7900 series MCUs | : M3T-NC79WA V.2.00 Release 1 -- V.4.10 Release 1 |
| For the 77xx series MCUs | : M3T-NC77WA V.3.10 Release 2 -- V.5.20 Release 4 |
- Description
When compiling switch-case statements, code that makes a jump only to the default or a specific case label may be generated no matter what case value meets the conditional expression.
- Conditions
- This problem occurs if condition (1) or (2) below is satisfied with compile option -fswitch_table[-fST] selected.
- (1) In the conditional expression of a switch statement, a variable of type unsigned char is used, and the sequence of the case values is any of the following three types:
- a. The case values are 255 consecutive numbers from 1 to 255.
- b. The case values are 255 consecutive numbers from 0 to 254.
- c. The total number of cases (excluding the default) is 143 or more with the minimum case value 0 and the maximum 255.
- (2) In the conditional expression of a switch statement, a variable of type signed char is used, and the sequence of the case values is any of the following three types:
- a. The case values are 255 consecutive numbers from -127 to +127.
- b. The case values are 255 consecutive numbers from -128 to +126.
- c. The total number of cases (excluding the default) is 143 or more with the minimum case value -128 and the maximum 127.
- Example
[In condition (1)-a]
--------------------------------------------------------------------
char c;
switch(c){
case 1:
case 2:
func(3);
break;
:
:
case 255:
func(255);
break;
default:
break;
}
--------------------------------------------------------------------
- Workaround
In the conditional expression of the switch statement, when the variable is of type unsigned char, cast it to an unisigned int value; when it is of type signed char, cast it to a signed int value.
[Example]
--------------------------------------------------------------------
char c;
switch((unsigned int)c){ /* Variable c of type unsigned char
case 1: is cast to of type unsigned int */
:
:
case 255:
:
}
--------------------------------------------------------------------
- Schedule of Fixing the Problem
We plan to fix this problem in our next release.
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