Q42597: Using Near Addresses in Interrupt Handlers in C

Article: Q42597
Product(s): See article
Version(s): 5.10
Operating System(s): MS-DOS
Keyword(s): ENDUSER | | mspl13_c
Last Modified: 17-MAY-1989

When the Microsoft C Compiler compiles the sample program below for
the small or medium memory model, it gives the following warning:

   warning C4058: address of frame variable taken, DS != SS

The sample program is as follows:

/* sample program */
void interrupt far handler (void) ;
void foo (char *) ;

void interrupt far handler (void)
{
char ch ;
foo (&ch) ;
}

void foo (char * ptr)   /* a trivial function */
{
*ptr = 'a' ;
}

The warning is generated because the Microsoft C Compiler assumes that
DS is equal to SS. Because the stack segment SS could possibly be
changed in an interrupt service routine, the compiler gives a warning
when a near address that refers to a stack location is passed to a
function.

In general, in the small and medium memory models, data pointers are
defaulted to be near unless the "far" keyword is used. In the example
above, the function "foo" is expecting a near address that is a 16-bit
offset. A function such as "foo" has no way to tell if the near
pointer passed to it is an offset relative to the data segment or the
stack segment. Therefore, the C compiler makes the assumption that an
offset by itself is always relative to the default data segment. This
is not a problem in the normal case, where we can depend on the
assumption that the C compiler makes SS equal to DS. However, in the
example above, the stack segment could be changed in the interrupt
handling routine; therefore, the compiler warns you that the code may
not work correctly.

If the memory model is large or compact, or if the "far" keyword is
used when foo's formal parameter is declared, the compiler will not
give any warnings. When "foo" is called, an address with the stack
segment and the offset will be passed to the function automatically.
Declaring the stack variable "ch" as static will also avoid the
problem. The corrected source code is as follows:

/* sample program */
void interrupt far handler (void) ;
void foo (char *) ;

void interrupt far handler (void)
{
static char ch ;
foo (&ch) ;
}

void foo (char * ptr)   /* a trivial function */
{
*ptr = 'a' ;
}

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