Q52101: /Zr Generates Error When Assigning to Video Memory

Article: Q52101
Product(s): See article
Version(s): 2.00 2.01
Operating System(s): MS-DOS
Keyword(s): ENDUSER | | mspl13_c
Last Modified: 17-JAN-1990

Question:

Why does the following code generate run-time error R6013 (illegal far
pointer use) for the second reference to farptr, but not the first,
when compiled with /Zr or when compiled within the QuickC environment
with pointer checking turned on?

Sample Code
-----------

void main(void)
{
    int far *farptr;
    int n=0;
    farptr=(int far *)0xb8000000lu;
    *(farptr+n)=177 | 0x0700; /* no error */
    n=12;
    n -= 11;
    *farptr=178 | 0x0700;      /* error  */
}

Response:

This is expected behavior from the /Zr option.

The code generated for pointer checking (/Zr) checks for out-of-range
pointers. As farptr is pointing to segment b800 (video memory), it
assumes that this is an error because b800 is larger than the variable
__asegh, which is defined as the highest segment in memory owned by
the program.

This variable gets updated if an _fmalloc, for example, allocates a
new segment higher than __asegh. When an assignment is made, for
instance "farptr=0xb8000000", __asegh is not modified.

Therefore, when the pointer assignment is made and the code is
generated to check for invalid pointers, the compiler thinks that this
pointer is invalid, since its segment (0xb800) is greater than
__asegh.

To use this code, you need to compile without /Zr (or turn off pointer
checking in the environment) when assigning pointers to memory higher
than the value __asegh. (Similarly, __aseglo marks the lowest segment
in use in memory.) This is valid code.

The pointer checking does not have any effect on the line *(farptr+n)
in the code because, since QuickC is a one pass compiler, QuickC
recognizes this line as one of incremental nature; its purpose in this
case is to increment the pointer. Due to the precedence of the "()"
over the "*" operator, and since this is a line of incremental nature,
QuickC does not generate the code to check the segment and offset with
_asegh and _aseglo.

It does generate the correct code to make the assignment; however, it
is too late to then generate the code for the segment:offset checking.

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