ID Number: Q42762
5.10 6.00 6.00a 6.00ax 7.00
MS-DOS
Summary:
SYMPTOMS
In Microsoft C versions 5.0, 5.1, 6.0, 6.0a, 6.0ax, and C/C++ version
7.0, memory overwrites occur in the library routine i8_input. While
debugging the program under CodeView with the assembler listing, you
may notice that after executing into the code, the first 2 bytes of
each instruction beginning with a hex CD are overwritten. The C
statement that generated the assembler code with the problem in it
was an fscanf() function call.
Below is an listing example of what is occurring:
Before
------
7E1F:4266 CD35C0 INT 35 ; FLD ST(0)
7E1F:4269 CD35E1 INT 35 ; FABS
7E1F:426C CD372E7425 INT 37 ; FLD TByte Ptr [__chbuf+5C (2574)]
After
-----
7E1F:4266 90 NOP
7E1F:4267 D9C0 FLD ST(0)
7E1F:4269 90 NOP
7E1F:426A D9E1 FABS
7E1F:426C 90 NOP
7E1F:426D DB2E7425 FLD TByte Ptr [__chbuf+5C (2574)]
CAUSE
This behavior is quite normal and is nothing to worry about. The
Microsoft C floating-point package works by generating the INT
instructions shown above. When these instructions are executed, the
routine they call replaces the INT instructions with either the
library calls to the emulator library or the actual 80x87
floating-point instructions, depending on whether or not a
coprocessor is installed.
When the instructions are executed again, there is no overhead for
determining whether on not a coprocessor is installed; the proper
instructions have been patched into place already. This patching
occurs even if the -FPi87 option has been selected.
More Information:
You can force in-line 8087 instructions to be put into your code. For
more information about this technique, query on the following keywords
in the Microsoft Knowledge Base:
in-line 8087 instructions
(There is no need to do this if your code is going to run under MS-DOS
or OS/2. Although this article mainly discusses FORTRAN, it applies to
C as well because the two languages use the same floating-point
library.)
This code modification occurs only under MS-DOS. Under OS/2, coprocessor
instructions (such as in the second listing) are always generated. If
a coprocessor is not present at execution, the instructions cause
exceptions that are handled by the floating-point emulator software.
Note: Such code modification is impossible under OS/2 because there is
no way to dynamically change a code segment under OS/2, although it is
possible to cause a data segment to be executed.
Additional reference words: 5.00 5.10 6.00 6.00a 6.00ax 7.00