Creating an Application w/ FORTRAN and Assembly Language

• Microsoft FORTRAN for MS-DOS, versions 4.0, 4.01, 4.1, 5.0, 5.1
• Microsoft FORTRAN for OS/2, versions 4.1, 5.0, 5.1
• Microsoft FORTRAN PowerStation for MS-DOS, versions 1.0 and 1.0a

When an application developed in FORTRAN calls a subprogram developed in assembly language, the subprogram must preserve the DS, SS, BP, SI and DI registers and clear the direction flag.

When an application uses the default optimizations, FORTRAN uses the SI and DI registers extensively. An assembly language subprogram that changes SI or DI must save the registers on the stack (with a PUSH instruction) and restore them from the stack (with a POP instruction) before it returns. The FORTRAN code also assumes that the direction flag, used by string functions in assembly language, is always clear. If your assembly language subprogram sets the direction flag (with an STD instruction), it must clear the flag (with a CLD instruction) before it returns.

As a general rule, an assembly language subprogram should also preserve the values of the DS, SS, and BP segment registers (using the PUSH and POP instructions as indicated above).