FIX: FNENI, FNCLEX Assembled Wrong After PROC USES

MS-DOS           | OS/2

The information in this article applies to:

• Microsoft Macro Assembler for MS-DOS, versions 6.0, 6.0a, and 6.0b
• Microsoft Macro Assembler for OS/2, versions 6.0, 6.0a, and 6.0b

SYMPTOMS

The Microsoft Macro Assembler (MASM) incorrectly assemble the FNENI or FNCLEX coprocessor instructions if the instructions are used in a procedure declared with the PROC USES directive. FNENI and FNCLEX used outsize of PROC USES procedures assemble correctly.

RESOLUTION

Several methods can be used to work around this problem, including:

   1. Using FENI or FCLEX instead of FNENI or FNCLEX.

   2. Using a PROC without the USES directive.

   3. Putting the opcode for FNENI or FNCLEX into the code directly.
      The sample code below demonstrates this workaround. This could
      also be done by setting up a macro that when invoked puts the
      opcode into the code directly.

STATUS

Microsoft has confirmed this to be a problem in MASM versions 6.0, 6.0a, and 6.0b. This problem was corrected in MASM for MS-DOS version 6.1.

MORE INFORMATION

FENI enables interrupts by clearing the interrupt-enable mask in the control word. Since the 80287, 80387, and 80486 coprocessors do not have an interrupt-enable mask, this instruction is recognized but ignored on these coprocessors. FENI is the wait version and FNENI is the no-wait version.

FCLEX clears all coprocessor exception flags, the busy flag, and bit 7 in the status word. Bit 7 is the interrupt-request flag on the 8087 and the error-status flag on the 80287, 80387 and 80486 coprocessors. FCLEX is the wait version and FNCLEX is the no-wait version.

If the following sample code is assembled, the FNENI and FNCLEX instruction will generate the following code:

   pop ax
   iretd

   pop ax
   iret

Sample Code

; Assemble options needed: none

.MODEL small, C .STACK 4096

.CODE

    .startup
    call func
    .exit 0

    fneni       ; This statement generates a pop ax and an iretd.
    fnclex      ; This statement generates a pop ax and an iret.
    DW 0E0DBh   ; This defines a FNENI statement.
    DW 0E2DBh   ; This defines a FNCLEX statement.
    ret