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Q81892: FIX: FNENI, FNCLEX Assembled Wrong After PROC USES

Article: Q81892
Product(s): Microsoft Macro Assembler
Version(s): 6.0,6.0a,6.0b
Operating System(s): 
Keyword(s): 
Last Modified: 06-MAY-2001

-------------------------------------------------------------------------------
The information in this article applies to:

- Microsoft Macro Assembler (MASM), versions 6.0, 6.0a, 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

  func PROC uses ax
      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
  func ENDP
  END

Additional query words: 6.00 6.00a 6.00b buglist6.00 buglist6.00a buglist6.00b fixlist6.10

======================================================================
Keywords          :  
Technology        : kbMASMsearch kbAudDeveloper kbMASM600 kbMASM600a kbMASM600b
Version           : :6.0,6.0a,6.0b
Solution Type     : kbfix

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