DOCERR: Control and Status Word Missing Abbreviation Info

• Microsoft Macro Assembler for MS-DOS, version 6.11

Page 148 of the "MASM 6.11 Reference Manual" has an alphabetical listing of math coprocessor instructions. Page 150 contains Figure 5.2, a diagram of the Control Word and Status Word. The manual is missing the descriptions of the different fields, which were included with the "MASM 6.0 Reference Manual."

The format of the Control-Word and Status-Word registers are as follows:

Control Word

Bit                                          Bit

 X  X  X IC RC RC RC PC PC IE    PM UM ZM DM IM

Status Word

Bit                                          Bit

 B C3 ST ST ST C2 C1 C0 ES SF PE UE OE ZE DE IE

   IC - Infinity Control
   0 = Projective (default on 8087 and 80287)
   1 = Affine
   Note: Used for 8087 and 80287 only, 80387 uses affine regardless
         of setting

   RC - Rounding Control
   00 = Round to nearest or even (default)
   01 = Round down toward -infinity
   10 = Round up toward +infinity
   11 = Chop by truncating toward 0

   PC - Precision Control
   00 = 24-bit mantissa
   10 = 53-bit mantissa
   11 = 64-bit mantissa

   IE - Interrupt Enable Mask
   Used for 8087 only; undefined on 80287 and 80387

   SF - Stack Flag
   Used for 80387 only; undefined on 80287 and 8087

Exception Masks and Flags

   PM/PE - Precision
   UM/UE - Underflow
   OM/OE - Overflow
   ZM/ZE - Zero Divide
   DM/DE - Denormalized Operand
   IM/IE - Invalid Operation
   For masks:
      1 = masked; 0 = unmasked
   For exceptions:
      1 = exceptions; 0 = no exception

   B - Busy
   1 = exception control unit active

   C3 C2 C1 C0 - Condition Codes

   ST - Stack Top Pointer
   Points to current top of stack

   ES - Error Summary (80287/387)
   On a 8087, the value is IR - Interrupt Request