SUMMARY
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
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
X X X IC RC RC RC PC PC IE PM UM ZM DM IM
Status Word
Bit Bit
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
B C3 ST ST ST C2 C1 C0 ES SF PE UE OE ZE DE IE
The abbreviations for the fields in the Control Word and Status Word are:
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