Computer Hardware User's Manual

Vol. 2A 3-399
INSTRUCTION SET REFERENCE, A-M
FSUB/FSUBP/FISUB—Subtract
The FISUB instructions convert an integer source operand to double extended-preci-
sion floating-point format before performing the subtraction.
Table 3-43 shows the results obtained when subtracting various classes of numbers
from one another, assuming that neither overflow nor underflow occurs. Here, the
SRC value is subtracted from the DEST value (DEST − SRC = result).
When the difference between two operands of like sign is 0, the result is +0, except for
the round toward −∞ mode, in which case the result is −0. This instruction also guaran-
tees that +0 − (−0)
= +0, and that −0 − (+0) = −0. When the source operand is an integer 0,
it is treated as a +0.
When one operand is ∞, the result is ∞ of the expected sign. If both operands are ∞ of
the same sign, an invalid-operation exception is generated.
This instruction’s operation is the same in non-64-bit modes and 64-bit mode.
Operation
IF Instruction = FISUB
THEN
DEST ← DEST
− ConvertToDoubleExtendedPrecisionFP(SRC);
ELSE (* Source operand is floating-point value *)
DEST ← DEST
− SRC;
FI;
Table 3-43. FSUB/FSUBP/FISUB Results
SRC
−∞ −F or −I −0 +0 +F or +I+∞ NaN
−∞ * −∞ −∞ −∞ −∞ −∞ NaN
−F+∞ ±F or ±0 DEST DEST −F −∞ NaN
DEST −0+
∞ −SRC ±0 −0 −SRC −∞ NaN
+0+∞ −SRC +0 ±0 −SRC −∞ NaN
+F+∞ +F DEST DEST ±F or ±0 −∞ NaN
+
∞ +∞ +∞ +∞ +∞ +∞ *NaN
NaN NaN NaN NaN NaN NaN NaN NaN
NOTES:
F Means finite floating-point value.
IMeans integer.
* Indicates floating-point invalid-arithmetic-operand (#IA) exception.