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INSTRUCTION SET REFERENCE
LDMXCSRLoad Streaming SIMD Extension Control/Status
(Continued)
The rounding control is set to round to nearest upon reset.
Bit 15 (FZ) is used to turn on the Flush-To-Zero mode (bit is set). Turning on the Flush-To-Zero
mode has the following effects during underflow situations:
zero results are returned with the sign of the true result
precision and underflow exception flags are set
The IEEE mandated masked response to underflow is to deliver the denormalized result (i.e.,
gradual underflow); consequently, the flush-to-zero mode is not compatible with IEEE Std. 754.
It is provided primarily for performance reasons. At the cost of a slight precision loss, faster
execution can be achieved for applications where underflows are common. Unmasking the
underflow exception takes precedence over Flush-To-Zero mode. This arrangement means that
an exception handler will be invoked for a Streaming SIMD Extension that generates an under-
flow condition while this exception is unmasked, regardless of whether flush-to-zero is enabled.
The other bits of MXCSR (bits 31-16 and bit 6) are defined as reserved and cleared; attempting
to write a non-zero value to these bits, using either the FXRSTOR or LDMXCSR instructions,
will result in a general protection exception.
The linear address corresponds to the address of the least-significant byte of the referenced
memory data.
Operation
MXCSR = m32;
Rounding ModeRC FieldDescription
Round to nearest
(even)
00B
Rounded result is the closest to the infinitely precise
result. If two values are equally close, the result is the
even value (that is, the one with the least-significant bit
of zero).
Round down (to
minus infinity)
01B
Rounded result is close to but no greater than the infi-
nitely precise result
Round up (toward
positive infinity)
10B
Rounded result is close to but no less than the infinitely
precise result.
Round toward
zero (truncate)
11B
Rounded result is close to but no greater in absolute
value than the infinitely precise result.
