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Alpha 21264/EV67 Hardware Reference Manual
Internal Architecture
2–37
Floating-Point Control Register
UNFD
[61]
RW
Underflow Disable. The 21264/EV67 hardware cannot generate IEEE compli-
ant denormal results. UNFD is used in conjunction with UNDZ as follows:
UNDZ
[60]
RW
Underflow to zero. When UNDZ is set together with UNFD, underflow traps
are disabled and the 21264/EV67 places a true zero in the destination register.
See UNFD, above.
DYN
[59:58]
RW
Dynamic rounding mode. Indicates the rounding mode to be used by an IEEE
floating-point instruction when the instruction specifies dynamic rounding
mode:
IOV
[57]
RW
Integer overflow. An integer arithmetic operation or a conversion from float-
ing-point to integer overflowed the destination precision.
INE
[56]
RW
Inexact result. A floating-point arithmetic or conversion operation gave a result
that differed from the mathematically exact result.
UNF
[55]
RW
Underflow. A floating-point arithmetic or conversion operation gave a result
that underflowed the destination exponent.
OVF
[54]
RW
Overflow. A floating-point arithmetic or conversion operation gave a result that
overflowed the destination exponent.
DZE
[53]
RW
Divide by zero. An attempt was made to perform a floating-point divide with a
divisor of zero.
INV
[52]
RW
Invalid operation. An attempt was made to perform a floating-point arithmetic
operation and one or more of its operand values were illegal.
OVFD
[51]
RW
Overflow disable. If this bit is set and a floating-point arithmetic operation gen-
erates an overflow condition, then the appropriate IEEE nontrapping result is
placed in the destination register and the trap is suppressed.
DZED
[50]
RW
Division by zero disable. If this bit is set and a floating-point divide by zero is
detected, the appropriate IEEE nontrapping result is placed in the destination
register and the trap is suppressed.
INVD
[49]
RW
Invalid operation disable. If this bit is set and a floating-point operate generates
an invalid operation condition and 21264/EV67 is capable of producing the
correct IEEE nontrapping result, that result is placed in the destination register
and the trap is suppressed.
Table 2–14 Floating-Point Control Register Fields (Continued)
Name
Extent
Type
Description
UNFD
UNDZ
Result
0
X
Underflow trap.
1
0
Trap to supply a possible denormal result.
1
1
Underflow trap suppressed. Destination is written with a
true zero (+0.0).
Bits
Meaning
00
Chopped
01
Minus infinity
10
Normal
11
Plus infinity
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