Derive Exponent - Analog Devices adsp-2100 Manual

SHIFTER
15

DERIVE EXPONENT

Syntax:
Permissible xops
SI
SR1
SR0
Example:
Description:
designated exponent operation. If the condition is not true then perform a
no-operation. Omitting the condition performs the exponent operation
unconditionally.
The EXP operation derives the effective exponent of the input operand to
prepare for the normalization operation (NORM). EXP supplies the source
operand to the exponent detector, which generates a Shift Code from the
number of leading sign bits in the input operand. The Shift Code, stored in
SE at the completion of the EXP instruction, is the effective exponent of the
input value. The Shift Code depends on which exponent detector mode is
used (HI, HIX, LO).
In the HI mode, the input is interpreted as a single precision signed
number, or as the upper half of a double precision signed number. The
exponent detector counts the number of leading sign bits in the source
operand and stores the resulting Shift Code in SE. The Shift Code will
equal the negative of the number of redundant sign bits in the input.
In the HIX mode, the input is interpreted as the result of an add or
subtract which may have overflowed. HIX is intended to handle shifting
and normalization of results from ALU operations. The HIX mode
examines the ALU Overflow bit (AV) in the Arithmetic Status Register: if
AV is set, then the effective exponent of the input is +1 (indicating that an
ALU overflow occurred before the EXP operation), and +1 is stored in SE.
If AV is not set, then HIX performs exactly the same operations as the HI
mode.
15 – 56
[ IF cond ] SE = EXP xop
Permissible conds (see Table 15.9)
AR EQ
MR2 NE
MR1 GT
MR0 GE
LT
IF GT SE = EXP MR1 (HI) ;
Test the optional condition and, if true, perform the
(HI ) ;
(LO)
(HIX)
LE AC
NEG NOT AC
POS MV
AV NOT MV
NOT AV NOT CE