Ad = Exp (As - Lucent Technologies DSP1617 Information Manual

DSP1611/17/18/27/28/29 DIGITAL SIGNAL PROCESSOR
Instruction Set Summary

aD = exp (aS)

(get exponent of aS)
(aDh) ← (# of redundant sign bits in aS)
(aDl) ← 0
The number of redundant sign bits present in the 36-bit value in aS is computed and placed in the high half of aD
(bits 35—16), and aDl (bits 15—0) is cleared.
A two's complement number is normalized (see
sign bits and then shifting the number to the left E times (see
There are three extra sign bits, so shift left three times in order that the last sign bit ends up in the MSB position.
The number (E) of redundant sign bits is found with respect to sign bit 31. If an overflow has occurred, E will be
negative and an arithmetic right shift will be done to normalize the number (see
the total number of bits that are the same starting from bit 35 and counting to the right. For example:
Bit Positions
Accumulator Contents
The instruction for exponentiation is aD = exp (aS) where the exponent (E) is placed in the high half of the destina-
tion accumulator (aD, bits 31—16) and the lower half (bits 15—0) is cleared.
The flags (described in Section 13.2.2, Shifting
Bit 15 14
Field 1 1
B-49
page
35—32 31—0
0000 0110001 . . . 0
0000 0001100 . . . 0
0000 1000000 . . . 0
0110 1100010 . . . 0
1111 1100101 . . . 0
Operations) are set based on the value written into aD.
13 12 11 10
1 1 0 D
Words: 1
Cycles: 1
Group: BMU
Addressing: Register
Flags affected: LMI, LEQ, LLV, LMV,
ODDP, EVENP, MNS1, NMNS1
Interruptible: Yes
Cacheable: Yes
Format: 3b
DRAFT COPY
B-50) by detecting the number (E) of extra (or redundant)
Section 13.2.3,
11110011
10011000
Normalization Action
K = 5, E = 0, no shifting required.
K = 7, E = 2, shift left twice.
K = 4, E = –1, shift right once.
K = 1, E = –4, shift right four times.
K = 6, E = 1, shift left once.
9 8 7 6
S 0 0 0
Information Manual
April 1998
Normalization). For example:
page B-50). E = K – 5 where K is
5 4 3 2
0 0 1 1
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1 0
0 0