LSH32
DEVICES INCORPORATED
32-bit Cascadable Barrel Shifter
LSH32
DEVICES INCORPORATED
32-bit Cascadable Barrel Shifter
DESCRIPTION
The
LSH32
is a 32-bit high speed
shifter designed for use in floating
point normalization, word pack/
unpack, field extraction, and similar
applications. It has 32 data inputs,
and 16 output lines. Any shift
configuration of the 32 inputs, includ-
ing circular (barrel) shifting, left shifts
with zero fill, and right shift with sign
extend are possible. In addition, a
built-in priority encoder is provided
to aid floating point normalization.
SHIFT ARRAY
The 32 inputs to the LSH32 are
applied to a 32-bit shift array. The 32
outputs of this array are multiplexed
down to 16 lines for presentation at
the device outputs. The array may be
configured such that any contiguous
16-bit field (including wraparound of
the 32 inputs) may be presented to the
output pins under control of the shift
code field (wrap mode). Alterna-
tively, the wrap feature may be
disabled, resulting in zero or sign bit
fill, as appropriate (fill mode). The
shift code control assignments and the
resulting input to output mapping for
the wrap mode are shown in Table 1.
Essentially the LSH32 is configured as
a left shift device. That is, a shift code
of 00000
2
results in no shift of the
input field. A code of 00001
2
provides
an effective left shift of 1 position, etc.
When viewed as a right shift, the shift
code corresponds to the two’s com-
plement of the shift distance, i.e., a
shift code of 11111
2
(–1
10
) results in a
right shift of one position, etc.
When not in the wrap mode, the
LSH32 fills bit positions for which
there is no corresponding input bit.
The fill value and the positions filled
depend on the RIGHT/LEFT (R/L)
direction pin. This pin is a don’t care
input when in wrap mode. For left
shifts in fill mode, lower bits are filled
with zero as shown in Table 2. For
right shifts, however, the SIGN input
is used as the fill value. Table 3
depicts the bits to be filled as a
function of shift code for the right shift
case. Note that the R/L input changes
only the fill convention, and does not
affect the definition of the shift code.
In fill mode, as in wrap mode, the shift
code input represents the number of
shift positions directly for left shifts,
but the two’s complement of the shift
code results in the equivalent right
shift. However, for fill mode the R/L
input can be viewed as the most
OE
MS/LS
FEATURES
u
32-bit Input, 32-bit Output Multi-
plexed to 16 Lines
u
Full 0-31 Position Barrel Shift
Capability
u
Integral Priority Encoder for 32-bit
Floating Point Normalization
u
Sign-Magnitude or Two’s Comple-
ment Mantissa Representation
u
32-bit Linear Shifts with Sign or
Zero Fill
u
Independent Priority Encoder
Outputs for Block Floating Point
u
68-pin PLCC, J-Lead
LSH32 B
LOCK
D
IAGRAM
SIGN I
31
-I
0
32
32:5
PRIORITY
ENCODE
32
5
2:1
SI
4
-SI
0
RIGHT/LEFT
FILL/WRAP
16
NORM
32-bit
BARREL
SHIFT
ARRAY
16
2:1
5
16
SO
4
-SO
0
Y
15
-Y
0
Special Arithmetic Functions
1
08/16/2000–LDS.32-Q
LOGIC [ LOGIC DEVICES INCORPORATED ]
