Analog Devices ADSP-BF53x Blackfin Reference page 1001

Typically, block floating-point format allows you to shift out non-signifi-
cant MSBs (most significant bits), increasing the precision available in
each value. Block floating-point format can also be used to eliminate the
possibility of a data value overflowing. See
data samples shown has at least two non-significant, redundant sign bits.
Each data value can grow by these two bits (two orders of magnitude)
before overflowing. These bits are called guard bits.
0x0FFF = 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1
0x1FFF = 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1
0x07FF = 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1
To detect bit growth into two guard bits, set SB = –2
Figure D-4. Data With Guard Bits
If it is known that a process will not cause any value to grow by more than
the two guard bits, then the process can be run without loss of data. Later,
however, the block must be adjusted to replace the guard bits before the
next process.
Figure D-5 shows the data after processing but before adjustment. The
block floating-point adjustment is performed as follows.
• Assume the output of the
as an argument in the
is +2, corresponding to the two guard bits. During processing, each
resulting data value is inspected by the
counts the number of redundant sign bits and adjusts
ADSP-BF53x/BF56x Blackfin Processor Programming Reference
2 Guard Bits
Sign Bit
SIGNBITS
instruction. Initially, the value of
EXPADJ
Numeric Formats
Figure D-4. Each of the three
instruction is and
SB
instruction, which
EXPADJ
is used
SB
SB
if the
SB
D-7