Analog Devices ADSP-BF53x Blackfin Reference page 1002

Block Floating-Point Format
number of redundant sign bits is less than two. In this example,
= +1 after processing, indicating the block of data must be
SB
shifted right one bit to maintain the two guard bits.
• If were 0 after processing, the block would have to be shifted
SB
two bits right. In either case, the block exponent is updated to
reflect the shift.
1. Check for bit growth
One Guard Bit
0x1FFF = 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1
0x3FFF = 0 0 1 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
Sign Bit
2. Shift right to restore guard bits
Two 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
0x03FF = 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1
Sign Bit
Figure D-5. Block Floating-point Adjustment
D-8 ADSP-BF53x/BF56x Blackfin Processor Programming Reference
EXPADJ instruction checks
exponent, adjusts SB
Exponent = +2, SB = +2
Exponent = +1, SB = +1
Exponent = +4, SB = +1