Analog Devices ADSP-BF53x Blackfin Reference page 810

Introduction
Opcode Bit Terminology
The following conventions describe the instruction opcode bit states.
Table C-6.
SYMBOL
0
1
x
Undefined Opcodes
Any and all undefined instruction opcode bit patterns are reserved, poten-
tially for future use.
Holes In Opcode Ranges
Holes may exist in the range of operation codes shown for some instructions.
For example, one version of the Zero Overhead Loop Setup instruction spans
the opcode range E080 0000 through E08F 03FF, as shown in the excerpt,
below. However, not all values within that range are valid opcodes; some bit
field values are fixed, leaving gaps or "holes" in the sequence of valid
opcodes. These undefined opcode holes are reserved for potential future use.
C-10 ADSP-BF53x/BF56x Blackfin Processor Programming Reference
MEANING
Binary zero bit, logical "low"
Binary one bit, logical "high"
"don't care" bit