Bit Shifts
Chapter 14
143
Upon false-true transition, bit A from a particular input word will be
shifted into the first bit of the bit shift register. Bit 1 will move to the left
and displace bit 2. Bit 2 will displace bit 3, etc. Each bit displaces the one
to its left until the last bit in the word (bit 16) is reached. Bit 16 then
replaces bit 00 in the next word. This bumping procedure continues
throughout the file until the last bit is ejected from the stack into bit B in
a particular output word.
If the shift register of Figure 14.1A had been 123 bits long it would have
ended at bit 12
8
of word 407
8
. In this case the bits to the left of 12
8
in word
407
8
would be unused for the bit shift register. However, they cannot be
used for any other purpose. The value in bit 123 would be shifted directly
into bit B when a bit shift occurred as shown by the dotted line in
Figure 14.1A.
The instruction operates in the complete mode. The status of the input bit
is shifted into the first bit in the register and the status of the last bit in the
register is shifted into the output bit in one scan.
Instruction Overview:
Output instruction
Key sequence: [SHIFT REG] 12
Counter modified by instruction
Operates in complete mode
Requires 6 words of user program
WARNING: The counter address specified for the Bit Shift
Left instruction should be reserved for that instruction. Do not
manipulate the counter preset or accumulated values.
Inadvertent changes to these values could result in hazardous or
unpredictable machine operation or a run-time error. Damage to
equipment and/or personal injury could result.
To program a bit shift left press [SHIFT REG] 12. A display represented
by Figure 14.2 will appear.
14.1.1
Programming Bit Shift Left
Instruction
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