4.3. Bit Manipulation Flashcards
Left shift (LSL #n)
- Bits are shifted to the left to multiply
- E.g. to multiply by four, all digits shift two places to left
Right shift (LSR #n)
- Bits are shifted to the right to divide
- E.g. to divide by four, all digits shift two places to right
Logical shift
Zzeros replace the vacated bit position
Arithmetic shift
Used to carry out multiplication and
division of signed integers represented by bits in the
accumulator by ensuring that the sign-bit (usually the
MSB) is the same after the shift.
Cyclic shift
The bit that is removed from one end by the
shift is added to the other end.
Bit Masking
- Each bit can represent an individual flag.
∴ by altering the bits, flags could be operated upon.
Bit manipulation operations: - Masking: an operation that defines which bits you
want to keep and which bits you want to clear. - Masking to 1: The OR operation is used with a 1.
- Masking to 0: The AND operation is used with a 0.
- Matching: an operation that allows the
accumulator to compare the value it contains to
the given value in order to change the state of the
status register.
Practical applications of Bit Masking
Setting an individual bit position:
* Mask the content of the register with a mask
pattern which has 0 in the ‘mask out’ positions and
1 in the ‘retain’ positions.
* Set the result with the match pattern by using the
AND command with a direct address.
Testing one or more bits:
* Mask the content of the register with a mask
pattern which has 0 in the ‘mask out’ positions and
1 in the ‘retain’ positions.
* Compare the result with the match pattern by
using the CMP command or by “Checking the
pattern”.
Checking the pattern
* Use AND operation to mask bits and obtain
resultant.
* Now subtract matching bit pattern from resultant.
* The final ‘non-zero’ result confirms the patterns
are not the same else vice versa.
