Bit-wise operations

Bit-wise operations

Bit-wise operators

• Instructions that manipulate individual bits given to them in their operands
• Generally best understood by considering values in their binary form
• Many of these will feel familiar from the first half of this unit

AND – Logical bit-wise AND

• Syntax:

AND R3, R1, R2

• Performs a bit-wise logical AND on the two inputs given in the second and third operands
• Stores the result in the destination, (first operand)
• Eg, If R1 = #0b0110 R2 = #0b1100 R1 “AND” R2”: R3 = #0b0100

ORR

Logical bit-wise OR

• Syntax:

ORR R3, R1, R2

• Performs a bit-wise logical OR on the two inputs given in the second and third operands
• Stores the result in the destination, (first operand)
• Eg, If R1 = #0b0110 R2 = #0b1100 R1 “ORR” R2”: R3 = #0b1110

EOR

Logical bit-wise Exclusive OR

• Syntax:

EOR R3, R1, R2

• Performs a bit-wise logical XOR on the two inputs given in the second and third operands
• Stores the result in the destination, (first operand)
• Eg, If R1 = #0b0110 R2 = #0b1100 R1 “EOR” R2”: R3 = #0b1010

Observations on AND,ORR,EOR

• Have clear analogies to the logical gates we have seen previously
• Generally useful for specific bit manipulations to control hardware, as well as bit masking (e.g, to extract part but not all of a 32 bit string).
• Also for performing comparisons between, or merging bit strings.

LSL

Logical Shift Left

• Syntax:

LSL R1, R1,#3

• Shifts each bit of the input value to the left, by the number of places specified in the third operand, losing the leftmost bits, and adding zeros on the right.
• Stores the result in the destination, (first operand) Eg, If R1 = #0b01101110, and shift = #3 R1 becomes #0b01110000

LS

Logical Shift Left

• Syntax:

LSR R1, R1,#3

• Shifts each bit of the input value to the right, by the number of places specified in the third operand, losing the rightmost bits, and adding zeros on the left.
• Stores the result in the destination, (first operand) Eg, If R1 = #0b01101110, and shift = #3 R1 becomes #0b00001101

LSL and LSR Observations

• Assuming Little Endian notation:
• A single LSL doubles the value represented by the bit string
• A single LSR halves the value represented by the bit string
• Both instructions invoke a shift register to perform task in hardware
• As with all bit-wise operators, they can be performed in one CPU cycle.
• Form the backbone for implementing multiplication and division operations, which typically require multiple CPU cycles to calculate (we’ll come back to this later)