Subtopic Notes
4.3 Bit manipulation
4. Processor Fundamentals
4.3 Bit manipulation
Bitwise Instructions
| Opcode | Operand | Explanation |
|---|---|---|
| AND | #n /Bn/&n | Bitwise AND operation of the contents of ACC with the operand |
| AND | <address> | Bitwise AND operation of the contents of ACC with the contents of <address> |
| XOR | #n/Bn/&n | Bitwise XOR operation of the contents of ACC with the operand |
| XOR | <address> | Bitwise XOR operation of the contents of ACC with the contents of <address> |
| OR | #n/Bn/&n | Bitwise OR operation of the contents of ACC with the operand |
| OR | <address> | Bitwise OR operation of the contents of ACC with the contents of <address> |
| LSL | #n | Bits in ACC are shifted logically n places to the left. Zeros are introduced on the right hand end |
| LSR | #n | Bits in ACC are shifted logically n places to the right. Zeros are introduced on the left hand end |
All questions will assume there is only one general purpose register available
(Accumulator)
ACC denotes Accumulator
IX denotes Index Register
<address> can be an absolute or symbolic address
# denotes a denary number, e.g. #123
B denotes a binary number, e.g. B0101010
& denotes a hexadecimal number, e.g. &4A
Shifts
Logical Shifts
- Moving a binary value to the left or the right
- Most significant bits (MSB) or the least significant bits (LSB) are lost
- Shifting 1100 1010 two places to the left = 0010 1000
- Each left shift multiplies the original number by 2 (Data may be lost)
- Shifting 1100 1010 two places to the right = 0011 0010
- Each right shift divides the original number by 2 (Data may be lost)
Arithmetic Shift: Shifts the bits of a binary number, while preserving the sign bit (usually the MSB)
Cyclic Shift: Bits which are removed from one end is added to the other
Bit Masking
- Definition: Process of getting CPU to ignore the bits we don't want to work on and only to process digits we want to process
- Used to check/change/remove individual bits within a binary value
- Advantage: Less processing needed to perform a task
- Application: Used in simple control systems where each bits represent a flag
- Operations:
- AND: Setting chosen bits to 0
- AND 0: Sets value to 0
- AND 1: Retains value
- OR: Setting chosen bits to 1
- OR 0: Retains value
- OR 1: Sets value to 1
- XOR: Inverting chosen bits or finding differences between
- XOR 0: Retains the Value
- XOR 1: Inverts the value
- AND: Setting chosen bits to 0