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### BITWISE operations

353156 – Microprocessor

Asst. Prof. Dr. ChoopanRattanapoka and Asst. Prof. Dr. SuphotChunwiphat

Objectives

- To understand
- Bitwise Logical Operations
- OR
- AND
- XOR

- Shift Operations
- Shift Left
- Shift Right
- Rotate

- Bitwise Logical Operations

Bitwise Operations (1)

- Up until now, we’ve done :
- Arithmetic (ADD, SUB, RSB, ..etc..)
- Some of data movement (MOV)

- All of these instructions view content of register as a single quantity
- But for bitwise operations, we will see the content of register as 32 bits rather than as a single 32-bit number

Bitwise Operations (2)

- Since register are composed of 32 bits, we may want to access individual bits rather than the whole.
- Introduction two new classes of instructions/operations :
- Logical Instruction
- Shift Operations

Logical Operations

- Operator Names:
- AND
- BIC
- ORR
- EOR

- Operands
- Destination : Register
- Operand1 : Register
- Operand2 : Register, Shifted Register, Immediate

- Example
- AND a1, v1, v2
- AND a1, v1, #0x40

Logical AND Operator

- AND (truth table)
- AND : bit-by-bit operation leaves a 1 in the result only if both bits of the operands are 1

Example : Logical AND Operator

- Assume that register A and B are 8-bit register
- Let A stores value 0011 1011
- Let B stores value 1001 0010
- Find A AND B
0 0 1 1 1 0 1 1 AND

1 0 0 1 0 0 1 0

0

0

0

1

0

0

1

0

Exercise 1: Logical AND Operator

- What is the value store in register R0 after the execution of program is done

(1)

(2)

AREA ex1_1, CODE, READONLY

ENTRY

start

MOV a2, #0x42

MOV a3, #0xFC

AND a1, a2, a3

END

AREA ex1_2, CODE, READONLY

ENTRY

start

MOV a2, #0xBC000000

AND a1, a2, #0xFF

END

Logical BIC (AND NOT) Operator

- BIC (BIt Clean) : bit-by-bit operation leaves a 1 in the result only if bit of the first operand is 1 and the second operands 0
- Example
0 0 1 1 BIC

0 1 0 1

0

0

1

0

Exercise 2: Logical BIC Operator

- What is the value store in register R0 after the execution of program is done

(1)

(2)

AREA ex2_1, CODE, READONLY

ENTRY

start

MOV a2, #0x42

MOV a3, #0xFC

BIC a1, a2, a3

END

AREA ex2_2, CODE, READONLY

ENTRY

start

MOV a2, #0xBC000000

BIC a1, a2, #0xFF

END

MASK

- AND and BIC normally use to create a mask
- Example : Given R0 stores 0x12345678
- if we want to keep only the value of the last 2 bytes and we want to clear the first 2 bytes to 0 (0x00005678)
- R1 stores 0xFFFF
- AND R0, R0, R1 ; short written constant value

- if we want to keep only the value of the first 2 bytes and we want to clear the last 2 bytes to 0 (0x12340000)
- R1 stores 0xFFFF0000
- AND R0, R0, R1 ; long written constant value

- R2 stores 0xFFFF
- BIC R0, R0, R2 ; better ?

- R1 stores 0xFFFF0000

- if we want to keep only the value of the last 2 bytes and we want to clear the first 2 bytes to 0 (0x00005678)

Logical OR Operator

- OR (truth table)
- OR : bit-by-bit operation leaves a 1 in the result if either bit of the operands is 1

Example : Logical OR Operator

- Assume that register A and B are 8-bit register
- Let A = 0011 1011, and B = 1001 0010
- Find A OR B
0 0 1 1 1 0 1 1 OR

1 0 0 1 0 0 1 0

- Example : ARM Instruction
- ORR R0, R0, #0x20

1

0

1

1

1

0

1

1

Exercise 3 : Logical OR Operator

- What is the value store in register R0 after the execution of program is done

AREA ex3_1, CODE, READONLY

ENTRY

start

MOV a2, #0x34

MOV a3, #0x86

ORR a1, a2, a3

END

Logical XOR Operator

- XOR (truth table)
- XOR : bit-by-bit operation leaves a 1 in the result if bit of the operands are different

Example : Logical XOR Operator

- Assume that register A and B are 8-bit register
- Let A = 0011 1011, and B = 1001 0010
- Find A XOR B
0 0 1 1 1 0 1 1 XOR

1 0 0 1 0 0 1 0

- Example : ARM Instruction
- EOR R0, R0, #0x20

1

0

1

0

1

0

0

1

Exercise 4 : Logical XOR Operator

- What is the value store in register R0 after the execution of program is done

AREA ex4_1, CODE, READONLY

ENTRY

start

MOV a2, #0x34

MOV a3, #0x86

EOR a1, a2, a3

END

Shift Operations

- Shift means move all the bits in a word to the left or right by number of bits
- In ARM, there are 3 types of shift operations
- Fill emptied bits with 0s (LSL, LSR)
- Fill emptied bits with sign bits (ASR)
- Fill emptied bits with the bits falling (rotation) (ROR)

Shift Operations (Type 1)

- Move all the bits in a word to the left or right by a number of bits, filling the emptied bits with 0s
- Example :
Given A an 8-bit register which stores data 1100 1010

1

1

0

0

1

0

1

0

0

0

0

0

1

1

0

0

1

0

1

0

Shift right by 4 bits

Shift left by

4 bits

1

1

0

0

1

0

1

0

0

0

0

0

LSL, LSR Instructions

- In ARM, we have 2 instructions for shift operation type 1
- LSL (Logical Shift Left)
- LSR (Logical Shift Right)

- We can use 2 formats of instruction
- Opcodedest_reg, reg_contains_number_of_bit_to_shift
- LSL R0, R1 ; R1 must store number of bit to shift

- Opcodedest_reg, src_reg, constant_number_of_bit_to_shift
- LSR R0, R1, #4 ; Shift right value in R1 for 4 bits and then store it to R0

- Opcodedest_reg, reg_contains_number_of_bit_to_shift
- ** Only R0-R7 can do Shift operations **

Exercise 5: LSL, LSR Operations

- What is the value store in register R0 after the execution of program is done

(1)

(2)

AREA ex5_1, CODE, READONLY

ENTRY

start

MOV a1, #0x01

MOV a2, #4

LSL a1, a2

END

AREA ex5_2, CODE, READONLY

ENTRY

start

MOV a1, #0xFF

LSR a1, a1, #0x4

END

Shift Operations (Type 2)

- Move all the bits in a word to the right by a number of bits, filling the emptied bits with sign bits
- Example :
- Given A an 8-bit register which stores data 1100 1010
- Given A an 8-bit register which stores data 0100 1010

1

1

0

0

1

0

1

0

1

1

1

1

1

1

0

0

1

0

1

0

Shift right by 4 bits

0

1

0

0

1

0

1

0

0

0

0

0

1

1

0

0

1

0

1

0

Shift right by 4 bits

ASR Instructions

- ASR (ArithmaticShift Right)
- We can use 2 formats of instruction
- Opcodedest_reg, reg_contains_number_of_bit_to_shift
- ASR R0, R1 ; R1 must store number of bit to shift

- Opcodedest_reg, src_reg, constant_number_of_bit_to_shift
- ASR R0, R1, #4 ; Shift right value in R1 for 4 bits and then store it to R0

- Opcodedest_reg, reg_contains_number_of_bit_to_shift
- ** Only R0-R7 can do Shift operations **

Exercise 6: ASR Operations

- What is the value store in register R0 after the execution of program is done

(1)

(2)

AREA ex6_1, CODE, READONLY

ENTRY

start

MOV a1, #0xFFFFFF10

MOV a2, #2

ASR a1, a2

END

AREA ex6_2, CODE, READONLY

ENTRY

start

MOV a1, #0xFF

ASR a1, a1, #0x2

END

Shift Operations (Type 3)

- Move all the bits in a word to the right by a number of bits, filling the emptied bits with the bits falling of the right
- Example :
Given A an 8-bit register which stores data 1100 1010

Rotate right by 3 bits

1

0

1

1

0

1

0

0

1

0

0

1

1

0

0

1

0

0

1

1

0

1

1

0

0

1

0

ROR Instructions

- ROR (ROtateRight)
- We can use 2 formats of instruction
- Opcodedest_reg, reg_contains_number_of_bit_to_shift
- ROR R0, R1 ; R1 must store number of bit to shift

- Opcodedest_reg, src_reg, constant_number_of_bit_to_shift
- ROR R0, R1, #4 ; Rorate right value in R1 for 4 bits and then store it to R0

- Opcodedest_reg, reg_contains_number_of_bit_to_shift
- ** Only R0-R7 can do Shift operations **

Exercise 7: ROR Operations

- What is the value store in register R0 after the execution of program is done

(1)

(2)

AREA ex7_1, CODE, READONLY

ENTRY

start

MOV a1, #0xFFFFFF10

MOV a2, #4

ROR a1, a2

END

AREA ex7_2, CODE, READONLY

ENTRY

start

MOV a1, #0xFF

ROR a1, a1, #0x4

END

Review : 2nd Operand

- 2ndoperand for operation :
- register
- immediate(numerical constant)
- shifted register

- Example :
- MOV R0, R1
- MOV R0, #0x12
- MOV R0, R1, LSL #4 ; R0 (R1 << 4)

Barrel Shifter : The Second Operand

- Register, optionally with shift operation applied
- Shift value can be either :
- 5 bit unsigned integer
- ADD R0, R1, R2, LSL #8 ;R0 R1 + (R2 << 8 bits)

- Specified in bottom of another register
- ADD R0, R1, R2, LSL R3 ;R0 R1 + (R2 << R3 bits)

- 5 bit unsigned integer

Operand 1

Operand 2

Barrel

Shifter

- Immediate value
- 8 bit number
- Can be rotated right through as even number of position
- Assembler will calculate rotate for you from constant
- ADD R0, R1, #10 ; R0 R1 + 10
- ADD R0, R1, #0xFF00 ; R0 R1 + (0xFF << 16 bits)
- ADD R0, R1, #0xFFF ; ERROR

ALU

Result

Assignment 5

- What is the value store in register R0 after the execution of program is done (explain the result of each instruction)

(1)

(2)

AREA hw5_1, CODE, READONLY

ENTRY

start

MOV a1, #0xFFFFFF18

MOV a2, #16

ROR a1, a2

MOV a2, #0xFF000000

AND a1, a1, a2, LSR #4

LSR a1, #16

END

AREA hw5_2, CODE, READONLY

ENTRY

start

MOV a1, #0xFF

MOV a2, #0xCC

MOV a3, #16

ADD a1, a1, a2, LSL a3

EOR a2, a2, #0x65

ORR a1, a1, a2, LSL #8

END

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