OR for setting bits in Embedded C

Introduction

We use OR to turn on specific bits in a number without changing the other bits.

Turning on a light in a group of lights controlled by bits.

Enabling specific features in a device by setting bits.

Marking options as active in a settings byte.

Setting flags in a status register without losing current flags.

Syntax

Embedded C

variable = variable | mask;

variable is the number where bits will be set.

mask has 1s in the bit positions you want to turn on.

Examples

Set the first bit of flags to 1.

Embedded C

flags = flags | 0x01;

Turn on the third bit of status.

Embedded C

status = status | 0x04;

Shortcut to set the fifth bit of control.

Embedded C

control |= 0x10;

Sample Program

This program starts with all bits off in flags. Then it sets bits 0 and 2 using OR with mask. The result shows which bits are now on.

Embedded C

#include <stdio.h>

int main() {
    unsigned char flags = 0x00;  // all bits off
    unsigned char mask = 0x05;   // bits 0 and 2 set (00000101)

    // Set bits in flags using OR
    flags = flags | mask;

    printf("Flags after setting bits: 0x%02X\n", flags);
    return 0;
}

Important Notes

Using OR to set bits never turns any bit off.

The mask should have 1s only where you want to set bits.

Use hexadecimal (like 0x01, 0x04) to easily see which bits you are setting.

Summary

OR operation sets bits to 1 without changing others.

Use a mask with 1s in the bit positions you want to set.

Shortcut: variable |= mask; does the same as variable = variable | mask;.

Practice

1. What does the OR operation do when used to set bits in a variable in embedded C?

easy

A. It toggles specified bits from 0 to 1 or 1 to 0.

B. It clears specified bits to 0 without changing other bits.

C. It sets specified bits to 1 without changing other bits.

D. It shifts bits to the left by one position.

Solution

1. Step 1: Understand OR operation effect on bits

   The OR operation sets bits to 1 if either bit is 1, so it can set bits without clearing others.

2. Step 2: Apply to setting bits

   Using OR with a mask having 1s in desired positions sets those bits to 1, leaving others unchanged.

3. Final Answer:

   It sets specified bits to 1 without changing other bits. -> Option C

4. Quick Check:

   OR sets bits to 1 without clearing others [OK]

Hint: OR with mask sets bits to 1, leaves others unchanged [OK]

Common Mistakes:

• Confusing OR with AND operation
• Thinking OR clears bits instead of setting
• Assuming OR toggles bits
• Mixing OR with bit shifting

2. Which of the following is the correct syntax to set bits using OR in embedded C?

easy

A. variable |= mask;

B. variable = variable & mask;

C. variable = variable ^ mask;

D. variable &= ~mask;

Solution

1. Step 1: Identify syntax for setting bits with OR

   The operator '|=' is the shorthand for 'variable = variable | mask;', which sets bits.

2. Step 2: Check other options

   variable = variable & mask; uses AND, which clears bits; variable = variable ^ mask; uses XOR, which toggles bits; variable &= ~mask; clears bits using AND with negated mask.

3. Final Answer:

   variable |= mask; -> Option A

4. Quick Check:

   Use '|=' to set bits quickly [OK]

Hint: Use '|=' operator to set bits easily [OK]

Common Mistakes:

• Using '&' instead of '|' to set bits
• Confusing XOR (^) with OR (|)
• Forgetting shorthand '|=' syntax
• Using negation incorrectly

3. What is the value of flags after executing the following code?

unsigned char flags = 0x12;  // binary 00010010
unsigned char mask = 0x05;   // binary 00000101
flags = flags | mask;

medium

A. 0x05 (binary 00000101)

B. 0x17 (binary 00010111)

C. 0x10 (binary 00010000)

D. 0x07 (binary 00000111)

Solution

1. Step 1: Convert values to binary and apply OR

   flags = 00010010, mask = 00000101. OR operation sets bits where either is 1.

2. Step 2: Perform bitwise OR

   00010010 OR 00000101 = 00010111 (binary), which is 0x17 in hex.

3. Final Answer:

   0x17 (binary 00010111) -> Option B

4. Quick Check:

   0x12 | 0x05 = 0x17 [OK]

Hint: OR bits: 1 if either bit is 1 [OK]

Common Mistakes:

• Adding values instead of ORing
• Confusing OR with AND
• Misreading binary values
• Forgetting to convert hex to binary

4. Identify the error in the following code snippet that tries to set bits using OR:

unsigned int reg = 0x0F;
unsigned int mask = 0x30;
reg = reg & mask;

medium

A. Using '&' instead of '|' to set bits

B. Mask value is incorrect

C. Variable types should be 'char' not 'int'

D. Missing semicolon after mask declaration

Solution

1. Step 1: Understand the intended operation

   The code aims to set bits using OR, but uses '&' which clears bits instead.

2. Step 2: Identify correct operator

   To set bits, use '|' operator, so 'reg = reg | mask;' or 'reg |= mask;' is correct.

3. Final Answer:

   Using '&' instead of '|' to set bits -> Option A

4. Quick Check:

   Use '|' to set bits, '&' clears bits [OK]

Hint: Use '|' not '&' to set bits [OK]

Common Mistakes:

• Confusing AND with OR
• Assuming '&' sets bits
• Ignoring operator precedence
• Not testing code output

5. You have a 16-bit register reg initially set to 0x0000. You want to set bits 0, 3, and 15 to 1 without changing other bits. Which code correctly does this?

hard

A. reg = reg ^ ((1 << 0) | (1 << 3) | (1 << 15));

B. reg = reg & ((1 << 0) | (1 << 3) | (1 << 15));

C. reg |= (1 << 1) | (1 << 2) | (1 << 14);

D. reg |= (1 << 0) | (1 << 3) | (1 << 15);

Solution

1. Step 1: Identify bits to set and create mask

   Bits 0, 3, and 15 correspond to (1<<0), (1<<3), and (1<<15). OR them to create mask.

2. Step 2: Use OR assignment to set bits without changing others

   Use 'reg |= mask;' to set bits. reg |= (1 << 0) | (1 << 3) | (1 << 15); matches this.

3. Step 3: Check other options

   reg = reg & ((1 << 0) | (1 << 3) | (1 << 15)); uses AND which clears bits; reg |= (1 << 1) | (1 << 2) | (1 << 14); sets wrong bits; reg = reg ^ ((1 << 0) | (1 << 3) | (1 << 15)); toggles bits which may not be desired.

4. Final Answer:

   reg |= (1 << 0) | (1 << 3) | (1 << 15); -> Option D

5. Quick Check:

   Use OR with mask of bits to set [OK]

Hint: Use '|=' with bit-shifted mask for setting bits [OK]

Common Mistakes:

• Using '&' instead of '|' to set bits
• Setting wrong bit positions
• Using XOR which toggles bits
• Not combining bits into one mask