Question

What does this: packed_data = (packed_data & ~(0xff << 18)) | ((n & 0xff) << 18); do?

Answer 1

The code clears and then sets an 8-bit segment in an integer variable. It uses bitwise operations to manipulate bits 18-25 of 'packed_data' by taking the least significant 8 bits of another integer 'n' and inserting them at the same position.

Step-by-step explanation:

The code packed_data = (packed_data & ~(0xff << 18)) | ((n & 0xff) << 18) performs a bitwise operation that updates an 8-bit segment of an integer variable named packed_data. Specifically, it clears the bits 18-25 of packed_data and then sets them to the least significant 8 bits of another integer variable named n.

Here is the step-by-step explanation:

1. The expression 0xff is a hexadecimal literal which represents the decimal value 255. In binary, it corresponds to 8 '1' bits (11111111).
2. The operation (0xff << 18) shifts this binary value to the left by 18 positions. This will result in eight '1' bits starting from bit position 18 to 25 of the 32-bit integer.
3. The bitwise NOT operator ~ inverts all the bits of (0xff << 18), thus creating a mask to clear bits 18-25 in packed_data.
4. The & operator is used to apply this mask to packed_data, clearing bits 18-25.
5. The value (n & 0xff) extracts the least significant 8 bits from the variable n.
6. These bits are then shifted to the left by 18 positions with ((n & 0xff) << 18).
7. Finally, the bitwise OR operator | is used to set the previously cleared bits 18-25 in packed_data to the value obtained from n.

This code is typically used in programming when you need to manipulate individual bits or sections of an integer for storing compact data or for bit masking operations.