How to set values ​​for a range of bits in a variable?

Question

C++ / C#

Hello!
It is required to make the bit values ​​equal to 1 in a certain range in a 64-bit unsigned long, leaving the remaining bits unchanged. For example, 0x000100100fe80010 should become 0xffffffffffffff , i.e. the range of fe in the number remains unchanged. Tell me how to do it?

Answer 1

Set bit 15 to 1:
var |= (1 << 14);
Set bit 15 to 0:
var &= ~(1ul << 14);
Install as you requested:
var= 0x000100100fe80010ul | 0xffffffff00ffffful;