Hardware Interfacing: bit operations and explicit addresses

If you're interfacing with hardware and need to operate on bits you can use bitset but you may prefer to use C++'s low-level operations.

Setting a bit :-

Suppose you wanted to set bit 6 of i (a long, say) to 1. First you need to create a mask that has a 1 in the 6th bit and 0 elsewhere by doing `1L<<6' which shifts all the bits of the long 1 left 6 bits. Then you need to do a bit-wise OR using `i = i | (1L<<6)'.

Unsetting a bit :-

Suppose you wanted to set bit 6 of i (a long, say) to 0. First you need to create a mask that has a 0 in the 6th bit and 1 elsewhere by doing `1L<<6' then inverting the bits using the ~ operator. Then you need to do a bit-wise AND using the & operator. The whole operation is `i =i & ~(1<<6)' which can be contracted to `i &= ~(1<<6)'.

Creating a mask :-

In graphics, masks are often created each of whose bits represent a option that is to be selected in some way. Each bit can be referred to using an alias that has been set up in an include file. For example, a mask which could be used in a call to make a window sensitive to key presses and buttonpresses could be set up by doing

unsigned int mask = KeyPressMask | ButtonPressMask ;

Suppose because of hardware considerations the i variable needed to be the value of memory location 2000. The following code will do what you want.

long *memory_pointer = reinterpret_cast<long*>(2000);
long i = *memory_pointer;