CF = bit(dest, source) ; extract bit numbered by source from dest
The bt instruction uses the source value as the bit number of the destination to test. The destination can be a 64, 32 or 16 bit register or memory location. The source can be either an immediate value or a register of the same size as the destination.
When testing based on a memory address using a register, the register can hold a larger integer which defines a doubleword in memory to test and a bit within the doubleword. So you can access an array of bits.
Some examples of using bt:
bt rax, 15 ; test bit 15 of rax bt eax, 10 ; test bit 10 of eax bt dx, cx ; cx contains bit number of dx to test bt [x], rdx ; rdx contains bit number of x to test