bt assembly instruction

When the book says "bit one of location TestMe+8", the "8" refers to an address offset, which is measured in bytes. There are 64 bits in 8 bytes, so the 65th bit is bit one of 8 bytes past TestMe.

• The byte at TestMe has bits 7..0
• The byte at TestMe+1 has bits 15..8
• The byte at TestMe+2 has bits 23..16
• ...
• The byte at TestMe+8 has bits 71..64

So "65" refers to "bit 1" (the second counting from the right) of the byte at address TestMe+8.

bt TestMe, bx where bx contains 65 is an access 8 bytes (64 bits plus 1) beyond the address of TestMe. It doesn't copy the byte there, only the second bit in that byte (to the carry flag, CF).