Bit Test

The BT x86 assembly language instruction stands for Bit Test and was added to the x86 instruction set with the 80386 processor. BT copies a bit from a given register to the carry flag.{{cite web |url=http://faydoc.tripod.com/cpu/bt.htm| title=BT – Bit Test|accessdate=2011-08-21}}

Example: copy the third least significant bit from EAX to the carry flag

BT EAX, 2

BTS (Bit Test and Set) operates the same, but also sets the bit in the register,{{cite web |url=http://faydoc.tripod.com/cpu/bts.htm| title=BTS – Bit Test and Set|accessdate=2011-08-21}} while BTR (Bit Test and Reset) resets it,{{cite web |url=http://faydoc.tripod.com/cpu/btr.htm| title=BTR – Bit Test and Reset|accessdate=2011-08-21}} and BTC (Bit Test and Complement) flips it.{{cite web |url=http://css.csail.mit.edu/6.858/2010/readings/i386/BTC.htm| title=BTC – Bit Test and Complement|accessdate=2012-11-04}}

Logical Explanation BT

BT SRC, POSITION

SRC

Represent as bits array

POSITION

Represent as numeric position

From Right to Left

Start at 0

Carry Flag

Result

CF = SRC[POSITION]

Logical Explanation BTC

BTC SRC, POSITION

SRC

Represent as bits array for CF Result

Represent as hex for SRC Result