Volume 3: General-Purpose and System Instructions - Stanford ...

24594 Rev. 3.10 February 2005 AMD64 TechnologyBTRBit Test and ResetCopies a bit, specified by a bit index in a register or 8-bit immediate value (secondoperand), from a bit string (first operand), also called the bit base, to the carry flag(CF) of the rFLAGS register, and then clears the bit in the bit string to 0.If the bit base operand is a register, the instruction uses the modulo 16, 32, or 64(depending on the operand size) of the bit index to select a bit in the register.If the bit base operand is a memory location, bit 0 of the byte at the specified addressis the bit base of the bit string. If the bit index is in a register, the instruction selects abit position relative to the bit base in the range –2 63 to +2 63 – 1 if the operand size is64, –2 31 to +2 31 – 1, if the operand size is 32, and –2 15 to +2 15 – 1 if the operand size is16. If the bit index is in an immediate value, the bit selected is that value modulo 16,32, or 64, depending on the operand size.This instruction is useful for implementing semaphores in concurrent operatingsystems. Such applications should precede this instruction with the LOCK prefix. Fordetails about the LOCK prefix, see “Lock Prefix” on page 10.Mnemonic Opcode DescriptionBTR reg/mem16, reg16 0F B3 /rBTR reg/mem32, reg32 0F B3 /rBTR reg/mem64, reg64 0F B3 /rBTR reg/mem16, imm80F BA /6 ibBTR reg/mem32, imm80F BA /6 ibBTR reg/mem64, imm80F BA /6 ibCopy the value of the selected bit to the carry flag, then clear theselected bit.Copy the value of the selected bit to the carry flag, then clear theselected bit.Copy the value of the selected bit to the carry flag, then clear theselected bit.Copy the value of the selected bit to the carry flag, then clear theselected bit.Copy the value of the selected bit to the carry flag, then clear theselected bit.Copy the value of the selected bit to the carry flag, then clear theselected bit.Related InstructionsBT, BTC, BTSBTR 83