BSR -- Bit Scan Reverse

BSR -- Bit Scan Reverse Opcode Instruction Clocks Description 0F BD BSR r16,r/m16 10+3n Bit scan reverse on r/m word 0F BD BSR r32,r/m32 10+3n Bit scan reverse on r/m dword Operation IF r/m = 0 THEN ZF = 1; register = UNDEFINED; ELSE temp = OperandSize - 1; ZF = 0; WHILE BIT[r/m, temp] = 0 DO temp = temp - 1; register = temp; OD; FI; Description BSR scans the bits in the second word or doubleword operand from the most significant bit to the least significant bit. The ZF flag is cleared if the bits are all 0; otherwise, ZF is set and the destination register is loaded with the bit index of the first set bit found when scanning in the reverse direction. Flags Affected ZF as described above Protected Mode Exceptions #GP(0) if the result is in a nonwritable segment; #GP(0) for an illegal memory operand effective address in the CS, DS, ES, FS, or GS segments; #SS(0) for an illegal address in the SS segment; #PF(fault-code) for a page fault Real Address Mode Exceptions Interrupt 13 if any part of the operand would lie outside of the effective address space from 0 to 0FFFFH Virtual 8086 Mode Exceptions Same exceptions as in Real Address Mode; #PF(fault-code) for a page fault