GATE CS 2006 Q43 — Machine Instructions and Addressing Modes

The bbs reg, pos, label (branch-on-bit-set) instruction branches to label if the bit at index pos of register reg is $1$.

The emulation performs a bitwise AND operation:

$$temp \leftarrow reg \ \& \ mask$$

And branches if temp is non-zero.

For the emulation to work correctly:

• If the bit at index pos in reg is $1$, then $reg \ \& \ mask$ must be non-zero.
• If the bit at index pos in reg is $0$, then $reg \ \& \ mask$ must be zero.

This requires the mask to have a binary $1$ at position pos and $0$ at all other bit positions. Such a mask is generated by shifting the value $1$ (represented in hex as 0x1) to the left by pos positions:

$$mask \leftarrow \text{0x1} \ll pos$$

Therefore, the correct option is A.