The remaining 12 bits in a 20-bit shift register can typically be used by any instruction, not just another shift or sequencer instruction. This is contingent on the design and architecture of the system in which the register operates.
In a 20-bit shift register, the function of the remaining 12 bits after a shift operation depends on the context and design of the register within a specific system. However, typically, any bits not used in a shift operation are not inherently constrained in their use and can potentially be accessed or manipulated by any instruction in the system if the design allows for it. Therefore, the correct answer is d) Can be used by any instruction.
This means that the remaining bits are part of the register's overall storage capability and may be utilized for other operations — not solely by another shift instruction or a sequencer instruction. Their use is subject to the system's architecture and the instructions supported by the processor or controller managing the shift register.
For instance, in a 32-bit processor, while a shift operation may only actively manipulate 8 bits in a 20-bit shift register, the remaining 12 bits are still part of the processor's memory space and can be used to hold or manipulate data needed for other instructions or operations beyond shifting.