Bangladesh Bank Assistant Director (IT) Preliminary Exam: 2016প্রশ্ন ও সমাধান

Correct answer: (b) Two stable states

A binary circuit element has to hold one of two values, 0 or 1, and keep it until it is deliberately changed. Such an element is called a bistable: it has exactly two stable states, for example a low and a high output voltage. Latches and flip-flops are the standard bistable elements, and each stores one bit. Feedback inside the circuit holds whichever state was last set.

Why the other options are wrong

• (a) One stable state: A circuit with only one stable state (a monostable) always returns to that state, so it cannot remember which of two values it was given and cannot hold a bit.
• (c) Three stable states: Three states would make a ternary (base-3) element. Binary means base 2, so there are exactly two states. A tri-state output's third condition is a disconnected (high-impedance) output, not a third logic value.
• (d) None of these: Option (b) is correct.

Tip: Binary element = bistable = two stable states = 1 bit.

Sources: University of Cambridge, Digital Electronics: Sequential Logic, introduction, latches and flip-flops (slides); MIT 6.191 (Fall 2023), L06: Sequential Circuits and Finite State Machines (slides)

Correct answer: (b) Secondary storage unit

The secondary storage unit (hard disk, SSD, optical disc, magnetic tape, USB flash drive) is non-volatile: it keeps data and programs when the computer is switched off, so it is where data is held permanently until deliberately deleted.

Why the other options are wrong

• (a) Input unit: Devices such as the keyboard and mouse only bring data into the computer; they do not store it.
• (c) Output unit: Devices such as the monitor and printer only present results; they do not keep data.
• (d) Primary Memory unit: Main memory (RAM) is volatile, so its contents are lost when power is cut; it is only a temporary working area.

Tip: Permanent → secondary storage; temporary working area → primary memory (RAM).

Source: What is primary storage? (IBM)

Correct answer: (d) All of them

Magnetic tape is a read/write medium, so it can fill all three roles:

• Secondary storage: it holds data outside main memory and keeps it when power is off. Today it is used mainly for backup and long-term archives because it is cheap and holds a lot.
• Input media: data recorded on tape is read back into the computer by a tape drive. Early systems such as UNIVAC I read their input from tape.
• Output media: the computer writes results to tape for storage or for transfer to another system.

Its main drawback is sequential access: to reach one file the drive must wind through the tape, so it is slow for random access.

Why the other options are wrong

• (a) Secondary storage media: correct, but only one of its roles.
• (b) Output media: correct, but only one of its roles.
• (c) Input media: correct, but only one of its roles.

Tip: Any medium a drive can both write and read (tape, magnetic disk, writable optical disc) counts as storage, input and output media.

Source: Magnetic Tape — CHM Revolution (Computer History Museum); Magnetic-tape data storage — Wikipedia