Combined Bank Senior Officer (IT/ICT) Written Test: 2019 (DU)প্রশ্ন ও সমাধান

A logic gate is a basic digital-circuit building block that performs one Boolean operation on one or more binary inputs to produce a single binary output. There are seven basic gates: AND, OR, NOT, NAND, NOR, XOR, XNOR.

graph LR
    A1((A))-->G1[AND]; B1((B))-->G1; G1-->O1((Y))
    A2((A))-->G2[OR]; B2((B))-->G2; G2-->O2((Y))
    A3((A))-->G3[NOT]; G3-->O3((Y))
    A4((A))-->G4[NAND]; B4((B))-->G4; G4-->O4((Y))
    A5((A))-->G5[NOR]; B5((B))-->G5; G5-->O5((Y))
    A6((A))-->G6[XOR]; B6((B))-->G6; G6-->O6((Y))
    A7((A))-->G7[XNOR]; B7((B))-->G7; G7-->O7((Y))

AND — output is 1 only when both inputs are 1.

A	B	Y
0	0	0
0	1	0
1	0	0
1	1	1

OR — output is 1 if at least one input is 1.

A	B	Y
0	0	0
0	1	1
1	0	1
1	1	1

NOT — single-input inverter; output is the complement of the input.

A	Y
0	1
1	0

NAND (NOT-AND) — complement of AND; output is 0 only when both inputs are 1.

A	B	Y
0	0	1
0	1	1
1	0	1
1	1	0

NOR (NOT-OR) — complement of OR; output is 1 only when both inputs are 0.

A	B	Y
0	0	1
0	1	0
1	0	0
1	1	0

XOR (Exclusive-OR) — output is 1 only when the inputs differ.

A	B	Y
0	0	0
0	1	1
1	0	1
1	1	0

XNOR (Exclusive-NOR) — output is 1 only when the inputs are equal.

A	B	Y
0	0	1
0	1	0
1	0	0
1	1	1

Exam takeaway: AND/OR/NOT are the fundamental (functionally complete) building blocks; NAND and NOR are each individually universal gates (any Boolean function can be built from NAND alone, or from NOR alone) — a fact frequently tested directly.

ACID Properties

ACID is the set of four guarantees a database transaction must satisfy to be considered reliable:

Property	Meaning
Atomicity	A transaction is "all or nothing" — either every operation in it completes, or none do (rolled back on failure).
Consistency	A transaction moves the database from one valid state to another, never violating defined rules/constraints.
Isolation	Concurrently executing transactions do not interfere with each other's intermediate results.
Durability	Once a transaction is committed, its effects survive permanently, even after a system crash.

Deadlock

A deadlock occurs when two or more transactions are each waiting for a resource (e.g., a row/table lock) held by another transaction in the same waiting group, so that none of them can ever proceed — a circular wait.

graph LR
    T1[Transaction 1] -->|holds lock on A, waits for B| T2[Transaction 2]
    T2 -->|holds lock on B, waits for A| T1

Conditions for deadlock (Coffman conditions) — all four must hold simultaneously: Mutual Exclusion, Hold-and-Wait, No Preemption, Circular Wait.

Deadlock prevention/avoidance techniques:

• Lock ordering: force all transactions to acquire locks on resources in the same predefined global order, eliminating circular wait.
• Timeout: abort a transaction if it waits beyond a set time, releasing its locks.
• Wait-Die / Wound-Wait schemes: use transaction timestamps to decide whether an older transaction may wait for a younger one, or must force it to abort.
• Deadlock detection + recovery: periodically build a wait-for graph; if a cycle is found, abort (roll back) one transaction in the cycle to break it.
• Avoid hold-and-wait: require a transaction to request all needed locks at once (all-or-nothing), rather than acquiring them incrementally.

Exam takeaway: ACID = Atomicity, Consistency, Isolation, Durability — the reliability contract of a transaction; deadlock = circular resource wait among transactions, prevented by consistent lock ordering or resolved by periodic detection-and-rollback.