ISCET Digital Systems & Microprocessors 3 — Questions and Answers
Question 1: What is the purpose of the stack pointer (SP) register in a microprocessor?
- Points to the next instruction in memory
- Holds the base address of the data segment
- Tracks the top of the LIFO stack in memory (Correct answer)
- Stores the return address of subroutines permanently
Correct answer: Tracks the top of the LIFO stack in memory
The stack pointer register always points to the current top of the hardware stack, adjusting automatically on PUSH and POP operations.
Question 2: A D-type latch differs from a D flip-flop in that the latch:
- Is edge-triggered while the flip-flop is level-triggered
- Is level-sensitive while the flip-flop is edge-triggered (Correct answer)
- Has two data inputs while the flip-flop has one
- Cannot hold its state without power
Correct answer: Is level-sensitive while the flip-flop is edge-triggered
A D latch is transparent (level-sensitive) when enabled, passing input to output continuously, whereas a D flip-flop captures data only on a clock edge.
Question 3: In hexadecimal, what is the result of adding 9F + 6C?
- 10B (Correct answer)
- 10C
- 10B hex
- 10Bh
Correct answer: 10B
9F + 6C: F+C=1B (write B carry 1), 9+6+1=10 hex, result is 10Bh.
Question 4: What does DMA (Direct Memory Access) allow in a microprocessor system?
- The CPU to access cache memory directly
- A peripheral device to transfer data to/from memory without CPU involvement (Correct answer)
- The ALU to bypass the bus interface
- Two CPUs to share the same memory simultaneously
Correct answer: A peripheral device to transfer data to/from memory without CPU involvement
DMA allows peripherals to transfer large blocks of data directly to or from memory, freeing the CPU for other tasks.
Question 5: How many states does a 3-bit Gray code counter have?
- 6
- 7
- 8 (Correct answer)
- 16
Correct answer: 8
A 3-bit counter can represent 2³ = 8 unique states; Gray code provides all 8 states with only one bit changing at a time.
Question 6: Which logic family is known for the lowest power consumption and highest noise immunity, making it preferred for battery-operated devices?
- TTL
- ECL
- CMOS (Correct answer)
- DTL
Correct answer: CMOS
CMOS logic draws near-zero static power because complementary transistors ensure one is always off, and its output swings fully between rails for high noise immunity.
Question 7: In interrupt-driven I/O, what is a vectored interrupt?
- An interrupt that cannot be masked
- An interrupt where the device supplies the address of its service routine (Correct answer)
- An interrupt that triggers a DMA transfer
- An interrupt generated by a software instruction
Correct answer: An interrupt where the device supplies the address of its service routine
A vectored interrupt allows the interrupting device to supply a vector (address or pointer) pointing to its specific interrupt service routine.
ISCET Digital Systems & Microprocessors 3
What is the purpose of the stack pointer (SP) register in a microprocessor?