Microprogrammed Control MCQ Questions and Answers

1. The primary function of control memory in a microprogrammed control unit is to store:
A. Instruction opcodes
B. Microinstructions that generate control signals
C. Register contents
D. Cache lines
Answer: B

2. Which register typically holds the address of the next microinstruction to be fetched from control memory?
A. Program Counter (PC)
B. Memory Address Register (MAR)
C. Microprogram Counter (MPC) or Sequencer Register
D. Instruction Register (IR)
Answer: C

3. In microprogrammed control, a horizontal microinstruction is characterized by:
A. Highly encoded fields and few bits per control signal
B. Bit-position control fields directly driving many signals in parallel
C. Single-bit micro-operations only
D. Being stored only in writable control stores
Answer: B

4. A vertical microinstruction format is most valued for:
A. Fast parallel control signal assertion
B. Compact encoding and fewer bits per microinstruction
C. Direct mapping to hardware control lines
D. Large control memory usage
Answer: B

5. Which addressing technique allows branching to an address obtained by concatenating a small field of microinstruction with part of the opcode?
A. Absolute addressing
B. Relative addressing
C. Indexed addressing
D. Field concatenation (or opcode-based) sequencing
Answer: D

6. A microinstruction that specifies “goto address X unconditionally” is called a:
A. Conditional transfer microinstruction
B. Unconditional microbranch or jump microinstruction
C. Fetch microinstruction
D. Return microinstruction
Answer: B

7. The control store implemented as ROM implies:
A. Microprogram can be modified by user at runtime
B. Microprogram is fixed and cannot be changed after manufacturing
C. Microinstructions are interpreted slower
D. It is cheaper to update microcode frequently
Answer: B

8. Which structure in the control unit selects the next microinstruction address based on tests and current microinstruction fields?
A. Arithmetic Logic Unit (ALU)
B. Sequence Generator / Address Sequencer
C. Cache Controller
D. Interrupt Controller
Answer: B

9. A nanoprogram is best described as:
A. A microprogram written for nanoscale devices only
B. A lower-level microcode layer that further decomposes microinstructions
C. Microcode that controls nanomachines
D. A microprogram in vertical format only
Answer: B

10. Microinstruction fields that directly set control lines (no decoding) are known as:
A. Encoded fields
B. Vertical fields
C. Bit-slice control fields (horizontal style)
D. Relative offsets
Answer: C

11. Which microinstruction sequencing method uses a dedicated field to specify the next address unconditionally?
A. Next-address field (direct sequencing)
B. Increment by one only
C. Microbranch table lookup
D. Implicit sequencing via ALU result
Answer: A

12. Conditional microbranches in microprogramming depend on:
A. Tests of status flags or external condition signals
B. Only the current microinstruction index
C. The value stored in data memory
D. The phase of the clock only
Answer: A

13. A microprogrammed control unit offers which key advantage over hardwired control?
A. Higher execution speed always
B. Easier modification and maintainability of control logic
C. Lower control memory cost always
D. No need for sequencing logic
Answer: B

14. Which of the following is a disadvantage of horizontal microinstructions?
A. Reduced parallelism in control signals
B. Large control store size due to many bits per microinstruction
C. Complex decoding overhead
D. Difficulty to represent control signals directly
Answer: B

15. In a typical microinstruction, the “next address generation” field may include:
A. Only the current PC value
B. A selection among increment, microbranch, subroutine call, or return
C. Only opcode bits
D. Only a conditional flag
Answer: B

16. Microprogram control that uses a writable control store (WCS) is useful because:
A. It prevents any firmware updates
B. It allows microcode updates after manufacturing
C. It is always faster than ROM-based control store
D. It requires no protection mechanisms
Answer: B

17. The microinstruction format that uses symbolic names and requires an assembler is called:
A. Machine code
B. Micro-assembly (symbolic microprogramming)
C. Binary microformat only
D. Hardwired encoding
Answer: B

18. Which microinstruction addressing mode uses the opcode of the machine instruction to choose the starting microaddress?
A. Absolute address mode
B. Opcode indexed sequencing (entry-point per opcode)
C. Relative mode only
D. Stack-based addressing
Answer: B

19. When designing a control unit, the set of micro-operations that can be performed in a single microinstruction word are determined primarily by:
A. The size of the ALU only
B. How many control lines the microinstruction can assert in parallel
C. The width of the data path exclusively
D. The instruction cache size
Answer: B

20. A microprogram subroutine call in a microprogrammed control unit typically uses:
A. A hardware stack or a micro-return address register to save return address
B. The main program stack automatically
C. No return mechanism; subroutines cannot be called
D. DMA controller to store addresses
Answer: A

21. Which microsequencing technique is best when many instructions share common entry sequences?
A. Fully hardwired next-addressing only
B. Using shared micro-routines and entry points indexed by opcode
C. Storing each instruction’s entire microsequence separately without sharing
D. Randomized microaddressing
Answer: B

22. Control-store addressable units are typically addressed by:
A. Byte addressing only
B. Microinstruction addresses (microaddress) which may be word-addressable
C. Physical memory frames only
D. Cache tags
Answer: B

23. The “microinstruction cycle” typically consists of:
A. Single clock only
B. Fetching a microinstruction, decoding its fields, executing the micro-operations, and computing the next address
C. Flushing the pipeline only
D. Bus arbitration exclusively
Answer: B

24. Which of the following best defines a horizontal microprogram advantage?
A. Lower code density
B. Fast execution due to minimal decoding and many control signals asserted concurrently
C. Easier to compress microcode
D. Eliminates the need for sequencer logic
Answer: B

25. Which structure can reduce control memory size by encoding control signals into fields that are decoded at runtime?
A. Horizontal microprogramming
B. Vertical microprogramming with a microinstruction decoder
C. Using ROM for every control line bit
D. No encoding possible
Answer: B

26. In microprogram design, “microinstruction packing” refers to:
A. Combining microinstructions into long macroinstructions in the ISA
B. Placing multiple functional control fields into one word to save space or improve locality
C. Compressing the main program into microcode
D. Linking microinstructions externally
Answer: B

27. Which of the following is true for hardwired control units vs microprogrammed control?
A. Hardwired control is typically simpler to modify than microprogrammed control
B. Hardwired control can be faster but less flexible than microprogrammed control
C. Microprogrammed control cannot implement complex ISAs
D. Hardwired control always uses less hardware
Answer: B

28. The return from microprogram subroutine typically uses which register?
A. Main PC register
B. Micro Return Address Register (or stack top saved address register)
C. MAR directly
D. IR as return register
Answer: B

29. A “next-address” field value that selects “increment” means:
A. Jump to a non-sequential microaddress
B. Go to the next microinstruction address (MPC + 1)
C. Halt execution
D. Return from subroutine
Answer: B

30. Which microinstruction field would you use to implement conditional branching based on the Zero flag?
A. Control line field only
B. Test or condition field combined with branch target field
C. Opcode field only
D. Data field only
Answer: B

31. The minimum hardware required for microinstruction fetch includes:
A. An MPC, a control memory read port, and a microinstruction register (MIR)
B. ALU only
C. Cache controller and TLB only
D. Main memory and instruction pointer only
Answer: A

32. In microprogram flow control, “vectoring” refers to:
A. Using microinstructions that perform vector arithmetic
B. Jumping to a microaddress computed from a vector table, often indexed by opcode or condition
C. Using vector registers to store microaddresses
D. Storing microprograms in multiple dimensions
Answer: B

33. Which design goal encourages using a vertical microinstruction format?
A. Maximum parallelism in control signals
B. Minimizing control store capacity (compact microcode)
C. Direct unencoded control of hardware lines
D. Eliminating decoding logic entirely
Answer: B

34. A sequencer that computes next microaddress by selecting among increment, branch target, and subroutine return is called:
A. A simple program counter only
B. A microaddress sequencer with multiple source selection logic
C. A FIFO controller only
D. An ALU without registers
Answer: B

35. Microinstruction “packing density” is most affected by:
A. Number of available general-purpose registers only
B. Width of microinstruction and encoding choices for control fields
C. Cache miss rate
D. Clock frequency only
Answer: B

36. Which is a typical role of the microinstruction register (MIR)?
A. Store the current instruction being executed from main memory
B. Hold the fetched microinstruction for decoding and control signal application
C. Act as the general-purpose accumulator
D. Manage virtual memory translation
Answer: B

37. Microprogram debugging tools often include:
A. Source-level debugger for C only
B. Microassembler, stepwise execution, microtrace, and breakpoint support at microinstruction level
C. Only hardware probes; no software tools
D. None — microprograms cannot be debugged
Answer: B

38. Which of the following improves microprogram execution speed without changing microinstruction width?
A. Increasing microinstruction count
B. Caching frequently used microinstructions in a control cache
C. Switching from ROM to slower memory
D. Removing the sequencer logic
Answer: B

39. In implementing complex instruction sets, microprogramming helps mainly by:
A. Eliminating the need for an ALU
B. Mapping complex machine instructions to sequences of simpler micro-operations in control memory
C. Replacing main memory with control memory
D. Ensuring single-cycle execution of all instructions
Answer: B

40. Which microinstruction format would most likely include an explicit “call” field for subroutine entry?
A. Horizontal with dedicated call bit/field
B. Vertical without any sequencing fields
C. Undefined microformat only
D. Only ROM-based microinstructions lack call fields
Answer: A

41. Control memory organized as associative memory would allow:
A. Only sequential microfetching
B. Selecting microinstructions by content rather than by explicit address (rare for control stores)
C. No branching possible
D. Only ROM usage permitted
Answer: B

42. Microinstructions that allow multiple simultaneous micro-operations on different parts of datapath are said to be:
A. Sequential micro-operations
B. Exploiting microparallelism (horizontal style)
C. Single-threaded only
D. Serial microinstructions
Answer: B

43. The key trade-off in microinstruction encoding is between:
A. Clock speed and transistor count only
B. Control store width (bit count per microinstruction) versus control store depth (number of microinstructions)
C. Compiler complexity only
D. Memory latency and cache size only
Answer: B

44. Which of the following is NOT normally stored in control memory?
A. Microinstructions (control vectors)
B. Microsequence entry points for opcodes
C. Main program data variables
D. Next-address fields and condition descriptors
Answer: C

45. A “sequencer with conditional tests” typically reads condition signals from:
A. External devices only
B. Status flags, condition latches, and sometimes external inputs or instruction fields
C. Disk storage only
D. The control store content only
Answer: B

46. Which microprogramming technique reduces control memory by using compressed microinstructions and decoding at fetch?
A. Horizontal microcode
B. Microinstruction compression or encoding (vertical approach with decoder)
C. Direct bit control with no decoding
D. Microcache only
Answer: B

47. During microinstruction execution, the order is:
A. Decode → Fetch → Execute → Next-address generation
B. Fetch → Decode → Execute micro-ops → Compute next microaddress
C. Execute → Fetch → Decode
D. Only fetch and execute; no decode stage
Answer: B

48. In a microprogrammed design, a return from micro-subroutine often requires which hardware mechanism?
A. A dedicated return-address stack or register
B. Using main memory PC to return
C. Resetting the sequencer to zero always
D. Storing return address in ALU internal register only
Answer: A

49. Which statement about the control unit synthesis is correct?
A. Hardwired control is synthesized from microcode only
B. One can synthesize a control unit either as a microprogram (control store) or as combinational/sequential logic (hardwired) depending on trade-offs
C. Synthesis always uses PLA exclusively
D. Control unit cannot be synthesized automatically
Answer: B

50. A microinstruction field that enables clocked transfer of register contents is called:
A. Transfer control field or register-transfer control bits
B. Opcode field only
C. Memory management field
D. Cache control field
Answer: A

51. Which approach eases support for multiple instruction set architectures (ISAs) on the same hardware?
A. Hardwired control only
B. Microprogramming with an easily updatable control store (emulation via microcode)
C. Fixed microcode in ROM that cannot be updated
D. No control unit at all
Answer: B

52. The mechanism that allows microinstructions to examine external events or input signals for branching decisions is called:
A. Microfetch buffer only
B. Test or status input lines used by the sequencer
C. Data cache controller
D. Memory management unit
Answer: B

53. Which microprogram storage is most secure from user modifications but inflexible to patches?
A. Writable control store (WCS)
B. ROM or mask-programmed control store
C. Flash-based WCS only
D. SRAM-based WCS with protection disabled
Answer: B

54. The microinstruction addressing method that uses concatenation of opcode bits with a fixed prefix is beneficial for:
A. Randomizing microaddresses
B. Directly mapping each opcode to a microprogram entry with fewer bits in address field
C. Only vertical microcode
D. Avoiding any decoding at runtime
Answer: B

55. In microprogram sequencing, a “conditional call” differs from a “conditional branch” in that:
A. Call saves return address before jumping, branch does not
B. Branch saves return address, call does not
C. Both always save addresses
D. Neither can be used in microprograms
Answer: A

56. The component that converts encoded microinstruction fields into individual control signals is the:
A. Microinstruction decoder or control signal decoder
B. Main CPU ALU
C. Cache decoder only
D. Memory management unit
Answer: A

57. When measuring control unit performance, which metric is most relevant?
A. Number of general-purpose registers only
B. Average number of microinstructions per machine instruction and microinstruction cycle time
C. L1 cache size only
D. Disk I/O rate
Answer: B

58. Which design would likely produce the smallest control store size?
A. Horizontal microcode with direct control bits
B. Vertical microcode with compact encoding and decoder
C. A microprogram with no encoding at all
D. One that duplicates micro-sequences for every instruction variant
Answer: B

59. If microinstructions are wide and many control signals are asserted simultaneously, the likely effect is:
A. Smaller microprogram size always
B. Faster per-instruction control generation, but larger control memory usage
C. Slower microcycle time always
D. Need for more microassembler passes only
Answer: B

60. Microprograms are typically written using:
A. High-level languages like Java only
B. Microassembly language or a symbolic microcode language specific to the control store
C. Binary only with no assembly toolchain
D. SQL-like queries
Answer: B

61. A “microinstruction cache” (control cache) stores:
A. Frequently used data from RAM
B. Recently used microinstructions to speed up sequencing and reduce control store access time
C. Only macroinstructions from main memory
D. The translation lookaside buffer entries
Answer: B

62. The most important reason for using microprogram subroutines in control memory is:
A. To increase redundancy in microcode
B. To share common micro-sequences among several machine instructions, reducing code size
C. To make microcode execution serial only
D. To avoid using an ALU
Answer: B

63. A microinstruction that includes a mask for enabling selected control lines is an example of:
A. Encoded vertical microinstruction only
B. Horizontal microinstruction style where a control field directly maps to hardware lines via mask bits
C. A branch microinstruction only
D. Data memory microinstruction only
Answer: B

64. Which microsequencing method is well-suited when the next microaddress depends on ALU computation results?
A. Only increment-based sequencing
B. Using the ALU result to form the next microaddress (computed branching)
C. ROM-only sequencing with no inputs
D. Static concatenation only
Answer: B

65. In microprogramming, “threaded microcode” typically refers to:
A. Microcode that runs on multiple CPU threads
B. Using shared micro-sequences linked by small jump fields to create many instruction behaviors with less memory
C. Microcode written in threaded assembly language
D. Parallel processing microcode only
Answer: B

66. The control memory interface must guarantee that a microinstruction is available to the MIR in:
A. Undefined time only
B. One microcycle (or known number of cycles) so sequencing is deterministic
C. Only when main memory is free
D. After user process context switch
Answer: B

67. Which of the following best describes “microinstruction redundancy reduction”?
A. Duplicating identical microinstructions across control store
B. Factoring repeated micro-sequences into shared subroutines or using encoding to avoid repetition
C. Using longer microinstructions always
D. Storing microinstructions in multiple caches simultaneously
Answer: B

68. Microcode that emulates another machine’s instruction set is often called:
A. Virtual microcode only
B. Emulation microcode (microprogram-based emulator)
C. Hardwired control
D. Kernel microcode exclusively
Answer: B

69. Which hardware structure speeds up access to frequently executed microinstructions and reduces control store bandwidth?
A. Instruction TLB
B. Control or microinstruction cache (control cache)
C. Data cache only
D. Disk swap area
Answer: B

70. Which microprogram sequencing field would most likely be used to implement loop repetition for certain micro-operations?
A. Next-address increment only
B. Conditional branch target combined with a counter or condition test
C. Opcode field only
D. Stack pointer only
Answer: B

71. The process of translating symbolic microcode into binary control store contents is called:
A. Compilation only for high-level languages
B. Microassembly (assembling the microprogram)
C. Linking only
D. Interpretation at runtime only
Answer: B

72. Which of these is a typical microinstruction format field?
A. Memory management unit table index
B. Control field(s), next address field, condition/test field, and sometimes subroutine field
C. File system pointer only
D. Network address only
Answer: B

73. In designing a control unit for pipelined architectures, microprogram designers must take care to:
A. Ignore hazards entirely
B. Ensure microprogram sequencing accounts for pipeline stages, hazards, and control interlocks
C. Always use hardwired control for pipeline stages only
D. Use opcode-only sequencing always
Answer: B

74. Which microprogram design technique helps when a control store is small but many control signals exist?
A. Use horizontal microcode only
B. Use vertical microcode with compact fields and a decoder to expand into many control signals
C. Duplicate control signals physically to reduce decoding
D. Store microinstructions on disk
Answer: B

75. In a writable control store, which mechanism is essential for reliability and security?
A. Allowing unrestricted writes from any process
B. Protection and privileged modes for microcode updates (write protection and authentication)
C. No need for write verification
D. Only ROM can be used for protection
Answer: B

76. The “fetch-execute” cycle of a microprogram differs from machine instruction fetch-execute because:
A. Microprogram fetches microinstructions from control memory rather than machine instructions from main memory
B. Microprograms never perform ALU operations
C. Machine instruction fetch is always faster than microfetch
D. Microprograms use the PC directly for micro-ops
Answer: A

77. Microcode compression schemes trade off:
A. Speed vs space — compressed microinstructions save space but require extra decoding time
B. Power vs cache size only
C. Reliability vs functionality only
D. Nothing; compression always better
Answer: A

78. Which field in a microinstruction would you examine to determine whether a micro-branch should occur on the negative flag?
A. Micro-op field only
B. Condition/test field that references the negative flag and branch target field
C. Data field only
D. Opcode field only
Answer: B

79. A “two-level microprogram” architecture (nanoprogram) uses:
A. Only one level of microinstructions with no further breakdown
B. A higher-level microinstruction set implemented by another lower-level microprogram (nanocode) for very fine control
C. No microsequencing at all
D. Only ROM-based control stores at both levels
Answer: B

80. Which of the following improves maintainability of microcode?
A. Writing microcode only in binary
B. Using symbolic microassembly, comments, modular subroutines, and structured entry points
C. Embedding comments into hardware only
D. Avoiding subroutines in microcode entirely
Answer: B

81. The next-address generation logic that chooses between increment, branch target, and a register-specified address is usually implemented using:
A. A multiplexer controlled by condition/test fields and microinstruction bits
B. A stack exclusively
C. Hard-coded wiring without logic
D. A separate CPU core only
Answer: A

82. In a microprogrammed control unit, the condition tests used for branching are most often read from:
A. The microinstruction itself only
B. Status registers and external condition signals (zero, carry, ready, etc.)
C. Disk status only
D. Floating point unit exclusively
Answer: B

83. Which of the following could be a reason to choose hardwired control over microprogrammed control?
A. Need for easy field updates and patches
B. Requirement for the highest possible instruction throughput with minimal microinstruction overhead
C. Desire to run multiple ISAs on the same hardware easily
D. Need for runtime microcode modification
Answer: B

84. When microinstructions are executed faster than the control store can supply them, designers may add:
A. Larger microinstructions only
B. A control cache or buffer to decouple fetch from microexecution
C. More main memory only
D. Migrate to hardwired control exclusively
Answer: B

85. A microprogram “entry point table” maps machine opcodes to:
A. Register values only
B. Starting microaddresses (entry points) in control memory for each opcode
C. Disk block addresses only
D. ALU micro-ops only
Answer: B

86. Which microinstruction feature makes interruptible micro-sequences easier to implement?
A. No next-address field at all
B. Saving and restoring the microaddress and relevant state (context saving) on interrupt entry/exit
C. Using only ROM microstores that cannot be paused
D. Avoiding subroutines always
Answer: B

87. In microprogram design, “fan-out” of control lines refers to:
A. Number of microinstructions that call a subroutine only
B. Number of hardware signals a single microinstruction bit can drive or propagate to
C. Number of pipeline stages only
D. Cache associativity only
Answer: B

88. Microprograms that implement virtual machines or emulators most critically require:
A. No sequencer logic at all
B. High flexibility and often writable control stores to update emulation strategies or bugfixes
C. ROM with no update capability always
D. No condition testing capability
Answer: B

89. Which of the following is most likely to reduce time spent in control-store fetches?
A. Increasing control word width only
B. Implementing a small, fast control cache (micro-cache) holding hot microinstructions
C. Using vertical microcode only
D. Deleting tests and branch fields from microinstructions
Answer: B

90. Which of these is crucial for a microprogram that must run in real-time constrained systems?
A. Random microaddress fetching only
B. Deterministic worst-case microinstruction execution time and bounded control-store access latency
C. Only compressing microcode without performance analysis
D. Frequent writable control store updates at runtime
Answer: B

91. A technique to make microcode smaller by replacing repeated sequences with calls is called:
A. Unrolling microcode loops
B. Factoring into micro-subroutines (procedural decomposition)
C. Duplicating sequences across the store
D. Removing all branches
Answer: B

92. In microprogram control, “vertical microcode” typically requires:
A. No decoder logic at all
B. A decoder to expand compact fields into many actual control signals
C. Wider microinstructions than horizontal formats
D. Direct hardware wiring for each bit
Answer: B

93. The main cost of implementing a very wide horizontal microinstruction is:
A. Complexity of decoding only
B. Large physical control store area and increased memory bandwidth to fetch wide words
C. Increased symbolic expressiveness only
D. Decreased parallelism always
Answer: B

94. Which microprogramming facility simplifies writing microcode by allowing symbolic names for control bits?
A. Direct binary entry only
B. Microassembler that provides symbols and labels for control signals and addresses
C. No facilities exist for symbolic control bit names
D. Disk-based microcode editor only
Answer: B

95. A microinstruction tested on an “I/O ready” signal before branching is an example of:
A. Data cache coherency only
B. Using external condition signals to synchronize microprogram flow with peripheral readiness
C. Ignoring external signals entirely
D. Hardwired control exclusively
Answer: B

96. Which technique helps mitigate large control store access time when using wide microinstructions?
A. Decrease microinstruction width only
B. Use pipelined control-store access or microinstruction prefetching and buffering
C. Avoid using subroutines entirely
D. Use only ROM and no caches
Answer: B

97. Microprogrammed control units can implement complex addressing modes by:
A. Ignoring addressing modes and only using registers
B. Sequencing micro-operations that perform effective address computation step-by-step under microcode control
C. Replacing ALU entirely
D. Storing all possible addresses in control memory permanently
Answer: B

98. Which of these is true about microcode portability across different hardware implementations?
A. Binary microcode is always portable across machines of different datapaths
B. Symbolic microcode with an appropriate microassembler and mapping layer is more portable than raw binary microcode
C. No microcode can ever be ported between machines
D. Microcode portability is irrelevant to design choices
Answer: B

99. During microprogram verification, important checks include:
A. Only checking syntax, not runtime behavior
B. Ensuring correctness of sequencing, absence of unreachable or infinite-loop micropaths, and correct condition tests and return mechanics
C. Only running the entire system in hardware without simulation
D. Ignoring concurrency hazards entirely
Answer: B

100. Which microsequencing feature most directly supports implementing multi-word machine instructions (instruction with multiple fetch cycles)?
A. Static single-address only
B. Sequencer support for looping, waiting for memory-ready, and branching to fetch subroutines ensures multi-step instruction fetch and execution sequences
C. Removing conditional branches entirely
D. Only vertical encoding support
Answer: B