GATE CSE 1987 | Question: 1-vi

Question

A microprogrammed control unit

• A. Is faster than a hard-wired control unit.
• B. Facilitates easy implementation of new instruction.
• C. Is useful when very small programs are to be run.
• D. Usually refers to the control unit of a microprocessor.

Comments

Microprogrammed control unit (CU) vs hard-wired CU — what to remember:

• How they work:

  • A microprogrammed CU implements instruction sequencing and control signals by storing microinstructions (control words) in a small control memory; the control memory is read to generate control signals.

  • A hard-wired CU implements the same control logic using fixed combinational/sequential logic (no control memory).

• Performance:

  • Because microprogrammed CU adds an extra level of interpretation (fetch microinstruction → generate control signals), it is usually slower than a carefully designed hard-wired CU. So microprogramming trades some raw speed for flexibility. 

• Flexibility / extensibility:

  • Microprogrammed CU makes it easy to add or change instructions — you just update the control memory contents (microcode) rather than redesigning hardware. This is the primary advantage.

• Use for small programs / small systems:

  • If the system runs only very small or simple programs and the instruction set is limited and stable, a hard-wired CU may be simpler and faster; microprogramming’s overhead and control memory may not be justified. So the statement “useful when very small programs are to be run” is not generally an advantage of microprogramming. 

• Association with microprocessors:

  • The term “microprogrammed control unit” does not necessarily mean “the control unit of a microprocessor.” A microprocessor’s control unit might be implemented either way (hardwired or microprogrammed). So equating “microprogrammed CU” with “control unit of a microprocessor” is not generally correct. 

---

2) Apply to the options

The options given:

1. Is faster than a hard-wired control unit.

   • False. Microprogrammed CU is normally slower because it uses a control memory and an interpretation step; hardwired can be faster.

2. Facilitates easy implementation of new instruction.

   • True. New instruction sequences can be implemented by changing microcode (control memory), so this is the classic advantage of microprogramming.

3. Is useful when very small programs are to be run.

   • False (not generally true). For very small/simple instruction sets or tiny systems, hard-wired control is often simpler/faster and more economical.

4. Usually refers to the control unit of a microprocessor.

   • False. “Microprogrammed CU” is an implementation style; it does not automatically mean “control unit of a microprocessor” — microprocessors can use either style.

---

A microprogrammed control unit is a type of control unit in a CPU that generates control signals by executing a sequence of tiny instructions called microinstructions.

Instead of using complex, fixed physical logic gates to figure out what to do (which is how a hardwired control unit works), it uses a small, dedicated memory inside the CPU to store these microinstructions.

Answer 1

• A. is wrong. Microprogrammed Control Unit (CU) can never be faster than hardwired CU. Microprogrammed CU it has an extra layer on top of hardwired CU and hence can only be slower than hardwired CU.
• B. is a suitable answer as we can add new instruction by changing the content of control memory.
• C. is not correct as when only small programs are there, hardwired control makes more sense.
• D. control unit can also be hardwired, so this is also not correct.

Reference: Slides

Correct Answer: $B$

Comments

@Arjun Sir

above reference link is not working...

---

I cannot really understand the relation between  the length of programs and the use of hardwired/microprogrammed control units.

 

few points I thought about.

if we have only small length of programs then  will have less complexity of the hardwired circuit.because of less number of instructions.

but having small length programm doesnot guarantee us that we will have less number of instructions .In total we might have a large number of different  instructions.(making the circuit complex )

 

if we have large length programs then we will have more complexity  of the  hardwired circuit.because of more number of instructions.

  but having large length program doesnot guarantee us that we will have large number of instructions .In total we might have a programs using only a small set of instructions.thus less hardwire required.

 

so the length of program doesnot determine the which type of control unit to use.

---

https://web.archive.org/web/20151211075340/http://www2.mta.ac.il/~carmi/Teaching/Architecture/SlidesOriginal/ch07%20-%20Microprogrammed%20Control.pdf

Answer 2

B. Facilitates easy implementation of new instruction.

Comments

can u explain this option more ...what does it means?

---

Gate1987-1-vi follow this link....

---

it will clear

---

@shruti gupta1 Thank You

Answer 3

Microprogrammed control unit (CU) vs hard-wired CU — what to remember:

• How they work:

  • A microprogrammed CU implements instruction sequencing and control signals by storing microinstructions (control words) in a small control memory; the control memory is read to generate control signals.

  • A hard-wired CU implements the same control logic using fixed combinational/sequential logic (no control memory).

• Performance:

  • Because microprogrammed CU adds an extra level of interpretation (fetch microinstruction → generate control signals), it is usually slower than a carefully designed hard-wired CU. So microprogramming trades some raw speed for flexibility. 

• Flexibility / extensibility:

  • Microprogrammed CU makes it easy to add or change instructions — you just update the control memory contents (microcode) rather than redesigning hardware. This is the primary advantage.

• Use for small programs / small systems:

  • If the system runs only very small or simple programs and the instruction set is limited and stable, a hard-wired CU may be simpler and faster; microprogramming’s overhead and control memory may not be justified. So the statement “useful when very small programs are to be run” is not generally an advantage of microprogramming. 

• Association with microprocessors:

  • The term “microprogrammed control unit” does not necessarily mean “the control unit of a microprocessor.” A microprocessor’s control unit might be implemented either way (hardwired or microprogrammed). So equating “microprogrammed CU” with “control unit of a microprocessor” is not generally correct. 

---

2) Apply to the options

The options given:

1. Is faster than a hard-wired control unit.

   • False. Microprogrammed CU is normally slower because it uses a control memory and an interpretation step; hardwired can be faster.

2. Facilitates easy implementation of new instruction.

   • True. New instruction sequences can be implemented by changing microcode (control memory), so this is the classic advantage of microprogramming.

3. Is useful when very small programs are to be run.

   • False (not generally true). For very small/simple instruction sets or tiny systems, hard-wired control is often simpler/faster and more economical.

4. Usually refers to the control unit of a microprocessor.

   • False. “Microprogrammed CU” is an implementation style; it does not automatically mean “control unit of a microprocessor” — microprocessors can use either style.