GATE Overflow Test Series | Computer Organization and Architecture | Test 1 | Question: 7

Question

Which of the following is/are correct?

1. In RISC, control circuit is simpler than CISC kind of architectures
2. In vertical microprogramming, control signals cannot be directly generated after fetching of microinstructions
3. Horizontal microprogrammed control unit requires longer control word compared to vertical microprogramming
4. Horizontal microprogrammed control unit is more flexible than vertical microprogrammed.

• A. (a)(b)(c)(d)
• B. (a)(b)(c)
• C. (a)(b)
• D. (b)

Answer 1

A simpler control circuit is a main feature of RISC architecture.
   
   In vertical microprogramming some of the control signals are encoded to reduce the size of the control word. So, these must be decoded using special circuits before generating the corresponding control signals.
   
   In horizontal microprogramming for each control signal we use a separate bit. So, it requires longer control words than vertical microprogramming.
   
   Horizontal micrprogrammed control unit is the most flexible as each control signal has a bit.
   Ref: https://inst.eecs.berkeley.edu/~cs150/fa05/CLD_Su
   So, all $a,b,c,d$ are correct. Option A.

Comments

I am confused here a bit. Please clarify meaning of Flexible in the above context. Horizontal micro programing computers are Hardwired. So once we build the internal circuitry and map control signals. We can not change them later. So how could it be more flexible?

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Control Unit Design is done in two ways:

1. Hardwired – done using SOP Expressions.
2. Microprogrammed Control Unit

Vertical and Horizontal are ways of Microprogrammed Control Unit

See above link for vertical and horizontal programming

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optn d: vertical is more flexible than horizontal and hardwired is the least flexible.

d is incorrect.