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Authors:
Linda Null ,julia Lobur
Chapter:
A Closer Look At Instruction Set Architectures
Exercise:
Exercises
Question:11 | ISBN:9780763704445 | Edition: 3

Question

11. a) In a computer instruction format, the instruction length is 11 bits and the size of an address field is 4 bits. Is it possible to have

5 2-address instructions

45 1-address instructions

32 0-address instructions

using the format? Justify your answer.

b) Assume that a computer architect has already designed 6 two-address and 24 zero-address instructions using the instruction format given in Problem 11. What is the maximum number of one-address instructions that can be added to the instruction set?

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Answer

a)

In a computer instruction format with an instruction length of 11 bits and a 4-bit address field, we can determine if it is possible to have the given number of instructions in each category:

  • 5 2-address instructions: Each 2-address instruction requires two address fields. With a 4-bit address field, we have a total of 4 bits available for addresses. Since each address field requires 4 bits, the maximum number of 2-address instructions that can be accommodated is (4 bits / 4 bits) = 1 instruction. Therefore, it is not possible to have 5 2-address instructions using this format.

  • 45 1-address instructions: Each 1-address instruction requires one address field. With a 4-bit address field, we can have a maximum of 41 = 4 different addresses. Therefore, it is not possible to have 45 1-address instructions using this format.

  • 32 0-address instructions: 0-address instructions do not require any address fields. Since the instruction format has an instruction length of 11 bits, it can accommodate any number of 0-address instructions as long as they fit within the 11-bit instruction length. Therefore, it is possible to have 32 0-address instructions using this format.

b)

If the computer architect has already designed 6 two-address instructions and 24 zero-address instructions using the given format, we can calculate the maximum number of one-address instructions that can be added:

  • Total number of instructions = Total instruction slots - Two-address instructions - Zero-address instructions
  • Total number of instructions = 211 (total possible instruction combinations) - 6 (two-address instructions) - 24 (zero-address instructions)
  • Total number of instructions = 2048 - 6 - 24
  • Total number of instructions = 2018

 

Hence, the maximum number of one-address instructions that can be added to the instruction set is 2018.

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