ITPEC FE Morning April 2020 Question 19

Topic: Software

32-bit virtual address space is mapped to 4 k-byte pages. How many pages can be created? Here, 1k bytes represent 1,024 bytes.

Explanation

1,048,576 pages — that is 220, the 32-bit address space divided by the 4 KB page size.

  • •Virtual address space: 232 bytes (4 GB)
  • •Page size: 4 KB = 4 × 1,024 = 22 × 210 = 212 bytes
  • •Number of pages: 232 ÷ 212 = 220

Equivalently, the 32-bit address splits into a 12-bit offset within the page and a 20-bit page number.

Answer (c)

Why not others:

  • •(a) — 24 pages of 4 KB cover only 64 KB of the 4 GB space
  • •(b) — 210 pages of 4 KB cover only 4 MB; to fill 4 GB with that many pages each would have to be 4 MB
  • •(d) — 232 is the number of byte addresses, which would mean pages of one byte

Key rule: The number of pages is the address space divided by the page size; with powers of two, subtract the exponents, which equals the address bits minus the offset bits.

Software · AI-written, may contain errors

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