ITPEC FE Morning October 2018 Question 12
When a 1-megabyte memory is connected to the MPU as shown in the figure below, what is the minimum number of signal lines (represented as n) required by address buses? Here, the memory is accessed in units of bytes. In addition, 1 megabyte = 1,024 kilobytes and 1 kilobyte = 1,024 bytes.
Explanation
20 lines — each byte of the 1 MB memory needs its own address, and 220 bytes need 20 address bits.
- •Memory size: 1,024 × 1,024 = 210 × 210 = 220 bytes
- •The data bus is 8 bits wide (D0 to D7), so one address selects one byte
- •n address lines give 2n addresses, so 2n ≥ 220 gives n = 20, lines A0 to A19
Answer (c)
Why not others:
- •(a) 18 — 218 addresses reach only 256 KB, a quarter of the memory
- •(b) 19 — 219 addresses reach only 512 KB, half of the memory
- •(d) 21 — 221 addresses cover 2 MB; it works, but it is one line more than the minimum
Key rule: The number of address lines is the exponent of the number of addressable units: 2n units need n lines, and the addressing unit (here a byte) sets what one unit is.
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