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4 GB = 2^30 * 2 ^ 2 = 2^32 bytes Then the answer would be 32 but is there a difference when we talk about RAM/ROM ?

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There is no difference: on a typical system RAM and ROM share the same address space, and each byte needs to have a unique address, so regardless of whether you have 4 GB of RAM or ROM you will need to have 32 address lanes.

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It depends on the processor, and its RAM interface. Some processors will transmit data to and from RAM 128 bits at a time, for example, and individual bytes are sorted out within the processor. 128 bits = 16 bytes, so you need 4 address lines less (28 instead of 32).

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    Conventional DRAM uses a chip-select, bank, row/column interface. The row/column lines are partially reused, first to select the row and later the column. Furthermore, besides the interface width itself, there is a burst length (8 is standard for DDR3) which can further reduce the number of address lines needed. (On the other hand, each channel has separate addressing lines.) Some interfaces do not use distinct address lines (e.g., Hybrid Memory Cube). Feb 17 '18 at 14:16

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