Composing Memory

Computer Organization & Embedded System — Computer Arithmetic and Memory System, NEC licence examination syllabus (Nepal Engineering Council).

Composing Memory: building the size you actually need

Real memory chips are small. Real systems need much more. Here's how you bridge that gap.

A memory chip might only offer, say, 1K words × 8 bits. But a system may need 4K words × 16 bits. You combine multiple small chips to build the memory size you actually need — in two independent directions.

Increasing word size: connect chips side by side, all sharing the same address lines — each chip contributes some bits to a wider word (e.g., two 8-bit chips → one 16-bit word).

Increasing number of words: stack chips so each handles a different range of addresses — an extra high-order address bit (via a decoder) selects which chip is active for a given address.

💡 The standard numerical: "Design a memory system of size X words × Y bits using chips of size A words × B bits — how many chips do you need, and how are they arranged?" Answer as: (chips for width) × (chips for depth) = total chips. Practice at least 2-3 of these.

Syllabus points

Increasing word size (data width)
Increasing number of words (address space) + decoding
Combined expansion numericals

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