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Compose 1K x 8 ROMs into 4K x 8 ROM

Short answer

Use 4 1K×8 ROMs. A11,A10 feed a 2-to-4 decoder to select one row. A9..A0 connect to all modules in parallel. 8-bit data outputs share a tri-state bus. Result: 4K×8 ROM.

Compose 1K × 8 ROMs into a 4K × 8 ROM.

Given: base module 1K x 8 ROM; required: 4K x 8 ROM. Number of modules = (depth ratio) x (width ratio) = 4 x 1 = 4.

Method

  • Width: the word width is unchanged (8 bits), so no width expansion is needed.
  • Depth: 4 times as many words are needed, so 4 rows of modules are used. The two extra high-order address lines A11 and A10 go to a 2-to-4 decoder whose outputs enable one row at a time. The low-order 10 address lines (A9..A0) go to every module in parallel. The rows share the same data lines (tri-state outputs).
  • Total address lines = 10 + 2 = 12; total data lines = 8.

Block diagram

 Address bits: 12 lines
   A11 A10 --> [2-to-4 decoder] --> row enables E0..E3
   A9..A0 --> to every module (parallel)

 row 0: [M00]  <- E0
 row 1: [M10]  <- E1
 row 2: [M20]  <- E2
 row 3: [M30]  <- E3

 Data: 8 lines (all rows share them)

Address ranges (row selected by the high bits)

A11 A10Row enabledAddress range
00Row 0 (E0)000-3FFH
01Row 1 (E1)400-7FFH
10Row 2 (E2)800-BFFH
11Row 3 (E3)C00-FFFH

Each 1K x 8 ROM has 10 address lines (A9..A0) and 8 data lines.

✓ Answer checked by Claude Opus 5.5 (high effort)

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