Memory read

Also called: read operation, read cycle, memory read cycle, reading memory

The operation that copies the word stored at an address onto the data lines. The stored word is left unchanged.

A memory read fetches the word at a given address. The steps, with active-high controls:

Put the address on the address lines.
Set R/W = 1 (read) and activate CS (and OE, if the chip has it).
Inside, the address decoder activates one word line, and that row's cells are connected to the data outputs.
After the access time, the word is valid on the data lines. Copy it into a register.
Deactivate CS. The chip releases the data lines (back to Z).

The key property: reading doesn't change what's stored. You can read the same address a thousand times and get the same word. (Inside a DRAM the read drains the cell, but the chip immediately writes the value back, so from the outside nothing changes. See destructive read.)

A read from a ROM works the same way, minus the R/W line.

In the diagram, the chip drives the bus while CS is active with R/W = 1, and stops as soon as CS drops.

Worked examples

Example

Reading twice

Word 6 of a RAM holds 0101. The CPU reads address 6, then reads it again.

1.
First read: the data lines show 0101. The word stays stored.
2.
Second read: 0101 again.
3.
Only a write to address 6 could change what the next read returns.

Example

Reading in a multi-chip system

Put the events of a read from a memory built of several chips in order.

1.
The CPU puts the address on the address bus.
2.
The decoder on the high address bits activates one chip select.
3.
The selected chip's address decoder activates one word line inside it.
4.
After the access time, that chip drives the word onto the data bus; the others stay at Z.
5.
The CPU copies the word into a register.

Common mistakes

Thinking a read erases or moves the word. The stored word is unchanged.
Grabbing the data the instant CS goes active. The word is valid only after the access time.
Forgetting R/W. With R/W = 0, the same signals would be a write.

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