Memory control signals

Also called: memory control lines, memory control inputs, memory control pins

The inputs that tell a memory chip whether and how to act: chip select (CS), read/write (R/W) or write enable (WE), and often output enable (OE).

Besides its address and data lines, a memory chip has a few control signals:

Chip select (CS): "this access is for you". With CS inactive the chip ignores everything and lets go of the data lines.
Read/write (R/W): chooses the direction. On this site R/W = 1 means read and 0 means write. Some chips use a separate write enable (WE) instead.
Output enable (OE): allows the chip to drive the data lines during a read.

With active-high signals, CS = 1 and R/W = 1 means "read": the chip drives the selected word onto the data lines. CS = 1 and R/W = 0 means "write": the chip stores the word on the data lines. CS = 0 means do nothing, and the outputs go to high impedance.

In equations: Drive = C·R and Store = C·R', with C the chip select and R the read/write line.

In this course the controls are active-high unless a question says otherwise. Real chips usually make them active low, marked with a bar. Then "selected" is the complement of the pin: .

The order matters in a real cycle: set up the address first, then assert CS and the write signal, so the chip never stores into a wrong row while the address is still changing.


0	0	0	0
0	1	0	0
1	0	0	1
1	1	1	0

Worked examples

Example

What does the chip do?

Active-high chip select C and read/write R (1 = read). Read the diagram row by row.

1.
C R = 00 or 01: not selected. No read, no write; outputs at Z.
2.
C R = 10: selected, R = 0 → write. The word on the data lines is stored.
3.
C R = 11: selected, R = 1 → read. The chip drives the stored word onto the data lines.

Example

Active-low version

Now chip select C and write enable W are both active-low. When does the chip write, and when does it read?

1.
Selected means C = 0: . Writing means W = 0: .
2.
Write = : both pins low.
3.
Read = : selected, with write not enabled (W high).

Common mistakes

Reading R/W = 1 as write. On this site, and on most R/W pins, 1 means read; the bar sits over W.
Thinking an unselected chip outputs 0. It disconnects its outputs (high impedance) so other chips can use the bus.
Ignoring active-low markings. With a barred CS, the chip is selected when the pin is 0.

Practice Memory control signals

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Memory lesson full course

And 1 more in the Memory lesson.

Basic CPU / Computer Architecture lesson full course

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Learn it step by step

Memory control signals is taught in Memory and Basic CPU / Computer Architecture.

Memory →Basic CPU / Computer Architecture →

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