CS251 - Computer Organization and Design - Spring 2008

Lecture 4 - Gates and Transistors


Practical Details


Gates

We can draw them

Make a one bit adder out of NAND, AND and OR

Transistors

How a MOS-FET transistor works

There are two types of MOS-FET transistor

  1. nMOS
  2. pMOS

Problems:

  1. When nMOS or pMOS conduct they draw current: not good because it drains the battery and creates heat

Solution: cMOS

More complex patterns of transistors create more complex patters of gates

From now on, whew, we can leave the transistors to the electrical engineers.

Back to Gates

Multiple gates on a chip is the level of technology we call medium scale integration, MSI.

Here is the sort of thing you could buy in that era

Decoder/Encoder

Example: 2 to 4 decoder.

Where are decoders used

  1. Buses: read/write line
  2. Bus arbitration
  3. DRAM decoding

Multiplexor/Demultiplexor

Example: 2 to 1 multiplexor

Where are multiplexors used

  1. Driving a bus, together with 3-state outputs
  2. Inserting sprites in video games
  3. USARTs

Drawing a bus with a slash

General Logic

ROM

PLA


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