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Address Decoder

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A2:
Address Decoder
Hardwired or Software Controlled

Decoders

These are circuits where ...

The input is a binary number.
Each output goes high for only one binary number input.
Only one output is ever high.
The 4017 chip has a decoded output which counts from 0 to 9 and each of its outputs goes high in turn depending on the counter.
Address decoders activate a device when the correct unique address is detected.
Counter chips which reset when a certain number is reached must "decode" the reset number and use the signal to reset the counter.
Multiplexer chips decode the binary address input. So you can have a four to sixteen decoder with a four bit binary input and sixteen outputs where only ever one is active.

Detecting a Unique Address

The parallel printer port in older IBM compatible personal computers is located at address 378 (hexadecimal) 888 (decimal). In binary, this address is 0000 0011 0111 1000. The circuit below decodes this address. That means it responds to this address and to no other. Hexadecimal numbers are often preceded by "0x". So the printer port is located at address 0x0378.

Decoder Circuit

Computer memory locations have addresses.

Computer Input/Output I/O ports have addresses too.

The addresses start at zero and go up to 4 000 000 000 or more. (Some types of computer have negative addresses too.)

Each memory location or I/O port has its own unique address.

Memory Mapping

Memory locations are used for I/O instead of data storage.
Hardware devices can be located at particular memory addresses.
Writing data to the correct address address sends the data to the hardware device.
The processor uses the address bus to select which device or memory location to read or write.
The processor uses a control bus wire to determine if the data goes to a memory location or an I/O port.
The processor uses another control bus wire to determine if data is being written to the address or read from the address.

I/O Mapping

Each I/O port has a port number.
Writing data to the correct port number sends the data to the appropriate I/O device.
The processor uses the address bus to select which I/O device to read or write.
The processor uses a control bus wire to determine if the data goes to a memory location or an I/O port.
The processor uses another control bus wire to determine if data is being written to the port or read from the port.