Commands to I/O Devices
Giving Commands to I/O Devices
°Two methods are used to address the device:
• Special I/O instructions
• Memory-mapped I/O
°Special I/O instructions specify:
• Both the device number and the command word
- Device number: the processor communicates this via a
set of wires normally included as part of the I/O bus
- Command word: this is usually send on the bus’s data lines
°Memory-mapped I/O:
• Portions of the address space are assigned to I/O device
• Read and writes to those addresses are interpreted
as commands to the I/O devices
• User programs are prevented from issuing I/O operations directly:
- The I/O address space is protected by the address translation
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