I/O Devices

• Two main types
  • Block: random access to data organized in fixed-size blocks
  • Serial/Character: Individual bytes, not addressable, can be one-way
• Data Rates
  • Raw: Total number of physically transferred bits per second
  • Effective data: Total number of physically transferred bits per second, excluding the protocol overhead
  • Burst data rate: Maximum data rate at which a communication channel can deliver data within a short time frame
  • Sustained data rate: The average transfer rate of a device over time

Handling

	CPU	Co-Proc
Memory	MMIO	DMA
Isolated	PMIO	Channels

• Memory Mapped IO (MMIO)
  • IO corresponds to a spot in memory
• Port Mapped IO (PIO)
  • Separate memory spaces for each device
• Direct Memory Access (DMA)
  • CPU tells co-processor to read and write, and the coprocessor handles the modification
  • Prevents the CPU from constantly having to deal with interrupts (the DMA controller handles them instead)
  • Very common
• Channels
  • High performance bus
  • Rarely used

Kernel’s Role

• Programmed IO with Pooling
  • Wait for the device to be ready and then write to it
  • Would need to block that entire time
• Programmed IO with Interrupts
  • Device alerts CPU when it is ready
  • Does not spin wait but still blocks the user until the interrupt is fired
• Direct memory access
  • The controller handles the IO for the CPU
  • The CPU needs to program the DMA which takes a considerable amount of time upfront