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Device Management

COMP124 Lectures

Devices

Peripheral devices connect to ports on the computer.

Data and commands to/from the devices may travel along a shared set of wires called a bus:

flowchart TB
    CPU <--> Bus
    1[Device] <--> Bus
    2[Device] <--> Bus
    3[Device] <--> Bus
  • Devices ignore messages not intended for them.
  • Problem of bus contention.

Communication

Devices usually have several registers:

  • Status register
    • Indicated busy/ready.
  • Command/Control register
    • To pass commands to device.
  • Data register
    • To send/receive data.

CPU may have special I/O instructions to alter/inspect device registers:

  • Very specialised applications.

Often registers are mapped onto memory locations:

  • Writing to location 100 might send a command to a device.
  • Makes I/O easier because we treat it just the same as writing to memory.
  • Need to protect these memory locations from malicious use or corruption.

I/O Handling

flowchart TB
    subgraph Software
    Application <--> Kernel
    subgraph Kernel
    dd[Device Driver]
    end
    end
    subgraph Hardware
    dd <--> dc[Device Controller]
    dc <--> Device
    end
  • The kernel talks to the device driver as a result of system calls.
  • The device driver translates high-level abstract system calls into signals that can be recognised by the device.

Application I/O Interface

I/O devices can be categorised by their behaviours into generic classes:

  • Each general type is accessed through an interface, which is a standard set of functions (though system calls may vary between OSs).

Device driver layer hides differences among I/O controllers from the kernel.

Devices vary on many dimensions:

  • Character-stream vs. block.
  • Sequential vs. random access.
  • Sharable vs. dedicated.
    • Multiple vs. single process.
  • Speed of operation.
  • Read-write, read only, or write only.

Device Handling

Device driver converts system calls to low-level commands to control the device.

  • It may have to be installed for a new device.
  • Treated as privileged code by the OS for kernel integration.
  • Poorly written drivers may lead to system instability or security issues.

Device controller converts commands to electronic signals operating the hardware.

Application interface:

  • At the application level, we want to treat devices as if they were files/streams.
  • Accessing a special file activates the device driver.
  • System call ioctl() can be used to pass arbitrary commands to the device driver.
    • This bypasses its file.

Example - Unix Terminal

flowchart LR
	Process <-->|read/write| OS
	subgraph OS
	t[Terminal Device Driver]
	end
	t --> oq[Output Queue]
	oq --> Terminal
	Terminal --> iq
	iq[Input Queue] --> t
  • Characters typed at keyboard are entered into input queue by device driver.
  • To echo, driver copies input queue to output queue.
  • Some characters require further processing by device driver:
    • Backspace removes an item from the input queue.
  • When read request is made, pass contents of input queue to process.