I/O Subsystem

* I/O Subsystem
- Interfaces between a device and the main processor occur in two ways: port mapped and memory mapped.
- I/O devices are classified as either character-mode devices or block-mode devices. The classification refers to how the device handles data transfer with the system.
- I/O devices could be active(issue interrupts periodically or aperiodically) or passive(no interrupts, CPU needs polling for read).
- DMA controllers allows data transfer bypassing the main processor.
- I/O Modes
1) Interrupt-driven I/O (active)
in which an input buffer is filled at device interrupt time(Ready) with DMA and is emptied by processes that read the device probably triggered by DMA complete interrupt; an output buffer is filled by processes that write to the device and is emptied at device interrupt time(Ready) with DMA. This might overload the CPU if the I/O traffic is heavy.
2) Polling in real-time (passive)
Within the regular interrupt of a timer, CPU would poll every I/O.
- Task Assignments for I/O
1) ISR associated tasks for active I/O devices
2) Timer triggered tasks for polling of passive I/O devices
3) Resource control task to control a shared I/O device or a group devices
4) Request dispatcher to multiple tasks from devices
- I/O subsystem associates ISR closely.
- I/O subsystems must be flexible enough to handle a wide range of I/O devices, which hides device peculiarities from applications.
- The I/O subsystem maintains a driver table that associates uniform I/O calls, like create, delete, read, write, ioctrl, etc. with driver-specific I/O routines.
- The I/O subsystem maintains a device table and forms an association between this table and the driver table.