Input and Output
Input and Output
An interface to one or more external devices through data links, and an interface betwee nthe CPU and memory via the system bus.
| Requirement | Description |
|---|---|
| Asynchronous timing | Handles timing differences between the device and the system. |
| Command decoding | Interprets commands sent from the system to the device. |
| Data exchange | Manages the transfer of data between the system and the device. |
| Status reporting | Provides information about the device's current state. |
| Address recognition | Identifies the correct device for communication. |
| Data buffering | Temporarily stores data to handle speed mismatches between components. |
| Error detection and correction | Identifies and resolves errors during data transmission. |
The CPU operates I/O devices by reading/writing from/to the devices’ status/control/data registers. The I/O device registers are mapped in two ways:
- Memory-mapped: mapped into main memory address space, accessed as if memory locations.
- Port-mapped: mapped into separate address space, accessed using special I/O instructions.
I/O Techniques
The CPU starts by sends a command to the I/O devices. There are three types of I/O techniques:
The CPU executees a program to control I/O devices, and actively waits for the device to complete, wasting CPU time.
The CPU will carry on with other tasks. When the I/O module finishes, it sends an interrupt signal to the CPU. The CPU will then handle the interrupt and return to its tasks.
The CPU sends a interrupt acknowledge signal (INTA) to the I/O module to indicate receipt of the interrupt signal. The last instruction of the interrupt routine is a return from interrupt instruction (RETI).
Use Input-Output Processor (IOP) that steals cycles from CPU. CPU sees elongated cycle and wait until cycle is over. Interruptions handled within one instruction cycle.
The “interruption” in DMA is within one instruction execution cycle, while the interruption in Interrupt-Driven I/O is after the nstruction execution cycle.
