- Input-Output interface provides a method for transferring information between internal storage (such as memory and CPU registers) and external I/O devices.
- Peripherals connected to a computer need special communication links for interfacing them with the central processing unit.
- The communication link resolves the following differences between the computer and peripheral devices.
o Devices and signals
Peripherals - Electromechanical Devices
CPU or Memory - Electronic Device
o Data Transfer Rate
CPU or Memory- Usually faster than peripherals
Some kinds of Synchronization mechanism may be needed
o Unit of Information
CPU or Memory - Word
o Operating Modes
Peripherals - Autonomous, Asynchronous
CPU or Memory – Synchronous
- To resolve these differences, computer systems include special hardware components (Interfaces) between the CPU and peripherals to supervise and synchronize all input and output interfaces.
I/O Bus and Interface Modules
- The I/O bus consists of data lines, address lines and control lines.
Fig: Connection of I/O bus to input-output devices
- Interface performs the following:
- Decodes the device address (device code)
- Decodes the commands (operation)
- Provides signals for the peripheral controller
- Synchronizes the data flow and supervises the transfer rate between peripheral and CPU or Memory
- I/O commands that the interface may receive:
- Control command: issued to activate the peripheral and to inform it what to do.
- Status command: used to test various status conditions in the interface and the peripheral.
- Output data: causes the interface to respond by transferring data from the bus into one of its registers.
- Input data: is the opposite of the data output.
I/O versus Memory Bus
- Computer buses can be used to communicate with memory and I/O in three ways:
- Use two separate buses, one for memory and other for I/O. In this method, all data, address and control lines would be separate for memory and I/O.
- Use one common bus for both memory and I/O but have separate control lines. There is a separate read and write lines; I/O read and I/O write for I/O and memory read and memory write for memory.
- Use a common bus for memory and I/O with common control line. This I/O configuration is called memory mapped.
Isolated I/O versus Memory Mapped I/O
- Isolated I/O
- Separate I/O read/write control lines in addition to memory read/write control lines
- Separate (isolated) memory and I/O address spaces
- Distinct input and output instructions
- A single set of read/write control lines (no distinction between memory and I/O transfer)
- Memory and I/O addresses share the common address space which reduces memory address range available
- No specific input or output instruction so the same memory reference instructions can be used for I/O transfers
- Considerable flexibility in handling I/O operations
Example of I/O Interface
- Information in each port can be assigned a meaning depending on the mode of operation of the I/O device
- Port A = Data; Port B = Command; Port C = Status
- CPU initializes (loads) each port by transferring a byte to the Control Register
- Allows CPU can define the mode of operation of each port
- Programmable Port: By changing the bits in the control register, it is possible to change the interface characteristics