I/O Interface (Interrupt and DMA Mode)

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I/O Interface 

(Interrupt and DMA Mode)

Peripheral Devices

• The I/O system provides an efficient mode of communication between the central system and the outside environment.
• Programs and data must be entered into computer memory for processing end results obtained from computations must be displayed for the user. The most familiar means of entering information into a computer is through a type writer-like keyboard. On the other hand, the central processing unit is an extremely fast device capable of performing operations at very high speed.
• To use a computer efficiently, a lot of programs and data must be prepared in advance and transmitted into a storage medium such as magnetic tapes or disks. The information in the disk is then transferred into a high-speed storage, such as disks.
• Input or output devices attached to the computer are called the peripheral devices. The most common peripherals are keyboards, display units, and printers. Peripherals that provide auxiliary storage for the system are magnetic disks and tapes.

Input-Output Interface

• Input-Output interface provides a method for transferring information between internal storage and external I/O devices.
• Peripherals are connected to the central processing unit with a special communication link (I/O bus).
• The I/O bus from the processor is attached to all peripheral interfaces.

Asynchronous Data Transfer

• The two units such as CPU and I/O interface, are designed independently of each other. If the registers in the interface do not have a common clock (global clock) with the CPU registers, then the transfer between the two units is said to be asynchronous.

• The asynchronous data transfer requires the control signals that are being transmitted between the communicating units to indicate the time at which data is being transmitted.

Strobe Control

• Strobe is a pulse signal supplied from one unit to another unit to indicate the time at which data is being transmitted.

• Strobe may be activated by either the source or the destination unit.
• The strobe pulse is controlled by the clock pulses in the CPU. The data bus carries the binary information from the source unit to the destination unit. In source initiated strobe for data transfer, the strobe is a single line that informs the destination unit when a valid data word is available on the bus.
• But in destination initiated for data transfer, it informs the source to provide the data. Then source unit places the data on the data bus.

Handshaking

• The disadvantage of the strobe method is that the source unit has no information whether the destination unit has actually received the data item if the source unit initiates the transfer. But if the destination unit initiates the transfer it has no way of knowing whether the source unit has actually placed the data on the bus. The handshake method solves this problem.
• The basic approach of handshaking is as follows. In handshaking method, there are two control signals, unlike strobe control method. One control signal is in the same direction as the data flow in the bus from the source to the destination. This signal is used to inform the destination unit whether there are valid data on the bus. The second control signal is in the other direction from the destination to the source. It is used to inform the source whether it can accept data.

Synchronous Data Transfer

In synchronous data transfer, a global or shared clock is provided to both sender and receiver. The sender and receiver work simultaneously.

Modes of Transfer

The information from an external device is stored in memory. Information transferred from the central computer into an external device via memory unit. Hence, this data transfer between the central computer and I/O devices is handled in various modes.

1. Programmed I/O
2. Interrupt- initiated I/O
3. Direct Memory Access (DMA)

Programmed I/O: In this mode, each data item is transferred by an instruction in the program. The CPU issues a command then waits for I/O operations to be complete.

As the CPU is faster than the I/O module, the problem with programmed I/O is that the CPU has to wait a long time for the I/O module of concern to be ready for either reception or transmission of data.

Programmed I/O basically works in these ways:

• CPU requests I/O operation
• I/O module performs operation
• I/O module sets status bits
• CPU checks status bits periodically
• I/O module does not inform CPU directly
• I/O module does not interrupt CPU
• CPU may wait or come back later

The CPU, while waiting, must repeatedly check the status of the I/O module, and this process is known as Polling. As a result, the level of the performance of the entire system is severely degraded.

Interrupt initiated I/O: This mode removes the drawback of the programmed I/O mode. The CPU issues commands to the I/O module then proceeds with its normal work until interrupted by I/O device on completion of its work.

For input, the device interrupts the CPU when new data has arrived and is ready to be retrieved by the system processor. The actual actions to perform depend on whether the device uses I/O ports, memory mapping.

For output, the device delivers an interrupt either when it is ready to accept new data or to acknowledge a successful data transfer. Memory-mapped and DMA-capable devices usually generate interrupt to tell the system they are done with the buffer.

Although Interrupt relieves the CPU of having to wait for the devices, but it is still inefficient in the data transfer of large amount because the CPU has to transfer the data word by word between I/O module and memory.

Below are the basic operations of Interrupt:

• CPU issues read command
• I/O module gets data from peripheral whilst CPU does other work
• I/O module interrupts CPU
• CPU requests data
• I/O module transfers data

Direct Memory Access (DMA)

Direct Memory Access (DMA) means CPU grants I/O module authority to read from or write to memory without involvement. DMA module controls an exchange of data between main memory and the I/O device. Because of DMA device can transfer data directly to and from memory, rather than using the CPU as an intermediary, and can thus relieve congestion on the bus. CPU is only involved at the beginning and end of the transfer and interrupted only after entire block has been transferred.

DMA is efficient for moving large amounts of data between I/O devices and main memory. It is considered efficient because it removes the CPU from being responsible for transferring data. DMA instructs the device controller to move data between the devices and main memory.

The CPU initiates the transfer by supplying the interface with the starting address and the number of words needed to be transferred and then proceeds to execute other tasks. When the transfer is made, the DMA requests memory cycles through the memory bus. When the request is granted by the memory controller, the DMA transfers the data directly into memory.

The Bus Request (BR) input is used by the DMA controller to request the CPU to get the control of buses. When this input is active, the CPU terminates the execution of the current instruction and places the address bus and the data bus. The CPU activates the Bus Grant (BG) output to inform the external DMA that the buses are available. The DMA now takes the control of the buses to conduct the memory transfer. When DMA terminates the transfer, it disables the bus request line. The CPU disables the bus grant, takes the control of the buses.

Hardware design is complicated because the DMA controller must be integrated into the system, and the system must allow the DMA controller to be a bus master. Cycle stealing may also be necessary to allow the CPU and DMA controller to share use of the memory bus.

During a block input byte transfer, the following sequence occurs as the data byte is sent from the interface to the memory:

• The interface sends the DMA controller a request for DMA service.
• A Bus request is made to the HOLD pin (active High) on a processor and the controller gains control of the bus.
• A Bus grant is returned to the DMA controller from the Hold Acknowledge (HLDA) pin (active High) on the processor.
• The DMA controller places contents of the address register onto the address bus.
• The controller sends the interface a DMA acknowledgment, which tells the interface to put data on the data bus. (For an output it signals the interface to latch the next data placed on the bus.)
• The data byte is transferred to the memory location indicated by the address bus.
• The interface latches the data.
• The Bus request is dropped, the HOLD pin goes Low, and the controller relinquishes the bus.
• The Bus grant from the processor is dropped and the HLDA pin goes Low.
• The address register is incremented by 1.
• The byte count is decremented by 1.
• If the byte count is non-zero, return to step 1, otherwise stop.

Types of Data transfers: The DMA Controller has several options available for the transfer of data.  

They are:

1. Cycle Steal: A read or write signal is generated by the DMAC, and the I/O device either generates or latches the data. The DMAC effectively steals cycles from the processor in order to transfer the byte, so single byte transfer is also known as cycle stealing.
2. Burst Transfer: To achieve block transfers, some DMAC’s incorporate an automatic sequencing of the value presented on the address bus. A register is used as a byte count, being decremented for each byte transfer, and upon the byte count reaching zero, the DMAC will release the bus. When the DMAC operates in burst mode, the CPU is halted for the duration of the data transfer.
3. Hidden DMA: It is possible to perform hidden DMA, which is transparent to the normal operation of the CPU. In other words, the bus is grabbed by the DMAC when the processor is not using it. The DMAC monitors the execution of the processor, and when it recognizes the processor executing an instruction which has sufficient empty clock cycles to perform a byte transfer, it waits till the processor is decoding the opcode, then grabs the bus during this time. The processor is not slowed down but continues processing normally. Naturally, the data transfer by the DMAC must be completed before the processor starts.

Regards 

Amrut Gupta