Unit–V Input-Output Organization

1. Define Peripherals. Explain I/O Bus and Interface Modules.

Peripherals:
 Input-output device attached to the computer are also called peripherals.

 A typical communication link between the processor and several peripherals is shown in
figure.
 The I/O bus consists of data lines, address lines, and control lines.
 The magnetic disk, printer, and terminal are employed in practically any general purpose
computer.
 Each peripheral device has associated with it an interface unit.
 Each interface decodes the address and control received from the I/O bus, interprets them
for the peripheral, and provides signals for the peripheral controller.
 It also synchronizes the data flow and supervises the transfer between peripheral and
processor.
 Each peripheral has its own controller that operates the particular electromechanical
device.
 For example, the printer controller controls the paper motion, the print timing, and the
selection of printing characters.
 The I/O bus from the processor is attached to all peripheral interfaces.
 To communicate with a particular device, the processor places a device address on the
address lines.
 Each interface attached to the I/O bus contains an address decoder that monitors the
address lines.
 When the interface detects its own address, it activates the path between the bus lines
and the device that it controls.
 All peripherals whose address does not correspond to the address in the bus are disabled
by their interface selected responds to the function code and proceed to execute it.
 The function code is referred to as an I/O command.
 There are four types of commands that an interface may receive. They are classified as
control, status, data output, and data input.
 A control command is issued to activate the peripheral and to inform it what to do.

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 Unit–V Input-Output Organization

 For example, a magnetic tape unit may be instructed to backspace the tape by one record,
to rewind the tape, or to start the tape moving in the forward direction.
 A status command is used to test various status conditions in the interface and the
peripheral.
 For example, the computer may wish to check the status of the peripheral before a
transfer is initiated.
 During the transfer, one or more errors may occur which are detected by the interface.
 These errors are designated by setting bits in a status register that the processor can read
at certain intervals.
 A data output command causes the interface to respond by transferring data from the bus
into one of its registers.
 The computer starts the tape moving by issuing a control command.
 The processor then monitors the status of the tape by means of a status command.
 When the tape is in the correct position, the processor issues a data output command.
 The interface responds to the address and command and transfers the information from
the data lines in the bus to its buffer register.
 The interface that communicates with the tape controller and sends the data to be stored
on tape.
 The data input command is the opposite of the data output.
 In this case the interface receives an item of data from the peripheral and places it in its
buffer register.
 The processor checks if data are available by means of a status command and then issues a
data input command.
 The interface places the data on the data lines, where they are accepted by the processor.

2. Explain I/O interface with example.[Winter-2016]

 An example of an I/O interface units is shown in block diagram from in figure.
 It consists of two data registers called ports, a control register, a status register, bus buffers,
and timing and control circuit.
 The interface communicates with the CPU through the data bus.
 The chip select and register select inputs determine the address assigned to the interface.
 The I/O read and writes are two control lines that specify an input or output, respectively.
 The four registers communicate directly with the I/O device attached to the interface.
 The I/O data to and from the device can be transferred into either port A or port B.
 If the interface is connected to a printer, it will only output data, and if it services a
character reader, it will only input data.
 A magnetic disk unit transfers data in both directions but not at the same time, so the
interface can use bidirectional lines.
 A command is passed to the I/O device by sending a word to the appropriate interface
register.
 The control register receives control information from the CPU. By loading appropriate bits
into the control register, the interface and the I/O device attached to it can be placed in a
variety of operating modes.
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 Unit–V Input-Output Organization

 For example, port A may be defined as an input port and port B as an output port.
 A magnetic tape unit may be instructed to rewind the tape or to start the tape moving in
the forward direction.
 The bits in the status register are used for status conditions and for recording errors that
may occur during the data transfer.
 For example, a status bit may indicate that port A has received a new data item from the
I/O device.
 Another bit in the status register may indicate that a parity error has occurred during the
transfer.
 The interface registers communicate with the CPU through the bidirectional data bus.
 The address bus selects the interface unit through the chip select and the two register
select inputs.
 A circuit must be provided externally (usually, a decoder) to detect the address assigned to
the interface registers.
 This circuit enables the chip select (CS) input when the interface is selected by the address
bus.

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 Unit–V Input-Output Organization

 The two register select inputs RS1 and RS0 are usually connected to the two least
significant lines of the address bus.
 These two inputs select one of the four registers in the interface as specified in the table
accompanying the diagram.
 The content of the selected register is transfer into the CPU via the data bus when the I/O
read signal is enables.
 The CPU transfers binary information into the selected register via the data bus when the
I/O write input is enabled.

3. What do you mean by Asynchronous data transfer? Explain Strobe control in

detail.[Winter-2016, Summer-2016, Winter-2015]
Asynchronous data transfer
 Data transfer between two independent units, where internal timing in each unit is
independent from the other. Such two units are said to be asynchronous to each other.
Strobe Control
 The Strobe control method of asynchronous data transfer employs a single control line to
time each transfer.
Source-initiated strobe for data transfer
 The strobe may be activated by either the source or the destination unit. Figure shows a
source-initiated transfer.
 The data bus carries the binary information from source unit to the destination unit.
 The strobe is a single line that informs the destination unit when a valid data word is
available in the bus.
 The source unit first places the data on the data bus.
 After a delay to ensure that the data settle to a steady value, the source activates the
strobe pulse.
 The information on the data bus and the strobe signal remain in the active state for a
sufficient time period to allow the destination unit to receive the data.
 The source removes the data from the bus a brief period after it disables its strobe pulse.

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 Unit–V Input-Output Organization

Destination-initiated strobe for data transfer

 Figure shows a data transfer initiated by the destination unit. In this case the destination
unit activates the strobe pulse, informing the source to provide the data.
 The source unit responds by placing the requested binary information on the data bus.
 The data must be valid and remain in the bus long enough for the destination unit to accept
it.
 The falling edge of the strobe pulse can be used again to trigger a destination register.
 The destination unit then disables the strobe. The source removes the data from the bus
after a predetermined time interval.
 The transfer of data between the CPU and an interface unit is similar to the strobe transfer
just described.
Disadvantage of Strobe method
 The disadvantage of the strobe method is that the source unit that initiates the transfer has
no way of knowing whether the destination unit has actually received the data item that
was placed in the bus
 Similarly, a destination unit that initiates the transfer has no way of knowing whether the
source unit has actually placed the data on the bus.

4. Explain Asynchronous data transfer with Handshaking method.[Winter-2016]

 The handshake method solves the problem of Strobe method by introducing a second
control signal that provides a reply to the unit that initiates the transfer.
Source-initiated transfer using handshaking
 One control line is in the same direction as the data flow in the bus from the source to the
destination.
 It is used by the source unit to inform the destination unit whether there are valid data in
the bus.

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 Unit–V Input-Output Organization

 The other control line is in the other direction from the destination to the source.
 It is used by the destination unit to inform the source whether it can accept data.
 The sequence of control during the transfer depends on the unit that initiates the transfer.
 Figure shows the data transfer procedure initiated by the source.
 The two handshaking lines the data valid, which is generated by the source unit, and data
accepted, generated by the destination unit, the timing diagram shows the exchange of
signals between the two units.
 The sequence of events listed in figure shows the four possible states that the system can
be at any given time.
 The source unit initiates the transfer by placing the data on the bus and enabling its data
valid signal.
 The data accepted signal is activated by the destination unit after it accepts the data from
the bus.
 The source unit then disables its data valid signal, which invalidates the data on the bus.
 The destination unit then disables its data accepted signal and the system goes into its
initial state.
 The source does not send the next data item until after the destination unit shows its
readiness to accept new data by disabling its data accepted signal.
 This scheme allows arbitrary delays from one state to the next and permits each unit to
respond at its own data transfer rate.

Destination-initiated transfer using handshaking

 The destination-initiated transfer using handshaking lines is shown in figure.

 Note that the name of the signal generated by the destination unit has been changed to
ready for data to reflect its new meaning.
 The source unit in this case does not place data on the bus until after it receives the ready
for data signal from the destination unit.
 From there on, the handshaking procedure follows the same pattern as in the source
initiated case.

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 Unit–V Input-Output Organization

 Note that the sequence of events in both cases would be identical if we consider the ready
for data signal as the complement of data accepted.
 In fact, the only difference between the source-initiated and the destination-initiated
transfer is in their choice of initial state.

5. Explain in detail Asynchronous serial transfer OR

Explain serial asynchronous data transmission technique.[Winter-2016, Summer-
2015, Summer-2014]
 The transfer of data between two units may be done in serial or parallel.
 Four different types of data transfer:
o Asynchronous serial transfer
o Synchronous serial transfer
o Asynchronous parallel transfer
o Synchronous parallel transfer
 In serial data trans-mission, each bit in the message is sent in sequence one at a time. This
method requires the use of one pair of conductors or one conductor and a common ground.
 Serial transmission is slower but is less expensive since it requires only one pair of
conductors.
Asynchronous serial transfer
 In asynchronous transmission, binary information is sent only when it is available and the
line remains idle when there is no information to be transmitted.
 This is in contrast to synchronous transmission, where bits must be transmitted
continuously to keep the clock frequency in both units synchronized with each other.
 A serial asynchronous data transmission technique used in many interactive terminals
employs special bits that are inserted at both ends of the character code.
 With this technique, each character consists of three parts: a start bit, the character bits,
and stop bits.
 The convention is that the transmitter rests at the 1-state when no characters are
transmitted.
 The first bit, called the start bit, is always a 0 and is used to indicate the beginning of a
character. The last bit called the stop bit is always a 1.
 A transmitted character can be detected by the receiver from knowledge of the
transmission rules :
 When a character is not being sent, the line is kept in the 1-state.
 The initiation of a character transmission is detected from the start bit,
which is always 0.
 The character bits always follow the start bit.
 After the last bit of the character is transmitted, a stop bit is detected when
the line returns to the 1-state for at least one bit time.
 Using these rules, the receiver can detect the start bit when the line goes from 1 to 0.
 The receiver knows the transfer rate of the bits and the number of character bits to accept.
 After the character bits are transmitted, one or two stop bits are sent.

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 Unit–V Input-Output Organization

 The stop bits are always in the 1-state and frame the end of the character to signify the
idle or wait state.
 At the end of the character the line is held at the 1-state for a period of at least one or two
bit times so that both the transmitter and receiver can resynchronize.
 The length of time that the line stays in this state depends on the amount of time required
for the equipment to resynchronize.
Synchronous Serial Transmission :
 In synchronous transmission, the two units share a common clock frequency and bits are
transmitted continuously at the rate dictated by the clock pulses.
 In long distant serial transmission, each unit is driven by a separate clock of the same
frequency.
 Synchronization signals are transmitted periodically between the two units to keep their
clocks in step with each other.

6. Explain Programmed I/O with example. OR

Explain various modes of data transfer between central computer and
peripherals.[Winter-2016, Summer-2016, winter-2015]

 In the programmed I/O method, the I/O device does not have direct access to memory.
 An example of data transfer from an I/O device through an interface into the CPU is shown
in figure.
 When a byte of data is available, the device places it in the I/O bus and enables its data
valid line.
 The interface accepts the byte into its data register and enables the data accepted line.
 The interface sets a bit in the status register that we will refer to as an F or "flag" bit.
 The device can now disables the data valid line, but it will not transfer another byte until
the data accepted line is disables by the interface.
 A program is written for the computer to check the flag in the status register to determine
if a byte has been placed in the data register by the I/O device.
 This is done by reading the status register into a CPU register and checking the value of the
flag bit.
 Once the flag is cleared, the interface disables the data accepted line and the device can
then transfer the next data byte.

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 Unit–V Input-Output Organization

Example of Programmed I/O:

 A flowchart of the program that must be written for the CPU is shown in figure.
 It is assumed that the device is sending a sequence of bytes that must be stored in memory.
 The transfer of each byte requires three instructions :
1. Read the status register.
2. Check the status of the flag bit and branch to step 1 if not set or to step 3 if set.
3. Read the data register.
 Each byte is read into a CPU register and then transferred to memory with a store
instruction.
 A common I/O programming task is to transfer a block of words from an I/O device and
store them in a memory buffer.

7. Explain in detail Interrupt Initiated I/O OR

Explain various modes of data transfer between central computer and
peripherals.[Winter-2016, Summer-2016, winter-2015]
 In programmed initiated, CPU stays in a program loop until the I/O unit indicates that it is
ready for data transfer.
 This is a time consuming process since it keeps the processor busy needlessly.
 It can be avoided by using an interrupt facility and a special command to inform the
interface to issue an interrupt request signal when data are available from the device.

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 Unit–V Input-Output Organization

 In the meantime CPU can proceed to execute another program.

 The interface meanwhile keeps monitoring the device.
 When the interface determines that the device is ready for data transfer, it generates an
interrupt request to the computer.
 While the CPU is running a program, it does not check the flag. However, when the flag is
set, the computer is momentarily interrupted from proceeding with the current program
and is informed of the fact that the flag has been set.
 The CPU deviates from what it is doing to take care of the input or output transfer.
 After the transfer is completed, the computer returns to the previous program to continue
what it was doing before the interrupt.
 The CPU responds to the interrupt signal by storing the return address from the program
counter into a memory stack and then control branches to a service routine that processes
the required I/O transfer.
 The way that the processor chooses the branch address of the service routine varies from
one unit to another.
 In non-vectored interrupt, branch address is assigned to a fixed location in memory.
 In a vectored interrupt, the source that interrupts supplies the branch information to the
computer. The information is called vector interrupt.
 In some computers the interrupt vector is the first address of the I/O service routine.
 In other computers the interrupt vector is an address that points to a location in memory
where the beginning address of the I/O service routine is stored.

8. Write a detailed note on Direct Memory Access (DMA)[winter-2016]

Direct Memory Access
 Transfer of data under programmed I/O is between CPU and peripheral.
 In direct memory access (DMA), Interface transfers data into and out of memory through
the memory bus.
 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, DMA transfers the data directly
into memory.
DMA controller
 DMA controller - Interface which allows I/O transfer directly between Memory and Device,
freeing CPU for other tasks
 CPU initializes DMA Controller by sending memory address and the block size (number of
words).

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 Unit–V Input-Output Organization

Block Diagram of DMA Controller

 The DMA controller needs the usual circuits of an interface to communicate with the CPU
and I/O device.
 In addition, it needs an address register, a word count register, and a set of address lines.
 The address register and address lines are used for direct communication with the
memory.
 The word count register specifies the number of words that must be transferred.
 The data transfer may be done directly between the device and memory under control of
the DMA.
 Figure shows the block diagram of a typical DMA controller.
 The unit communicates with the CPU via the data bus and control lines.
 The register in the DMA are selected by the CPU through the address bus by enabling the
DS (DMA select) and RS (register select) inputs.
 The RD (read) and WR (write) inputs are bidirectional.
 When the BG (bus grant) input is 0, the CPU can communicate with the DMA registers
through the data bus to read from or write to the DMA registers.
 When BG= 1, the CPU has relinquished the buses and the DMA can communicate directly
with the memory by specifying an address in the address but and activating the RD or WR
control.
 The DMA communicates with the external peripheral through the request and
acknowledge lines by using a prescribed handshaking procedure
.

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 Unit–V Input-Output Organization

 The DMA controller has three registers: an address register, a word count register, and a
control register.
 The address register contains an address to specify the desired location in memory.
 The word count register holds the number of words to be transferred.
 This register is decremented by one after each word transfer and internally tested for zero.
 The control register specifies the mode of transfer.
 All registers in the DMA appear to the CPU as I/O interface registers.
 Thus the CPU can read from or write into the DMA register under program control via the
data bus.
 The DMA is first initialized by the CPU.
 After that, the DMA starts and continues to transfer data between memory and peripheral
unit until an entire block is transferred.
 The CPU initializes the DMA by sending the following information through the data bus
1. The staring address of the memory block where data are available (for read) or where
data are to be stored (for write)
2. The word count, which is the number of words in the memory block.
3. Control to specify the mode of transfer such as read or write.
4. The starting address is stored in the address register.

9. Explain daisy chain priority interrupt system.[winter-2016]

 Determines which interrupt is to be served first when two or more requests are made
simultaneously
 Also determines which interrupts are permitted to interrupt the computer while another is
being serviced
 Higher priority interrupts can make requests while servicing a lower priority interrupt.
Daisy Chaining Priority

 The daisy-chaining method of establishing priority consists of a serial connection of all

devices that request an interrupt.
 The device with the highest priority is placed in the first position, followed by lower
priority devices up to the device with the lowest priority, which is placed last in the chain.
 This method of connection between three devices and the CPU is shown in figure
 If any device has its interrupt signal in the low-level state, the interrupt line goes to the
low-level state and enables the interrupt input in the CPU.
 When no interrupts are pending, the interrupt line stays in the high-level state and no
interrupts are recognized by the CPU.
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 Unit–V Input-Output Organization

 The CPU responds to an interrupt request by enabling the interrupt acknowledge line.
 This signal passes on to the next device through the PO (priority out) output only if device
1 is not requesting an interrupt.
 If device 1 has a pending interrupt, it blocks the acknowledge signal from the next device
by placing a 0 in the PO output.
 It then proceeds to insert its own interrupt vector address (VAD) into the data bus for the
CPU to use during the interrupt cycle.
 A device with a 0 in its Pl input generates a 0 in its PO output to inform the next-lower
priority device that the acknowledge signal has been blocked.
 A device that is requesting an interrupt and has a 1 in its Pl input will intercept the
acknowledge signal by placing a 0 in its PO output.
 If the device does not have pending interrupts, it transmits the acknowledge signal to the
next device by placing a 1 in its PO output.
 Thus the device with Pl = 1 and PO = 0 is the one with the highest priority that is requesting
an interrupt, and this device places its VAD on the data bus.
 The daisy chain arrangement gives the highest priority to the device that receives the
interrupt acknowledge signal from the CPU.
 The farther the device is from the first position; the lower is its priority.

10. Explain Input- Output Processor (IOP)

 IOP is similar to a CPU except that it is designed to handle the details of I/O processing.
 Unlike the DMA controller that must be setup entirely by the CPU, the IOP can fetch and
execute its own instruction.
 IOP instructions are specifically designed to facilitate I/O transfers.
 In addition, IOP can perform other processing tasks, such as arithmetic, logic branching,
and code translation.
 The block diagram of a computer with two processors is shown in figure 8.12.
 The memory unit occupies central position and can communicate with each processor by
means of direct memory access.
 The CPU is responsible for processing data needed in the solution of computational tasks.
 The IOP provides a path of for transfer of data between various peripheral devices and
memory unit.
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 Unit–V Input-Output Organization

 The CPU is usually assigned the task of initiating the I/O program.
 From then, IOP operates independent of the CPU and continues to transfer data from
external devices and memory.
 The data formats of peripheral devices differ from memory and CPU data formats. The IOP
must structure data words from many different sources. For example, it may be necessary
to take four bytes from an input device and pack them into one 32-bit word before the
transfer to memory.
 Data are gathered in the IOP at the device rate and bit capacity while the CPU is executing
its own program.
 After the input data are assembled into a memory word, they are transferred from IOP
directly into memory by "stealing" one memory cycle from the CPU.
 Similarly, an output word transferred from memory to the IOP is directed from the IOP to
the output word transferred from memory to the IOP.
 In most computer systems, the CPU is the master while the IOP is a slave processor.
 The CPU is assigned the task of initiating all operations, but I/O instructions are executed in
the IOP.
 CPU instructions provide operations to start an I/O transfer and also to test I/O status
conditions needed for making decisions on various I/O activities.
 The IOP, in turn, typically asks for CPU attention by means of an interrupt.
 Instructions that are read from memory by an IOP are sometimes called commands, to
distinguish them from instructions that are read by the CPU.

11. Explain CPU-IOP Communication.[Winter-2016, Summer-2016, Summer-2015,

Summer-2014]
 The communication between CPU and IOP may take different forms, depending on the
particular computer considered.
 In most cases the memory unit acts.
 The sequence of operations may be carried out as shown in the flowchart of figure.
 The CPU sends an instruction to test the IOP path.
 The IOP responds by inserting a status word in memory for the CPU to check.
 The bits of the status word indicate the condition of the IOP and I/O device, such as IOP
overload condition, device busy with another transfer, or device ready for I/O transfer.
 The CPU refers to the status word in memory to device what do next.
 If all is in order, the CPU sends the instruction to start I/O transfer.
 The memory address received with this instruction tells the IOP where to find its program.
 The CPU can now continue with another program while the IOP is busy with the I/O
program.
 Both programs refer to memory by means of DMA transfer.
 When the IOP terminates the execution of its program, it sends an interrupt request to the
CPU.
 The CPU responds to the interrupt by issuing an instruction to read the status from the
IOP.

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 Unit–V Input-Output Organization

 The IOP responds by placing the contents of its status report into a specified memory
location.
 The status word indicates whether the transfer has been completed or if any errors
occurred during the transfer.
 From inspection of the bits in the status word, the CPU determines if the I/O operation
was completed satisfactorily without errors.
 The IOP takes care of all data transfers between several I/O units and the memory while
the CPU is processing another program.
 The IOP and CPU are competing for the use of memory, so the number of devices that can
be in operation is limited by the access time of the memory.

12. Explain Crossbar Switch Interconnection Structures.[Winter-2016]

 Figure shows a crossbar switch interconnection between four CPUs and four memory
modules.
 The small square in each cross point is a switch that determines the path from a processor
to a memory module.
 Each switch point has control logic to set up the transfer path between a processor and
memory.
 It examines the address that is placed in the bus to determine whether its particular
module is being addressed.
 It also resolves multiple requests for access to the same memory module on a
predetermined priority basis, figure shows the functional design of a crossbar switch
connected to one memory module.
 The circuit consists of multiplexers that select the data, address, and control from one CPU
for communication with the memory module.
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 Unit–V Input-Output Organization

 Priority levels are established by the arbitration logic to select one CPU when two or more
CPUs attempt to access the same memory.

 The multiplex are controlled with the binary code that is generated by a priority encoder
with in the arbitration logic.
 A crossbar switch organization supports simultaneous transfers from memory modules
because there is a separate path associated with each module.
 However, the hardware required to implement the switch can becomes quite large and
complex.

16 Dept: CE COA(3340705) Prof. Chintan N. Kanani