???The latter technique is more common and is 
referred to as cycle stealing, because the DMA module in effect steals a bus cycle
Figure 7.7 Typical DMA Block 
Diagram
 When the processor wishes to read or write a block of data, it issues a command to the DMA 
module, by sending to the DMA module the following information:  
Whether a read or write is requested, using the read or write control line between the 
processor and the DMA module.I/O controllers are commonly seen on microcomputers, whereas 
I/O channels are used on mainframe 
Figure 7.2 Block Diagram of an I/O Module 
7.3 Programmed I/O  
With programmed I/O, data are exchanged between the processor and the I/O module.Similarly, if the I/O device operates at a 
rate higher than the memory access rate, then the I/O module performs the needed 
buffering operation 
Finally, an I/O module is often responsible for error detection and for subsequently 
reporting errors to the processor.There are four types of I/O 
commands that an I/O module may receive when it is addressed by a processor:  
o Control: Used to activate a peripheral and tell it what to do.  
o Test: Used to test various status conditions associated with an I/O module and its peripherals.Typically, a buffer is associated with the transducer to temporarily hold data being 
transferred between the I/O module and the external environment                                                              KEYBOARD/MONITOR                                                                                                                                         The basic unit of exchange here is the character : 
?When the processor is executing a program and encounters an instruction relating to I/O, it 
executes that instruction by issuing a command to the appropriate I/O module.o Read: Causes the I/O module to obtain an item of data from the peripheral and place it in an 
internal buffer 
o Write: Causes the I/O module to take an item of data (byte or word) from the data bus and 
subsequently transmit that data item to the peripheral 
Figure 7.3a gives an example of the use of programmed I/O to read in a block of data from a 
peripheral device  into memory.The 
minimum information required is (a) the status of the processor, which is contained in a 
register called the program status word (PSW); and (b) the location of the next instruction to 
be executed,  
processor now loads the program counter with the entry location of the interrupt-handling 
program that will respond to this interrupt.As a result, the next 
instruction to be executed
 Figure 7.5
Figure 7.6 Changes in Memory and Registers for 
an Interrup
 7.5 Direct Memory Access
 When large volumes of data are to be moved, a more efficient technique is required: direct 
memory access (DMA).The number of words to be read or written, again communicated via the data lines and 
stored in the data count register
 Figure 7.8 shows where in the instruction cycle the processor may be suspended.INPUT/OUTPUT 
7.1 External Devices 
I/O operations are accomplished through a wide assortment of external devices that provide a 
means of exchanging data between the external environment and the computer.In a fixed-head disk, the transducer is capable of converting between the magnetic patterns 
on the moving disk surface and bits in the device's buffer (Figure 7.1).This system includes two basic 
components(which not only connects an I/O device with the system bus, but plays a very crucial 
role in between) :  
i- 
ii- 
the I/O devices and  
I/O module,.Command decoding: The I/O module accepts commands from the processor, 
typically sent as signals on the control bus.For example, an I/O module for a disk 
drive might accept the following commands: READ SECTOR, WRITE SECTOR, 
SEEK track number, and SCAN record ID. The latter two commands each include a 
parameter that is sent on the data bus.This communication involves commands, status information, and data ( 
An essential task of an I/O module is data buffering.An I/O module that takes on most of the detailed processing burden, presenting a high-level 
interface to the processor, is usually referred to as an I/O channel or I/O processor.To explain the programmed I/O technique, we view it first from the point of view of the I/O 
commands issued by the processor to the I/O module, and then from the point of view of the I/O 
instructions executed by the processor.I/O COMMANDS                                                                                                                                          To execute an I/O-related instruction, the processor issues an address, specifying the 
particular I/O module and external device, and an I/O command.I/O INSTRUCTIONS                               With programmed I/O, there is a close correspondence between the I/O-related instructions that 
the processor fetches from memory and the I/O commands that the processor issues to an I/O 
module to execute the instructions.Because the address space for I/O is isolated from that 
for memory, this is referred to as isolated I/O
 memory-mapped 
I/O
 isolated I/O
 7.4 Memory-Mapped and Isolated I/O 
7.4 Interrupt-Driven I/O  
The problem with programmed I/O is that the processor has to wait a long time for the I/O 
module of concern to be ready for either reception or transmission of data.When the interrupt from the I/O module occurs, the processor saves the context (e.g., program 
counter and processor registers) of the current program and processes the interrupt.The processor finishes execution of the current instruction before responding to the interrupt 
The processor tests for an interrupt, and sends an acknowledgment signal to the device that 
issued the interrupt.Control signals determine the function that the device will perform, such as send data to 
the I/O module (INPUT or READ), accept data from the I/O module (OUTPUT or 
WRITE), report status, or perform some control function particular to the device (e.g., 
position a disk head).Printable characters are the alphabetic, numeric, and special characters that can be 
printed on paper or displayed on a screen.On output, IRA code characters are transmitted to an external device from the I/O 
module.A moving-head disk must also be able to cause the disk arm to move radially in and out 
across the disk's surface 
7.2   I/O MODULES 
The computer consists of an I/O (input- output) system.Functions of I/O Module: The major functions of an I/O module are:  
Processor communication -- this involves the following tasks: 
?When a byte is received, the I/O module checks the parity to determine whether an 
error has occurred 
7.2.1  I/O module structure 
Figure 7.2 provides a general block diagram of an I/O module.The module must 
also be able to recognize and generate addresses associated with the devices it controls.An I/O 
module that is quite primitive and requires detailed control is usually referred to as an I/O 
controller or device controller.With programmed 
I/O, the I/O module will perform the requested action and then set the appropriate bits in the I/O 
status register .The I/O module takes no further action to alert the processor.The processor treats the status and data registers of I/O modules as memory 
locations and uses the same machine instructions to access both memory and I/O devices.In this case, a user program is interrupted after the 
instruction at location N. The contents of all of the registers plus the address of the next 
instruction are pushed onto the stack.When interrupt processing is complete, the saved register values are retrieved from the stack 
and restored to the registers (e.g., see Figure 7.6b).Figure 7.5 shows a typical sequence.????????3.?1.2.??2.3.?????????