1. I/O Organization
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2. Other components called I/O system
I/O Organization
Accessing I/O
The important parts of any computer system are, CPU, Memory & I/O devices (peripherals)
CPU fetches instructions (opcodes & operands) from memory, processes them and stores results in memory
Other components called I/O system
The main function of I/O system to transfer information between CPU or memory and outside world
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3. I/O Organization popo Accessing I/O
I/O devices cannot be connected directly to the system bus
Because
Different I/O with different methods of operation, so it would be impractical to incorporate to control a range of devices
Data transfer rate of peripherals is often much slower than that of memory / CPU, so it would be impractical to use high speed system bus to communicate
Different I/O with different data formats and word length that of CPU used
To overcome all these difficulties, it is necessary to use a module in between system bus and I/O devices called I/O module or I/O system or I/O interface
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4. Single bus structure to connect I/O
I/O Organization
Accessing I/O
Single bus structure to connect I/O
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5. Each I/O device is assigned a unique set of addresses
I/O Organization
Accessing I/O
Single-bus structure
The bus enables all the devices connected to it to exchange information
Typically, the bus consists of three sets of lines used to carry address, data, and control signals
Each I/O device is assigned a unique set of addresses
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6. The data bus lines are used to receive / send data from or to the I/O
I/O Organization
Accessing I/O
The data bus lines are used to receive / send data from or to the I/O
The address bus lines selects the I/O device for data transfer
The control bus lines the direction of data flow
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7. I/O devices are accessed through I/O interface
I/O Organization
Accessing I/O
I/O devices are accessed through I/O interface
The major parts of I/O interface are
1 Control & Timing
2 CPU Communication
3 Device Communication
4 Data Buffering
5 Error Detection
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8. I/O Organization
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9. I/O Organization popo Accessing I/O
Control & Timing  To coordinate the flow of traffic b/w internal resources (memory, system bus) and external devices
CPU Communication involves different types of signal transfers such as
Processor sends commands to the I/O, over control bus
Exchanges of data b/w processor & I/O, over data bus
Data transfer rate of I/O is often slower than the processor. So it is necessary to check the peripheral is ready or not for data transfer. If not processor must wait
No of peripheral devices may connected to the I/O interface. The I/O controls the communication of each peripherals with processor. So it must recognize one unique address of each peripheral
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10. I/O Organization Accessing I/O
Data buffering  Data transfer rate of each peripheral devices are quite high than that of processor & memory
The data coming from memory/processor are sent to I/O interface, buffered in I/O and sent to the peripheral device at its data rate
Data are buffered in I/O interface so as not to tie up the memory in a slow transfer operation.
I/O interface can able to operate at both peripheral & memory speed
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11. Electrical malfunctions Bad disk Transmission error etc
I/O Organization
Accessing I/O
I/O interface is also responsible for error detection and reporting errors to the processor
Type of errors are
Mechanical
Electrical malfunctions
Bad disk
Transmission error etc
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12. I/O Organization popo Accessing I/O
I/O interface consists of data register, status/control register, address decoder and external device interface logic.
Data register holds the data being transferred to or from the processor
Status/control register contains information relevant to the operation of I/O device
Both data register & Status/control register connected to data Bus
Address line drive the address decoder
Address decoder enables the device to recognize its address when address appears on the address lines.
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