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Accessing I/O devices V. Saranya AP/CSESri vidya College of Engg & Tech
• I/O devices accessed through I/O interface.• Requirements for I/O interface: – CPU communication – Device communication – Data buffering – Control and timing – Error detection.
CPU Communication:• Processor sends commands to the I/O system which are generally the control signals on the control bus.• Exchange of data between the processor and the I/O interface over the data bus.• Check whether the devices are ready or not.
Data Buffering:• Data transfer rate is too high .• Data from processor and memory are sent to an I/O interface, buffered and then sent to the peripheral device at its data rate.Error Detection:• I/O interface is responsible for error detection• Used to report errors to the processor.• Types of errors: – Mechanical, electrical malfunctions, bad disk track, unintentional changes.
I/O interface Block diagram Data Register Data Data Lines Status/Control RegisterAddress Lines Status Address Decoder External Device Interface Control Control Lines Logic
• Data Register: holds the data being transferred to or from the processor.• Status/Control Register: contains information relevant to the operation.• Data and status/control registers: are connected to the data bus.• Address decoder: enables the device to recognize its address.
I/O interface for Input Device Address LinesBUS Data Lines Control lines Data & Status Address Decoder Control Circuits Registers I/O interface Input Device
I/O interface for Output Device Address LinesBUS Data Lines Control lines Data & Status Address Decoder Control Circuits Registers I/O interface Output Device
I/O Ports• 4 registers - status, control, data-in, data-out – Status - states whether the current command is completed, byte is available, device has an error, etc – Control - host determines to start a command or change the mode of a device – Data-in - host reads to get input – Data-out - host writes to send output• Size of registers - 1 to 4 bytes 10
I/O devices can be interfaced to a computer system I/O in 2 ways:• Memory Mapped I/O• I/O mapped I/O
Memory-Mapped I/O (1) Memory Address Space I/O Address Space (a) Separate I/O and memory space (b) Memory-mapped I/O (c) Hybrid 12
Memory Mapped I/O• No need of special I/O instructions.• Memory related instructions are used for I/O related operations.
I/O Mapped I/O Memory Address Space I/O address SpaceTotal Address Space
I/O Mapped I/O• If we want to reduce the memory address space, we allot a different I/O address space, apart from total memory space.Memory related instructions do not work hereProcessor use these mode only for I/O Read, I/OWrite.
Difference between Memory Mapped I/O & I/O mapped I/O Memory Mapped I/O I/O Mapped I/OMemory & I/O share the entire address Processor provides separate address range of processor range for memory & I/OProcessor provides more address lines Less address lines for accessing I/O for accessing memory More Decoding is required Less decoding is requiredMemory control signals used to control I/O control signals are used to control Read & Write I/O operations Read & Write I/O operations
• I/O operation means – A data transfer between an I/O device & memory or – Between I/O device & Processor.• If any I/O operations are completely controlled by processor, then the system is said to be using “ Programmed I/O” – Processor has to check I/O system periodically until the operation completes “POLLING” – Microprocessor has to check if any device need service.
Programmed I/O j & Int A? Service & routine AAddressDecoder Int C? j & Service routine C Int Z? j Service routine Z
Priority:The Routines assigns priority to the different I/O devicesPort A is always checked 1st.Then Port BThen Port COrder may change by changing routine.
• When Programmed I/O techniques is used: – Processor fetches I/O related instructions from memory and Issues I/O commands to I/O system to execute the instruction. – Memory Mapped I/O & I/O mapped I/O technique may apply. – Processor has 2 separate instructions IN & OUT for data transfer. – When the I/O instruction is encountered by the processor the I/O port is expected to be ready to response. Processor is usually programmed to test the I/O device status before initiating a data transfer.