07_Input_Output

1. 2. 3. 4. 5. 6. 7. 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
I/O Module Diagram
Input Output Techniques
• Programmed • Interrupt driven • Direct Memory Access (DMA)
Programmed I/O
• CPU has direct control over I/O operation
– Module must claim the bus before it can raise interrupt
Example - PC Bus
• 80x86 has one interrupt line (INTR) • 8086 based systems use one 8259A interrupt controller • 8259A has 8 interrupt lines
Control and Timing
The control of the transfer of data from an external device to the processor might involve the following sequence of steps:
– CPU checks I/O module device status – I/O module returns status – If ready, CPU requests data transfer – I/O module gets data from device – I/O module transfers data to CPU
Chapter 7 Input/Output
Input/Output Problems
• Not connected directly to the system bus because
– Wide variety of peripherals with various methods of operation – Often slower than CPU and RAM – Some faster than CPU and RAM – Peripherals often use different formats and words length
CPU Communication
I/O modules must communicate with the CPU and with the external device:
– Command decoding
• I/O module accept command from CPU, sent as signals on the control bus
External device can be classified into 3 categories: • Human readable
– Screen, printer, keyboard
• Machine readable
– Monitoring and control, magnetic disk, tape systems
Intel 82C59A Interrupt Controller
Direct Memory Access
• Interrupt driven and programmed I/O require active CPU intervention
– Transfer rate is limited with which the processor can test and service a device – CPU is tied up in managing an I/O transfer
– Sensing device status – Read/write commands – Transferring data
• CPU waits for I/O module to complete operation • Wastes CPU time
Programmed I/O - detail
External Device Block Diagram
I/O Module Function
The major functions or requirements for an I/O module fall into the following categories:
– Control & Timing – CPU Communication – Device Communication – Data Buffering – Error Detection
– Data
• Data are exchanged between the processor and the I/O module over theting
• Because the peripherals are so slow, it is important to know the status of the I/O module. Ex: BUSY/READY
• Fetch data & store
Interrupt Driven I/O Basic Operation
Multiple Interrupts
• Each interrupt line has a priority • Higher priority lines can interrupt lower priority lines • If bus mastering only current master can interrupt
• CPU carries on with other work • DMA controller deals with transfer • DMA controller sends interrupt when finished
– Address recognition
• I/O module must recognize one unique address for each peripheral it controls
Device Communication
Communication involves:
– Commands – Status information – Data
Data Buffering
• Transfer rate into and out of main memory or the processor is quite high
– Processor to I/O
• Data are buffered in the I/O module • Sent to the peripheral devices at its data rate
Programmed I/O - detail
I/O Commands
• CPU issues address
– Identifies module (& device if >1 per module)
• CPU issues command
– Control – used to activate a peripheral and tell module what to do
• Communication
– Modem – Network Interface Card (NIC)
External Device Block Diagram
• Control signal – Input/read or output/write • Data – Set of bits • Status signals – State of the device (READY/NOT READY) • Control logic – Control the device’s operation • Transducer – Converts data from electrical to other forms of energy
– I/O to Processor
• Data are buffered so as not to tie up the memory in a slow transfer operation
Error Detection
• Error detection
– Must be able to report errors to the processor – E.g. paper jam, bad disk track
• 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
• e.g. spin up disk
– Test - check status
• e.g. power? Error?
– Read/Write
• Module transfers data via buffer from/to device