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I/O Methods

I/O – Transfer of data between memory of the system and the I/O device

Most devices operate asynchronously from the CPU

Most methods involve CPU in performing actual transfer 1. Direct I/O2. Polled/Programmed I/O3. Interrupt Driven I/O4. Direct Memory Access

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Direct I/O

Only suitable for some I/O devices Simple devices – LEDs, simple switches etc. Device always ready to transfer data No status checking required

Example: Simple Port I/O for Microchip PIC

PORTB = PORTB | 0x40;

x = PORTA;

PORTBBits.3 = 1;

6-3

Polled/Programmed I/O

Device indicates readiness to transfer data. Typically the device has a status flag that can be

interrogated by the CPU CPU, under program control, reads (polls) the

status waiting for the device to indicate readiness to transfer data

Wastes CPU cycles (busy-waiting) Simple to understand and implement. CPU in control and performs the actual transfer

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Interrupt Driven I/O

Device has an output signal that is connected to a special signal of the CPU

The device asserts the signal when it is ready to transfer data (Interrupt request)

The CPU may ignore the request – the interrupt is said to be masked)

Some CPUs have a special interrupt that is non-maskable. For very important interrupts such as alarms, power failure etc.

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Interrupt Driven I/O

Interrupt processing is unique to each CPU. Typical features Enabled interrupts checked after each machine instruction. Interrupt requests are usually prioritised Most CPUs use an interrupt vectoring system Each possible interrupt source is allocated a particular

vector number/address The CPU vectors to the address in the table and continues

executing instructions. The CPU returns to executing the interrupted program on

encountering the special machine instruction Return from Interrupt

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Typical Interrupt Processing

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normal program execution

ISR:

store regs.

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restore regs.

Return from Interrupt

Interrupt processing

VECTOR TABLE

# Address Instruction

0 0000H

1 0004H

2 etc

35 008CH JUMP ISR

36 0090H

Interrupt request

Return to interrupted program

NB Interrupting device allocated to vector # 35

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Interrupt Processing (contd.)

Program counter(PC) is automatically pushed on the stack – possibly other regs.

The vector address is generated by the interrupt hardware.

The interrupt service routine (ISR) should preserve any registers that are used in the ISR.

A special return from interrupt instruction should restore the PC and normal program execution should resume.

An ISR may be interrupt by a higher priority request. The interrupt request is usually cleared by the hardware

when it generates the vector.

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Interrupt Processing (contd.)

Potential problems Re-entrancy of functions Sharing of variables – mutual exclusion

Solutions Keep interrupt processing code to a minimum Do not call other functions in the ISR For shared variables use a data type that is accessed

with an indivisible instruction e.g. single bit Disable interrupts during shared variable access.

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Direct Memory Access (DMA)

Requires a DMA controller Data transferred without CPU being involved CPU programs the DMA controller

set transfer direction – input or output Start address in memory Number of bytes to transfer Can copy memory to memory

DMA controller performs the transfer between the memory and the I/O device

DMA controller interrupts CPU on completion

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DMA (Contd.)

DMA signals – bus request, bus grant Operation

DMA request asserted CPU generates DMA bus grant CPU bus accesses suspended (CPU tri-stated) DMA controller performs a memory transfer DMA request negated

Two modes Cycle stealing – DMA and CPU interleave accesses Burst mode

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Summary

Direct I/O – limited usage Polled/Programmed I/O

CPU initiates transfer request and data transferred by CPU between memory and I/O device.

Simple but wastes CPU time May miss input data if data rate is higher than input

sampling frequency. Good for well defined periodic slow devices Poor for input devices and non-periodic device

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Summary (Contd.)

Interrupts I/O device initiates transfer CPU via software still performs the data transfer Good for non-period, asynchronous devices Every interrupt incurs a time overhead Very high interrupt rates can overload the CPU Requires extra programming to set up the interrupt

and interrupt service routine. Can be difficult to test & debug Examples: Keyboard input.

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Summary (Contd.)

DMA Requires additional hardware Good for transferring large blocks of data CPU not involved in transferring data Good for high data rates Examples: Hard Disks, Network card