I/O Example: Disk Drives

• To access data:— seek: position head over the proper track (8 to 20 ms.

avg.)— rotational latency: wait for desired sector (.5 / RPM)— transfer: grab the data (one or more sectors) 2 to 15

MB/sec

Platter

Track

Platters

Sectors

Tracks

How CPU talk to IO device?I/O hardware must contains some logical circuit to

communicate with CPU• Typically I/O hardware have some internal memory, called

I/O registers– Those I/O registers can be accessed though some addresses

“burned into” the device in prior.– CPU can read/write those addresses– CPU treat them as memory addresses

• I/O registers can be– Control register: when CPU write a particular value to this

address, a particular I/O function will be done– Status register: CPU can read this address to see the current status

of the device.– Data register: contain data CPU want from the I/O device.

A typical I/O procedure to read one byte from a I/O device

• CPU send a command to the control register of a I/O device

• CPU keep reading the status register until data is ready.

• CPU read the data register

How CPU Interact with I/O Dev

• For a read, • CPU supplies address

and transaction type (read)– Yes, each I/O device has

an address, like memory slots.

• Device places data on data bus and indicates data ready (how?).

• Read serial port (RS-232) outport(0x2fb, 1); While (1){ status = inport (0x2fc); if (status==1){ y = inport (0x2fd);

break;

}

}

• Memory-Mapped I/O– Device is controlled by writing

certain commands to the device controller’s register

– The address for Device controller’s register overlapped with main memory

• MIPS I/O is done through lw and sw instruction.

• X86: The I/O instructions IN and OUT move data between I/O ports and CPU register

• Some CPUs only execute IO instructions in "supervisor mode".

Internal register

Main mem

IO Dev 1

Internal register

IO Dev 2

What is BUS

A bus is a shared communication linkIncluding data, address and control

A bus transaction includes two parts:• Sending the address and requested action

• Receiving or sending the data (blocks)

Mainmemory

I/Ocontroller

I/Ocontroller

I/Ocontroller

Disk Graphicsoutput

Network

Memory– I/O bus

Processor

Cache

Interrupts

Disk

Bus Adapter• The I/O code showed early is not efficient• User bus adapter to relieve CPU from low IO

operations.– Usually a simple chip that execute desired bus

transaction

• Many bus adapter are connected to a common physical bus

°the one start bus trans is called the bus master • The one response to the bus req is called the bus slave• Most IO controllers, your desktop motherboard contains

many such chips, can act as both master and slave

• A bus transaction still must be kicked off by your program– Typically several IO instructions

Type of IO Bus• Processor-Memory Bus, Connects directly to the processor

– Short and high speed

– Only need to match the memory system

• I/O Bus

– Usually is lengthy and slower,

– Need to match a wide range of I/O devices

– Connects to the processor-memory bus

– many subtypes: ISA, PCI, USB..

• Differ in the number of data lines

• Backplane Bus:

– Allow processors, memory, and I/O devices to coexist

• Arbitration: Support multiple device of same bus protocols• E.g., if the system has two network interface cards, how to determine which one

can use the shared bus?

Typical Desktop Bus

Support Efficient IO

— pollingalways work, IO controller is the most simply

— interrupts

IO controller must be able to raise a line when

data is ready

— DMA

Can initiate a bus transaction to transfer data

between the device and other components

(MEM or other IO device)

INTERRUPT I/OSteps involved in interrupt I/O• The external device, when ready and needs CPU service

gives an interrupt request signal to the CPU. • The CPU completes the current instruction cycle and

sends an interrupt acknowledgement signal back to the I/O device.

• The device provides an interrupt vector number to the CPU so that the processor generates the starting address of the ISR.

• The CPU saves PC, status register, and data registers on the stack and loads PC with the starting address of the ISR.

• The CPU completes the execution of the ISR, restores register values, and returns back to the main program

Steps involved in DMA

• The I/O device gives a DMA request to the CPU through the DMA controller.

• The CPU completes the current microinstruction cycle.

• The CPU loads the DMA controller with the starting address of the block of data that needs to be transferred, the size of the block, and the address of the device that has requested the transfer.

• The CPU then isolates from the system and sends a DMA acknowledge signal back to the DMA controller to let it know that the DMA controller can take control over the system buses.

DMA I/O

Example: what happens when you fwrite(fd, buf, 4096k):

• Your code: call the driver and move on

• the disk driver (upon interrupt)

• Calculate the phy disk block

• Write disk block # to IDE controller

• Write buf addr to the IDE controller

• Write a start command to IDE controller

• IDE controller:

• read one block from memory and write to disk (DMA)

• When done, raise interrupt and trigger the disk driver