Expansion Slots

DefinitionThe I/O bus or Expansion slots enables

CPU to communicate with peripheral devices.

The I/O bus is used to add many devices to computer to expand its capabilities

The expansion cards are connected to the motherboard through data, address, and control lines / buses on the expansion slots

They are thin long connectors on the motherboard.

Types of I/O BusesClassified on the basis of

Number of bits they transfer at a timeSpeedBus Architecture

BUS Layout

BUS ControllerChips Slotted I/O

Built inI/O

CPU

RAM

External Cache

I/O Bus

Slow speed

I/O Bus

Slow Speed

Processor Bus( High speed)

Memory bus

( High Speed)

Categories of Expansion SlotsBased on width and the technology:8bit ISA(Industry Standard Architecture)16 bit ISAMCA (Micro Channel Architecture)EISA (Extended ISA)VESA – Video Electronics Standard

AdaptersPCI –Peripheral Component InterconnectAGP – Accelerated Graphics PortPCI Express

8 Bit ISAIt is called classic AT busThe connection contains 8 bit data line,20 bit

address line and power and Control linesIt can handle 1 MB of memory. It supports

connections for 6 interrupts (IRQ2 – IRQ7 ) and 3 DMA channels ( DMA0 – DMA2)

This bus runs at a system speed of 4.77 MHzTotally 62 contacts at the bottom of adapter card

– 31 contacts on both sides of the cardThe main disadvantage is , it requires many

jumpers and DIP switch settings to connect a new device

8 Bit ISA

16 bit ISA

16 bit ISASince 80286 have a 16 bit data line, AT

motherboards have 16 bit ISA busIBM added 36 new connector slots ( 18

on each side) to the existing 8 bit ISA slots – makes the 8 bit ISA card compatible with 16 bit ISA slots

5 interrupts and 4 DMA channels were included. 4 more address lines are provided and several more control signals are added.

Limitations Of ISA bus standardOnly 16 data lines , limiting data transfer to 2

bytes at a timeOnly 8.33 MHz max clock speedNo sharing of interrupts or DMA channels

between cards in different slotsNo provision for being disabled by the system

in case of resource conflictPeak transfer rate of 5 MB/sMany jumper settings and DIP switches are

required when a new device is added

MCAIntroduced by IBM for their PS/2

machinesMCA is not compatible with existing ISA

– requires a completely new expansion card.

MCA is a proprietary system by IBM- hence expensive

Two technologies were introduced by MCA: Auto Configure and Bus Mastering

Auto configureThis allows the user to connect any

device to MCA bus without worrying about IRQ, Interrupt, DMA channel setup problems

Devices connected to these buses configure automatically – Plug-N-Play

Bus MasteringIt allows an expansion card to directly transfer

data to and from other bus mastered peripheral controller without the need to pass through CPU

It allows peripheral devices to take control of the bus from the CPU for a short time and transfer data from peripheral device directly to memory

This frees the CPU to perform other tasks making the system efficient

This allows the device to transmit or receive large blocks of data in a short burst mode

EISACreated by a group of 9 computers

manufacturers in response to IBM’s MCA bus design

It is a 32 bit bus technology compatible with 8 bit, 16 bit ISA adapters

It allows Auto configure and bus mastering options

Local BusA local bus slot allows the device

connected to it to communicate with the CPU at a speed the CPU is capable of.

They are:VESA Local Bus/ VL busPCI Local bus

VESA Local Bus/VL busVideo Electronics Standard Association – an

association of video adapters and monitor manufacturers to standardize PC video specifications

It connects toVideo adapterHDD adapterThese 2 devices require very fast access by

the CPU

Specifications of VESA Local busMax Speed limit – 33 MHzTransfer of data rate – 32 bit data

at a timeThroughput – 130 MB/s which is 16

times faster than ISAAt a time only 3 VL bus should be

connected to the motherboard for avoiding overloading

It is used in 486 machines

PCI Local busPeripheral Component Interconnect bus

was introduced with Intel Pentium Computer

Pentium processors have 64 bit data path and 60-200 MHz speed – Hence VL bus cannot be used. Hence Intel provided a different local bus- PCI

For Pentium processors PCI is the standard local bus

PCI Local bus

Specifications of PCIBandwidth – 133 MB/s Supports 32 bit and 64 bit interfacesPlug and play capabilityProcessor Independence – Clocking and

transfer on PCI bus is independent of processor clock

High speed bus: Earlier 33.33 MHz having throughput of 132 MB/s with 32 bit board – 264 MB/s for 64 bit board.

Later 66.66 MHz had a throughput of 533 MB/s with 64 bit PCI devices

Retain support for ISA busBus mastering

Bus Type

Max Speed

MHz

No. of Data bits

No. of address bits

Software setup

Bus Mastering

Peak Transfer Rate ( MBps)

ISA 8.33 16 24 No No 5

MCA 8.33 32 32 yes yes 20

EISA 8.33 32 32 Yes Yes 32

VESA 33 32 32 No yes 130

PCI 66 64 32 yes yes 533

Personal Computer Memory Card International Association- PCMCIA

Developed for portable computers

It has 16 bit data bus and a 33 MHZ upper limit on speed

It has 26 address lines which limits its memory space to 64 MB.

Peak transfer rate – 20 MBps

FeaturesPlug and PlayHot SwappingDoes not support Bus Mastering or DMA

It uses 68 pin socket that connects directly Computer system bus

Limitations of PCIPCI is a parallel bus and hence has BW

limitationsThe available BW is shared by all

devices on a PCI busPCI bus cannot be easily scaled up in

frequency or scaled down in Voltage.PCI clock freq is inadequateLack of isochronous data transferDoes not support Advanced Power

ManagementPCI does not support built in Hot

plugging / hot swapping of peripherals

PCI -XEnhanced version of PCI used on workstations and servers

PCI-X has bus width of 64 bitsIt maintains backward compatibility with PCI

Max clock speed of 133 MHz in PCI-X 1.0

PCI-X 1.0 introduced improved protocols

PCI -XHas throughput of 1 GBpsMore efficient bus operation

allowing better interfacing with memory controller bridges and other advanced I/O solutions

Improved error handlingSupports 4 or more slotsPCI-X 2.0 has bus freq of 266 MHZ

and 533 MHZ Improved performance in real

time applications

Advantages of PCI Express over PCI Serial technology providing

Scalable performance High BWPoint – Point link dedicated to each

device instead of PCI shared BusLow delay since PCI express

provided a more direct connection to chipset

Isochronous data transferAdvanced Power MgmtHot pluggability

PCI ExpressVery fast serial bus having backward

compatibility with current PCIPhysical connection over copper, optical

or other physical media to allow for future encoding schemes

High BW allows small form factors, reduced cost, simpler board design and reduced signal integrity issues

Embedded clocking scheme enables easy speed changes as compared to sync clocking

PCI ExpressThroughput increases with width

Isochronous data transferHot swapping and plugging capabilities

Advanced Power Management capabilities

PCI PCI Express

Parallel bus Serial

Data can be sent in one direction at a time

Data can be sent in 2 directions

BW is shared ( Slow) BW not shared ( Fast)

32 bit PCI bus has max 132 MB/s speed at 33 MHz

Data transfer rates are different for diff specification.

Bigger connector size Smaller connector size

PCI is shared bus Point-point dedicated to each device

No built in support for hot plugging

Built in support

Lack of isochronous data transfer Supports isochronous data transfer

BW can’t be scaled BW can be scaled

No Advanced Power mgmt. Advanced Power Mgmt.

AGPAccelerated Graphics Port – High speed

bus for display boards/ accelerating 3D graphics

A single AGP can control a single deviceFeatures: it allows the video board to use

the system memory (RAM)AGP transfers 32 bits wide but uses 66.66

MHz Clock speedAGP 1x transfers 1 bit per data line per

clock cycle – 266.66 MB/s ( similarly for 2x, 4x, 8x)

It is 4-8 times faster than PCI

AGP PCI

AGP is a port connecting nodes PCI is a bus

AGP is pipelined , requests are executed in parallel

PCI is non pipelined making execution slower

AGP’s address /data is demultiplexed

PCI bus’s address/ data is multiplexed

AGP does not share BW with other devices

PCI bus shares BW

Peak throughput at 533 MB/s in 32 bits

Peak throughput of 133 MB/s in 32 bits

Single target, single master Multi target, Multi master

Memory R/W only, no other I/O operation

Link to entire system

High/ low priority queues No priority queues

Clock speed upto 66 MHZ PCI uses fixed 33 MHz bus

Conceptual Diagram of PCI Bus

PCI Local Bus

Bridge / memory controller DRAM Audio

Video

Processor

Cache

LAN SCSI

ISA/ EISA micro channel

EXP BUSInterface

Graphics

I/O Fns