Computer Architecture
Part IV-B: I/O Buses
Chipsets


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Intelligent bus controller chips found on the
motherboard
Enable higher speeds on one or more buses and the
utilization of new facilities (e.g. faster, larger RAM)
Suppliers include Intel, SIS, Opti, Via, ALi
Chipsets and the Other Components
CPU
Caches
System Bus / FSB
Memory
Adapter
Peripheral Bus
Controllers
I/O Devices:
Disks
Displays
Keyboards
Networks
Internal Bus Organization

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Same bus for all (e.g. HP-PB)
Separate bus for CPU-memory and
I/O (whether I/O to CPU or I/O to
memory, e.g. CIO)
I/O Bus Architecture

Four (4) main I/O bus architectures in
the modern PC
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Industry Standard Architecture (ISA)
Peripheral Component Interconnect (PCI)
Accelerated Graphics Port (AGP)
Universal Serial Bus (USB)
Each may have internal or external
ports
May be used to connect other I/O
buses
Internal vs. External Ports

Internal I/O ports


LPT, COM1, COM2, EIDE, etc.
External ports

Includes expansions slots in motherboard
which can accept various types of
controllers
I/O Buses in the PC
Source: www.mkdata.dk
Devices on each I/O Bus
Source: www.mkdata.dk
ISA



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16-bits wide, 8 MHz
Works synchronously with the CPU
clock
If the system bus is faster than 10 MHz
then the ISA bus frequency is reduced
to a fraction of the system bus
frequency
Theoretical maximum: 8 Mbps, Reality:
Only 1 – 2 Mbps
ISA Internal and External Ports
Source: www.mkdata.dk
ISA Drawbacks

16-bits, 8 MHz


Too narrow and slow, not enough
bandwidth
No intelligence


Every component requires a specific IRQ
and possibly a DMA channel
Tuning of IRQ and DMA needs to be
done manually – limited or no Plug-andPlay (PnP) features
ISA Future

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Obsolete!
No longer used in current motherboards
Intel’s 810 chipset was the first not to
include any support for ISA
PCI


Started 32-bits wide, 33 MHz,
maximum data transfer rate of 132
Mbps -> PCI-X supports 64-bits, 66
MHz
Processor independent


Can be used with any 32- or 64-bit CPU
Buffered architecture
Buffered Architecture



CPU delivers data to the buffer and
proceeds with other tasks; PCI bus
handles the rest of the operation
PCI adapters transmits data to the
buffer, regardless of whether the CPU
is free to handle request, i.e. requests
are placed in a queue
Plug-and-play is supported in PCI
specifications
PCI Internal and External Ports
Source: www.mkdata.dk
PCI Future

PCI-X

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Supported by IBM, 3Com, Mylex,
Adaptec, HP and Compaq
PCI-X 1.0: 64-bits, 66/133 MHz (133
provides around 11 MB/s bandwidth)
PCI-X 2.0: 64-bits, 266/533 MHz
PCI-X 1066: 1066 MHz (available 2004),
8.5 GB/s bandwidth
PCI Future

PCI Express (Next Generation I/O or
NGIO)



Supported by Intel, Dell, NEC, Sun, etc.
Codenamed 3GIO
Can provide a theoretical maximum of 16
GB/s bandwidth
AGP

Designed exclusively for video



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Relieves PCI bus of graphics data
66 MHz, 64-bits
Introduced by Intel on Pentium II boards
(440LX and later).
AGP standards

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1x – 266 Mbps
2x – 533 Mbps
4x – 1.07 Gbps
8x – 2.1 Gbps
ISA, PCI and AGP slots

ISA
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PCI

AGP
USB

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
12 megabits per second (Mbps) bus
Can connect up to 127 devices in one
long “daisy chain”
Devices can be plugged and
unplugged easily
USB
USB Devices

Meant to connect devices such as the
keyboard, mouse, joystick, speakers,
printers, modems, scanners, camera,
etc.
Source: www.mkdata.dk
USB Versions

USB v1.1 (USB)


Data rates of 12 Mbps (full-speed) and
1.5 Mbps (low-speed)
USB v2.0 (Hi-Speed USB)


Theoretical maximum data transfer rate
of 480 Mbps
Supports the three data transfer rates for
backward compatibility
Enhanced IDE (EIDE)

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IDE – Intelligent/Integrated Drive Electronics
Also known as Advanced Technology
Attachment (ATA)
Each channel has a master and slave
device
Source: www.mkdata.dk
EIDE Devices

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Hard disks (ATA-33/66/100)
CD-R, CD-RW drives
DVD drives
Zip drive
Tape Backup units
SCSI
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Small Computer System Interface
Utilizes host adapter to control 7 (or
15) devices using only one IRQ
SCSI has its own CPU system  frees
the main CPU from the I/O workload
The SCSI Host Adapter


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
Intelligent controller at the heart of a
SCSI system
Controls several SCSI units including
various types of drives (hard disks,
CDROM, Zip disks, MO drives, etc.),
backup tape units, scanners
Has its own BIOS
Some motherboards feature an onboard SCSI host adapter
SCSI Chain



Regular SCSI systems can handle 8
devices (including the adapter); wide
SCSI can handle 15 devices
Each device has to be assigned a
unique number from ID0 to ID7; host
adapter is typically assigned ID7
SCSI devices can be internal (inside
the computer casing) or external
SCSI Chain Example
SCSI Terminators


The devices at
both ends of a
SCSI chain
must be
terminated.
The host
adapter is one
end of the
chain.
Source: www.mkdata.dk
SCSI Intelligence



SCSI utilizes its own protocol to assure
maximum utilization of the bandwidth.
Basis of SCSI is a set of commands
Each device has its own intelligent controller
which can interpret these commands.
Source: www.mkdata.dk
SCSI Standards


There are various other variations of SCSI
SCSI standards are confusing to say the
least!