COMP 1321
Digital Infrastructure
Richard Henson
University of Worcester
October 2015
Week 4: Motherboards

Learning Objectives:
Explain main purpose of the motherboard
Explain how a CPU can handle input &
output from slow & fast devices
Explain how motherboard architecture has
changed in response to miniaturisation,
energy-saving, and consumerisation
demands
Why a Motherboard?

CPU far too fragile to exist independently
 i.e. needs a “mother”

motherboard delivers power and control to
CPU via
 control bus
 timing chips
 ROM and RAM

harnesses power of CPU effectively so it can
interact effectively with i/o devices
Typical PC Motherboard
http://my.englishclub.com/profiles/blogs/computer-motherboard-parts
I/O devices sorted by
data transfer times
Joystick
Keyboard
Mouse
CD Rom
TV Camera
Audio
IDE HDD
Scanner
VDU
10 B/s 100 B/s 1kB/s 10 kB/s 100 kB/s 1 MB/s 10
MB/s 100 MB/s
Typical Slow devices
Keyboard
 Mouse
 Joystick
 Audio

Fast devices (connect
straight into motherboard)
VDU
 Hard drive
 CD-ROM
 Scanner

Even faster devices..
CPU
 Memory (static/dynamic RAM & ROM)
 Graphics processors (GPU)

i/o Connectors
 Slow
(off motherboard
via ports)
 Serial
 Parallel
 Firewire
 PS/2 mouse & keyboard (legacy)
 VGA
 RJ-45: Network connection
 USB: getting faster & becoming
the standard
Motherboard connectors
from http://www.techiwarehouse.com/cms/articles.php?cat=13
North Bridge & South Bridge

North: control chipset for fastest onboard
devices
 ROM & static RAM

South: control chipset for slower onboard
devices
 SATA & IDE connectors
» hard disk & CD-ROM/DVD
 RAM connectors
» many types, differing no. of pins
Layout of component
connections
Extremely fast components talking directly to each other
“North Bridge”
chipset
Connection
Fast components talking directly to each other
“South Bridge”
chipset
Connection
Slow components talking directly to each other
Buffers

Areas of memory/storage…
 where data is stored before forwarding

Message from slow device received and
gradually fill buffer…
then sent quickly to fast device

Message from fast device quickly fill buffer
then sent slowly to slow device
Resulting arrangement
Incredibly
Fast
Pentium
III
Very Fast
440
North
Bridge
Organizing components
by their speeds
Cache
Main
Memory
Graphics
PCI Bus
Fast
IDE
Disk
South
Bridge
USB
KBd
Mouse
SCSI
Graphics
ISA Bus
Slow
Network
Printer
Sound
Buses and on-board
communications
Bus - physical link between computer
components that electrical impulses (i.e.
data as 0s and 1s) can travel through
 Only one device can send a message at
any one time

other devices have to wait until the line is
clear before sending
Motherboards, CPUs and
Assembly Language
As mentioned last week, each CPU
family has its own “instruction set”
 That instruction set allows the CPU to:

perform calculations
move data about between memory
locations
Get data from or send data to peripheral
devices
Motherboards, Peripherals,
and i/o

Input-output is part of the vocabulary of
computer science
shortened to i/o

Using this same convention:
CPU, memory, and rest of motherboard
are collectively “the computer”
anything that happens outside this
environment is via “a peripheral”
Interrupts
This is a mechanism for a peripheral
device to interrupt a CPU in the middle
(well almost…) of a program
 Not possible in early computers; only
became possible with microprocessors
and their associated instruction sets
 Very useful feature of “solid state”
CPUs…

Operating System i/o calls
When a computer program (any
language…) needs i/o (either i or o!), it
invokes an “i/o call” to the CPU, via the
operating system
 The actual “call” invokes an assembly
language instruction for i/o:

with Intel 8086, this is INT 21
further instruction selects motherboard port
8086 (DOS) Interrupts

Some examples:
02
05
08
09
0D
write character to screen
write to printer
take character from keyboard
write string to screen
write buffer to disk
Use of i/o calls in Assembly &
Higher Level Languages

Essential to invoke two commands:
1. Get right nibble contents into AH register
2. Invoke INT 21
o
o
o
e.g.
MOV AH,02
INT 21
Shutting down a program
Should be done formally
 Use 4C as the trigger with INT 21
 As with all INT 21 needs moving into AH
register first

e.g.
MOVE AH, 4C
INT 21
Smaller Motherboards

So far…
considered Intel 8086 series CPU
large instruction set & power requirements

Other CPUs use much less power,
smaller instruction set
therefore motherboard requirements
miniaturised
» e.g. Intel Atom: Netbook & Tablet PCs
» ARM: Smartphones & Tablets
Motherboard-on-a-chip
Started with Smartphones
 Spread to Tablet PCs…

Architecture of
Motherboard chip

With further miniaturisation…
“motherboard” self-contained
» mounted on a board to provide i/o
connectors
Arduino Board (i/o)

Microcontroller… not “motherboard on a chip”
 therefore not expensive..

Based on i/o control, not apps
 used for “physical” computing (real devices)
» http://www.atmel.com/Images/doc8161.pdf
 EPROM for embedded programming (“C”)
 USB-computer for power (a few mA)
 USB provides scope to use i/o for many purposes
http://www.ladyada.net/learn/arduino/lesson3.html
Raspberry Pi (computer)

Similar size (and price) to Arduino BUT
 needs specific power supply (micro USB, 700 mA)
 i/o based on wider range of ports

Microprocessor…
 uses ARM
 with GPU
 read to run an
operating system
(Linux)
Can run apps…
Smaller Motherboards

So far…
considered Intel 8086 series CPU
large instruction set & power requirements

Other CPUs use much less power,
smaller instruction set
therefore motherboard requirements
miniaturised
» e.g. Intel Atom: Netbook & Tablet PCs
» ARM: Smartphones & Tablets
Motherboard-on-a-chip
Started with Smartphones
 Spread to Tablet PCs…

Architecture of
Motherboard chip

With further miniaturisation…
“motherboard” self-contained
» mounted on a board to provide i/o
connectors
Arduino Board (i/o)

Microcontroller… not “motherboard on a chip”
 therefore not expensive..

Based on i/o control, not apps
 used for “physical” computing (real devices)
» http://www.atmel.com/Images/doc8161.pdf
 EPROM for embedded programming (“C”)
 USB-computer for power (a few mA)
 USB provides scope to use i/o for many purposes
http://www.ladyada.net/learn/arduino/lesson3.html
Raspberry Pi (computer)

Similar size (and price) to Arduino BUT
 needs specific power supply (micro USB, 700 mA)
 i/o based on wider range of ports

Microprocessor…
 uses ARM
 with GPU
 read to run an
operating system
(Linux)
Can run apps…
BBC Micro:Bit