CS1104: Computer Organisation
http://www.comp.nus.edu.sg/~cs1104
School of Computing
National University of Singapore
Lecture 1: Introduction
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Computer Organisation/Architecture
Machine Organisation
Buses
Instruction Set Architecture (ISA)
Questions
Clock Cycles
Central Processing Unit (CPU)
Introduction
2
Lecture 1: Introduction
 Code Execution
 Memory
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Introduction
3
Computer Organisation/Architecture
 Computer organisation: electronics



engineer’s view of a computer system.
Computer architecture: assembly
programmer’s view of a computer system –
an abstract view.
In practice, difficult to distinguish the two.
Who needs to study this? Software
engineers, embedded systems programmers,
computer engineers.
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Introduction
4
Computer Organisation/Architecture (2)
 In-depth understanding of the inner-workings



of modern computers, and trade-offs present
at the hardware/software boundary.
Emphasis is on the concept understanding
and not on the hardware implementation.
Textbook: Computer Organizations and
Design (The hardware/software interface)
by David A. Patterson and John L. Hennessy,
Morgan Kaufmann, 2nd ed, 1998.
Read Chapter 1.2 and 1.3.
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Introduction
5
Machine Organisation
 von Neumann architecture: Programs and


data are stored in memory (stored-memory
concept).
Consists of processor, memory and devices.
Data are carried along buses between
components.
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Introduction
6
Machine Organisation (2)
Bus
Processor
Memory
Devices
Control
Cache
Input
Datapath
Output
Registers
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Buses
 Bus: A communication path between




components.
Data bus, control bus, address bus.
Bus width: the number of lines (bits).
Data bus width usually coincides with word
size, which is also usually the register size.
Address bus width determines the
addressable address range. A n-bit address
bus can address up to 2n locations.
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8
Instruction Set Architecture (ISA)
 Computer Architecture = Instruction Set +


Machine Organisation.
The Instruction Set serves as the interface
between hardware and software.
ISA: “… the attribute of a [computing] system as
seen by the programmer, i.e. the conceptual structure
and functional behavior, as distinct from the
organization of the data flows and controls the logic
design, and the physical implementation.” – Amdahl,
Blaaw and Brooks, 1964.
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Introduction
9
Instruction Set Architecture (ISA) (2)
Applications
Operating
System
Compiler
Processor
Firmware
Memory organization
Instruction Set
Architecture
I/O system
Datapath & Control
Digital Design
Circuit Design
Layout
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Introduction
PART 1
Electrical Engineering
10
Instruction Set Architecture (ISA) (3)
 Some examples of ISAs:
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Digital Alpha
HP PA-RISC
Sun Sparc
SGI MIPS
Intel
(v1, v3)
(v1.1, v2.0)
(v8, v9)
(MIPS I,II,III,IV,V)
(8086,80286,80286,
80486,Pentium,…)
Introduction
1992-97
1986-96
1987-95
1986-96
1978-96
11
Instruction Set Architecture (ISA) (4)
 Issues concerning an ISA design:
 Organisation of programmable storage
 Data types and data structures: encoding and
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representations
Instruction set
Instruction formats
Modes of addressing and accessing data items
and instructions
Exceptional conditions
Introduction
12
Questions
1)
Computer architecture is a study of
a)
b)
c)
d)
e)
2)
Instruction set architecture is an important
interface between
a)
b)
c)
d)
e)
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instruction set architecture only.
programming language only.
machine organisation only.
(a) and (c) [Answer]
(a), (b) and (c)
digital circuit and datapath control.
application software and operating system.
application software and hardware organisation. [Answer]
compiler and programming language.
high-level programming language and assembly
language.
Introduction
13
Questions (2)
3)
Which of the following does not belong to the
definition of an ISA?
a)
b)
c)
d)
e)
4)
Which of the following statements is true?
a)
b)
c)
d)
e)
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Instruction formats and types.
Compilation of a C program into machine code. [Answer]
Encoding and representation of data in memory.
Modes of addressing and accessing data in memory.
None of the above.
Hardware implementation of machine organization is
part of the instruction set architecture definition.
Instruction set architecture is an interface between the
assembly language and the machine language.
A Pentium II processor running at 450 MHz & a Pentium
II processor running at 500 MHz have the same ISA. [Ans]
Computer architecture is a subset of the instruction set
architecture.
None of the above.
Introduction
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Questions (3)
5)
Which of the following statements is true about
ISA?
a)
b)
c)
d)
e)
6)
If two machines can read and understand the same
piece of memory data, they must have the same ISA.
If two machines have the same ISA, they must have the
same performance.
In general, executable codes for one ISA cannot be run
on another, different ISA. [Answer]
The ISA of a processor defines the hardware
implementation of the processor.
None of the above.
State whether this statement is true or false:
“Changing the hardware machine organisation will
definitely change the instruction set architecture (ISA).”
[False]
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Introduction
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Questions (4)
7)
Which of the following can be considered as part of
the ISA design?
a)
b)
c)
d)
e)
Specification of high level languages such as JAVA or
“C”.
Compiler that translates high level language programs
into machine language codes.
Machine instruction types such as “ADD” or “LOAD”.
Data accessing method by the processor.
Implementation of hardware functional units in the
processor.
[Answer: (c), (d)]
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Clock Cycles
 A synchronous system is synchronised
according to a clock.
Rising edge
Falling edge
Clock period
 A clock cycle is the duration between two
consecutive rising (falling) edges, and its
duration is also known as the clock period.
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Introduction
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Clock Cycles (2)
 The clock frequency is the reciprocal of clock
period, measured in Hertz (Hz): number of
cycles per second. A clock with period of
250ns (nano-seconds) has a frequency of
4MHz.
A
B
 Clock A has twice the clock period than that of
clock B, and half the frequency of B’s.
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Introduction
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Central Processing Unit (CPU)
 CPU = Control Unit + ALU + Registers.
 Control Unit: monitors and directs sequences

of instructions.
ALU (Arithmetic-Logic Unit): performs simple
arithmetic and logical operations.
 Examples: Add, subtract, and, or, invert, increment, etc.
A
B
R = A op B
select
ALU
n-bits operations
R
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Introduction
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Central Processing Unit (CPU) (2)
 Registers: Fast memories in the CPU, storing

operands, temporary results and status
information.
General-purpose registers and special
registers:
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PC (program counter)
ACC (accumulator)
IR (instruction register)
MAR (memory address register)
MBR (memory buffer register) or MDR (memory data
register)
Introduction
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Code Execution
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Program in
High-level language
(C, Pascal, etc)
Link multiple
machine-language programs
to one program
Compile program
into assembly language
Load program into
computer’s memory
Assemble program
to machine language
Execute program
Introduction
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Code Execution (2)
 Instruction execution cycle:
fetch, decode, execute.
 Fetch: fetch next instruction


(using PC) from memory into
IR.
Decode: decode the
instruction.
Execute: execute instruction.
Instruction
Fetch
Instruction
Decode
Operand
Fetch
Execute
Result
Store
Next
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Introduction
Instruction
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Memory
 Memory stores programs and data.
 Definitions:
 1 byte = 8 bits
 1 word: in multiple of bytes; a unit of transfer

between main memory and registers, usually size
of register.
1 KB (kilo-bytes) = 210 bytes; 1 MB (mega-bytes) =
220 bytes; 1 GB (giga-bytes) = 230 bytes; 1TB
(tera-bytes) = 240 bytes.
 Desirable properties: fast access, large

capacity, economical cost, non-volatile.
However, most memory devices do not
possess all these properties.
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Memory (2)
 Memory hierarchy:
Fast, expensive
(small numbers),
volatile
registers
main memory
disk storage
magnetic tapes
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Introduction
Slow, cheap
(large numbers),
non-volatile
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Memory (3)
 Data transfer:
Processor
Up to 2k
addressable
locations.
Address
k-bit address bus
MAR
Memory
0
1
2
3
4
5
n-bit data bus
MDR
:
Control lines
(R/W, etc.)
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Memory (4)
 A memory unit stores binary information in groups of
bits called words.
 The data consists of n lines (for n-bit words). Data
input lines provide the information to be stored
(written) into the memory, while data output lines
carry the information out (read) from the memory.
 The address consists of k lines which specify which
word (among the 2k words available) to be selected
for reading or writing.
 The control lines Read and Write (usually combined
into a single control line Read/Write) specifies the
direction of transfer of the data.
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Memory (5)
 Block diagram of a memory unit:
n data
input lines
n
k address lines
k
Memory unit
2k words
n bits per word
Read/Write
n
n data
output lines
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Memory (6)
 The Write operation:
 Transfers the address of the desired word to the


address lines.
Transfers the data bits (the word) to be stored in
memory to the data input lines.
Activates the Write control line (set Read/Write to
0).
 The Read operation:
 Transfers the address of the desired word to the
address lines.
 Activates the Read control line (set Read/Write to
1).
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Memory (7)
 The Read/Write operation:
Memory Enable Read/Write
Memory Operation
0
X
None
1
0
Write to selected word
1
1
Read from selected word
 Two types of RAM: Static and dynamic.
 Static RAMs use flip-flops as the memory cells.
 Dynamic RAMs use capacitor charges to
represent data. Though simpler in circuitry, they
have to be constantly refreshed.
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Memory (8)
 A single memory cell of the static RAM has
the following logic and block diagrams:
Select
Select
R
Input
S
Q
Output
Read/Write
Logic diagram
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Introduction
Input
BC
Output
Read/Write
Block diagram
30
Memory (9)
 Logic construction
of a 4 x 3 RAM
(with decoder and
OR gates):
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Introduction
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Memory (10)
 An array of RAM chips: memory chips are

combined to form larger memory.
A 1K x 8-bit RAM chip:
RAM 1K x 8
Input data 8
Address 10
Chip select
Read/write
DATA (8)
ADRS (10)
CS
RW
(8)
8
Output data
Block diagram of a 1K x 8 RAM chip
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Memory (11)
Address
Lines
11 10
Input data
Lines
0–9
8 lines
DATA (8)
(8)
ADRS (10)
CS
1K x 8
RW
2x4
decoder
S0
S1
0
1
2
3
1024 – 2047
DATA (8)
(8)
ADRS (10)
CS
1K x 8
RW
2048 – 3071
Read/write
DATA (8)
(8)
ADRS (10)
CS
1K x 8
RW
3072 – 4095
 4K x 8 RAM.
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0–1023
DATA (8)
(8)
ADRS (10)
CS
1K x 8
RW
Introduction
Output
data
33
Memory (12)
21-bit
addresses
19-bit internal chip address
A0
A1
A19
A20
2-bit
decoder
512K x 8
memory chip
8-bit data
input/output
19-bit
address
Chip select
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512k X 8
memory chip
D31-24
D23-16
D 15-8
D7-0
Another example:
Organization of a 2M  32 memory module
using 512K  8 static memory chips.
Introduction
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