Basic Concepts
COE 205
Computer Organization and Assembly Language
Computer Engineering Department
King Fahd University of Petroleum and Minerals
Overview
 Welcome to COE 205
 Assembly-, Machine-, and High-Level Languages
 Assembly Language Programming Tools
 Programmer’s View of a Computer System
 Basic Computer Organization
 Data Representation
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 2
Welcome to COE 205
 Assembly language programming
 Basics of computer organization
 CPU design
 Software Tools
 Microsoft Macro Assembler (MASM) version 6.15
 Link Libraries provided by Author (Irvine32.lib and Irivine16.lib)
 Microsoft Windows debugger
 ConTEXT Editor
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 3
Textbook
 Kip Irvine: Assembly Language for Intel-Based Computers
 4th edition (2003)
 5th edition (2007)
 Read the textbook!
 Key for learning
and obtaining a
good grade
 Online material
 http://assembly.pc.
ccse.kfupm.edu.sa
/
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 4
Course Objectives
After successfully completing the course, students will be able to:
 Describe the basic components of a computer system, its instruction
set architecture and its basic fetch-execute cycle operation.
 Describe how data is represented in a computer and recognize
when overflow occurs.
 Recognize the basics of assembly language programming including
addressing modes.
 Analyze, design, implement, and test assembly language programs.
 Recognize, analyze, and design the basic components of a simple
CPU including datapath and control unit design alternatives.
 Recognize various instruction formats.
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 5
Course Learning Outcomes
 Ability to analyze, design, implement, and test assembly
language programs.
 Ability to use tools and skills in analyzing and
debugging assembly language programs.
 Ability to design the datapath and control unit of a
simple CPU.
 Ability to demonstrate self-learning capability.
 Ability to work in a team.
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 6
Required Background
 The student should already be able to program confidently in at
least one high-level programming language, such as Java or C.
 Prerequisite
 COE 202: Fundamentals of computer engineering
 ICS 102: Introduction to computing
 Only students with computer engineering major should be
registered in this course.
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 7
Grading Policy
 Programming Assignments
15%
 Quizzes
10%
 Exam I
15% (Sat. Nov. 3, 2007)
 Exam II
20% (Sat. Dec. 29, 2007)
 Laboratory
20%
 Final
20%
 Attendance will be taken regularly.
 Excuses for officially authorized absences must be presented no later
than one week following resumption of class attendance.
 Late assignments will be accepted but you will be penalized 10% per
each late day.
 A student caught cheating in any of the assignments will get 0 out of
15%.
 No makeup will be made for missing Quizzes or Exams.
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 8
Course Topics
 Introduction and Information Representation:
6 lectures
Introduction to computer organization. Instruction Set Architecture.
Computer Components. Fetch-Execute cycle. Signed number
representation ranges. Overflow.
 Assembly Language Concepts:
7 lectures
Assembly language format. Directives vs. instructions. Constants
and variables. I/O. INT 21H. Addressing modes.
 8086 Assembly Language Programming:
19 lectures
Register set. Memory segmentation. MOV instructions. Arithmetic
instructions and flags (ADD, ADC, SUB, SBB, INC, DEC, MUL,
IMUL, DIV, IDIV). Compare, Jump and loop (CMP, JMP, Cond.
jumps, LOOP). Logic, shift and rotate. Stack operations.
Subprograms. Macros. I/O (IN, OUT). String instructions. Interrupts
and interrupt processing, INT and IRET.
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 9
Course Topics
 CPU Design:
12 lectures
Register transfer. Data-path design. 1-bus, 2-bus and 3bus CPU organization. Fetch and execute phases of
instruction processing. Performance consideration.
Control steps. CPU-Memory interface circuit. Hardwired
control unit design. Microprogramming. Horizontal and
Vertical microprogramming. Microprogrammed control
unit design.
 Instruction Set Formats:
1 lecture
Fixed vs. variable instruction format. Examples of
instruction formats.
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 10
Next …
 Welcome to COE 205
 Assembly-, Machine-, and High-Level Languages
 Assembly Language Programming Tools
 Programmer’s View of a Computer System
 Basic Computer Organization
 Data Representation
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 11
Some Important Questions to Ask
 What is Assembly Language?
 Why Learn Assembly Language?
 What is Machine Language?
 How is Assembly related to Machine Language?
 What is an Assembler?
 How is Assembly related to High-Level Language?
 Is Assembly Language portable?
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 12
A Hierarchy of Languages
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 13
Assembly and Machine Language
 Machine language
 Native to a processor: executed directly by hardware
 Instructions consist of binary code: 1s and 0s
 Assembly language
 Slightly higher-level language
 Readability of instructions is better than machine language
 One-to-one correspondence with machine language instructions
 Assemblers translate assembly to machine code
 Compilers translate high-level programs to machine code
 Either directly, or
 Indirectly via an assembler
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 14
Compiler and Assembler
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 15
Instructions and Machine Language
 Each command of a program is called an instruction (it
instructs the computer what to do).
 Computers only deal with binary data, hence the
instructions must be in binary format (0s and 1s) .
 The set of all instructions (in binary form) makes up the
computer's machine language. This is also referred to as
the instruction set.
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 16
Instruction Fields
 Machine language instructions usually are made up of
several fields. Each field specifies different information
for the computer. The major two fields are:
 Opcode field which stands for operation code and it
specifies the particular operation that is to be performed.
 Each operation has its unique opcode.
 Operands fields which specify where to get the source
and destination operands for the operation specified by
the opcode.
 The source/destination of operands can be a constant, the
memory or one of the general-purpose registers.
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 17
Assembly vs. Machine Code
Instruction Address Machine Code Assembly Instruction
Basic Concepts
0005
B8 0001
MOV AX, 1
0008
B8 0002
MOV AX, 2
000B
B8 0003
MOV AX, 3
000E
B8 0004
MOV AX, 4
0011
BB 0001
MOV BX, 1
0014
B9 0001
MOV CX, 1
0017
BA 0001
MOV DX, 1
001A
8B C3
MOV AX, BX
001C
8B C1
MOV AX, CX
001E
8B C2
MOV AX, DX
0020
83 C0 01
ADD AX, 1
0023
83 C0 02
ADD AX, 2
0026
03 C3
ADD AX, BX
0028
03 C1
ADD AX, CX
002A
03 06 0000
ADD AX, i
002E
83 E8 01
SUB AX, 1
0031
2B C3
SUB AX, BX
0033
05 1234
ADD AX, 1234h
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 18
Translating Languages
English: D is assigned the sum of A times B plus 10.
High-Level Language: D = A * B + 10
A statement in a high-level language is translated
typically into several machine-level instructions
Intel Assembly Language:
Intel Machine Language:
mov eax, A
A1 00404000
mul
B
F7 25 00404004
add
eax, 10
83 C0 0A
mov D, eax
Basic Concepts
A3 00404008
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 19
Advantages of High-Level Languages
 Program development is faster
 High-level statements: fewer instructions to code
 Program maintenance is easier
 For the same above reasons
 Programs are portable
 Contain few machine-dependent details
 Can be used with little or no modifications on different machines
 Compiler translates to the target machine language
 However, Assembly language programs are not portable
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 20
Why Learn Assembly Language?
 Two main reasons:
 Accessibility to system hardware
 Space and time efficiency
 Accessibility to system hardware
 Assembly Language is useful for implementing system software
 Also useful for small embedded system applications
 Space and Time efficiency
 Understanding sources of program inefficiency
 Tuning program performance
 Writing compact code
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 21
Assembly vs. High-Level Languages
Some representative types of applications:
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 22
Next …
 Welcome to COE 205
 Assembly-, Machine-, and High-Level Languages
 Assembly Language Programming Tools
 Programmer’s View of a Computer System
 Basic Computer Organization
 Data Representation
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 23
Assembler
 Software tools are needed for editing, assembling,
linking, and debugging assembly language programs
 An assembler is a program that converts source-code
programs written in assembly language into object files
in machine language
 Popular assemblers have emerged over the years for the
Intel family of processors. These include …
 TASM (Turbo Assembler from Borland)
 NASM (Netwide Assembler for both Windows and Linux), and
 GNU assembler distributed by the free software foundation
 You will use MASM (Macro Assembler from Microsoft)
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 24
Linker and Link Libraries
 You need a linker program to produce executable files
 It combines your program's object file created by the
assembler with other object files and link libraries, and
produces a single executable program
 LINK32.EXE is the linker program provided with the
MASM distribution for linking 32-bit programs
 We will also use a link library for input and output
 Called Irvine32.lib developed by Kip Irvine
 Works in Win32 console mode under MS-Windows
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 25
Assemble and Link Process
Source
File
Source
File
Source
File
Assembler
Object
File
Assembler
Object
File
Linker
Assembler
Object
File
Link
Libraries
Executable
File
A project may consist of multiple source files
Assembler translates each source file separately into an object file
Linker links all object files together with link libraries
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 26
Debugger
 Allows you to trace the execution of a program
 Allows you to view code, memory, registers, etc.
 You will use the 32-bit Windows debugger
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 27
Editor
 Allows you to create assembly language source files
 Some editors provide syntax highlighting features and
can be customized as a programming environment
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 28
Next …
 Welcome to COE 205
 Assembly-, Machine-, and High-Level Languages
 Assembly Language Programming Tools
 Programmer’s View of a Computer System
 Basic Computer Organization
 Data Representation
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 29
Programmer’s View of a Computer System
Increased level
of abstraction
Application Programs
High-Level Language
Level 5
Assembly Language
Level 4
Operating System
Instruction Set
Architecture
Level 2
Microarchitecture
Level 1
Digital Logic
Basic Concepts
Level 3
Level 0
COE 205 – Computer Organization and Assembly Language – KFUPM
Each level
hides the
details of the
level below it
slide 30
Programmer's View – 2
 Application Programs (Level 5)
 Written in high-level programming languages
 Such as Java, C++, Pascal, Visual Basic . . .
 Programs compile into assembly language level (Level 4)
 Assembly Language (Level 4)
 Instruction mnemonics are used
 Have one-to-one correspondence to machine language
 Calls functions written at the operating system level (Level 3)
 Programs are translated into machine language (Level 2)
 Operating System (Level 3)
 Provides services to level 4 and 5 programs
 Translated to run at the machine instruction level (Level 2)
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 31
Programmer's View – 3
 Instruction Set Architecture (Level 2)
 Specifies how a processor functions
 Machine instructions, registers, and memory are exposed
 Machine language is executed by Level 1 (microarchitecture)
 Microarchitecture (Level 1)
 Controls the execution of machine instructions (Level 2)
 Implemented by digital logic (Level 0)
 Digital Logic (Level 0)
 Implements the microarchitecture
 Uses digital logic gates
 Logic gates are implemented using transistors
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 32
Instruction Set Architecture (ISA)
 Collection of assembly/machine instruction set of the
machine,
 Machine resources that can be managed with these
instructions
 Memory,
 Programmer-accessible registers.
 Provides a hardware/software interface
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 33
Next …
 Welcome to COE 205
 Assembly-, Machine-, and High-Level Languages
 Assembly Language Programming Tools
 Programmer’s View of a Computer System
 Basic Computer Organization
 Data Representation
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 34
Basic Computer Organization
 Since the 1940's, computers have 3 classic components:
 Processor, called also the CPU (Central Processing Unit)
 Memory and Storage Devices
 I/O Devices
 Interconnected with one or more buses
 Bus consists of
data bus
 Data Bus
registers
 Address Bus
 Control Bus
Processor
(CPU)
ALU
CU
Memory
I/O
Device
#1
I/O
Device
#2
clock
control bus
address bus
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 35
Processor
 Processor consists of
 Datapath
 ALU
 Registers
 Control unit
 ALU
 Performs arithmetic
and logic instructions
 Control unit (CU)
 Generates the control signals required to execute instructions
 Implementation varies from one processor to another
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 36
Clock
 Synchronizes Processor and Bus operations
 Clock cycle = Clock period = 1 / Clock rate
Cycle 1
Cycle 2
Cycle 3
 Clock rate = Clock frequency = Cycles per second
 1 Hz = 1 cycle/sec
1 KHz = 103 cycles/sec
 1 MHz = 106 cycles/sec
1 GHz = 109 cycles/sec
 2 GHz clock has a cycle time = 1/(2×109) = 0.5 nanosecond (ns)
 Clock cycles measure the execution of instructions
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 37
Memory
 Ordered sequence of bytes
 The sequence number is called the memory address
 Byte addressable memory
 Each byte has a unique address
 Supported by almost all processors
 Physical address space
 Determined by the address bus width
 Pentium has a 32-bit address bus
 Physical address space = 4GB = 232 bytes
 Itanium with a 64-bit address bus can support
 Up to 264 bytes of physical address space
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 38
Address Space
Address Space is
the set of memory
locations (bytes) that
can be addressed
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 39
Address, Data, and Control Bus
 Address Bus
 Memory address is put on address bus
 If memory address = a bits then 2a locations are addressed
 Data Bus: bi-directional bus
 Data can be transferred in both directions on the data bus
 Control Bus
 Signals control
transfer of data
 Read request
Processor
address bus
Address Register
Basic Concepts
a bits
data bus
Data Register
 Write request
 Done transfer
Memory
d bits
0
1
2
3
read
Bus Control
...
write
done
COE 205 – Computer Organization and Assembly Language – KFUPM
2a – 1
slide 40
Memory Read and Write Cycles
 Read cycle
1. Processor places address on the address bus
2. Processor asserts the memory read control signal
3. Processor waits for memory to place the data on the data bus
4. Processor reads the data from the data bus
5. Processor drops the memory read signal
 Write cycle
1. Processor places address on the address bus
2. Processor places the data on the data bus
3. Processor asserts the memory write control signal
4. Wait for memory to store the data (wait states for slow memory)
5. Processor drops the memory write signal
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 41
Reading from Memory
 Multiple clock cycles are required
 Memory responds much more slowly than the CPU
 Address is placed on address bus
 Read Line (RD) goes low, indicating that processor wants to read
 CPU waits (one or more cycles) for memory to respond
 Read Line (RD) goes high, indicating that data is on the data bus
Cycle 1
Cycle 2
Cycle 3
Cycle 4
CLK
Address
ADDR
RD
Data
DATA
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 42
Memory Devices
 Volatile Memory Devices
 Data is lost when device is powered off
 RAM = Random Access Memory
 DRAM = Dynamic RAM
 1-Transistor cell + trench capacitor
 Dense but slow, must be refreshed
 Typical choice for main memory
 SRAM: Static RAM
 6-Transistor cell, faster but less dense than DRAM
 Typical choice for cache memory
 Non-Volatile Memory Devices





Basic Concepts
Stores information permanently
ROM = Read Only Memory
Used to store the information required to startup the computer
Many types: ROM, EPROM, EEPROM, and FLASH
FLASH memory can be erased electrically in blocks
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 43
Processor-Memory Performance Gap
CPU: 55% per year
Performance
1000
“Moore’s Law”
100
Processor-Memory
Performance Gap:
(grows 50% per year)
10
DRAM: 7% per year
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
1
 1980 – No cache in microprocessor
 1995 – Two-level cache on microprocessor
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 44
The Need for a Memory Hierarchy
 Widening speed gap between CPU and main memory
 Processor operation takes less than 1 ns
 Main memory requires more than 50 ns to access
 Each instruction involves at least one memory access
 One memory access to fetch the instruction
 A second memory access for load and store instructions
 Memory bandwidth limits the instruction execution rate
 Cache memory can help bridge the CPU-memory gap
 Cache memory is small in size but fast
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 45
Typical Memory Hierarchy
 Registers are at the top of the hierarchy
 Typical size < 1 KB
Microprocessor
 Access time: 0.5 – 1 ns
Registers
 L2 Cache (512KB – 8MB)
L1 Cache
 Access time: 2 – 10 ns
L2 Cache
 Main Memory (1 – 2 GB)
 Access time: 50 – 70 ns
Faster
 Level 1 Cache (8 – 64 KB)
Memory Bus
Memory
 Disk Storage (> 200 GB)
 Access time: milliseconds
Basic Concepts
Bigger
 Access time < 0.5 ns
I/O Bus
Disk, Tape, etc
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 46
Magnetic Disk Storage
Disk Access Time =
Seek Time +
Rotation Latency +
Transfer Time
Read/write head
Sector
Actuator
Recording area
Seek Time: head movement to the
desired track (milliseconds)
Rotation Latency: disk rotation until
desired sector arrives under the head
Transfer Time: to transfer data
Basic Concepts
Track 2
Track 1
Track 0
Arm
Direction of
rotation
Platter
Spindle
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 47
Example on Disk Access Time
 Given a magnetic disk with the following properties
 Rotation speed = 7200 RPM (rotations per minute)
 Average seek = 8 ms, Sector = 512 bytes, Track = 200 sectors
 Calculate
 Time of one rotation (in milliseconds)
 Average time to access a block of 32 consecutive sectors
 Answer
 Rotations per second = 7200/60 = 120 RPS
 Rotation time in milliseconds = 1000/120 = 8.33 ms
 Average rotational latency = time of half rotation = 4.17 ms
 Time to transfer 32 sectors = (32/200) * 8.33 = 1.33 ms
 Average access time = 8 + 4.17 + 1.33 = 13.5 ms
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 48
Input Devices
Key Cap
Spring
Mechanical switch
c
d
e
f
8
9
a
b
4
5
6
7
0
1
2
3
Logical arrangement of keys
Basic Concepts
Conductor-coated membrane
Contacts
Membrane switch
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 49
Output Devices
Cleaning of
excess toner
Charging
Fusing of toner
Rotating
drum
Heater
Light from
optical
system
Rollers
Toner
Sheet of paper
Laser printing
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 50
I/O Controllers
 I/O devices are interfaced via an I/O controller
 I/O controller uses the system bus to communicate with processor
 I/O controller takes care of low-level operation details
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 51
Next …
 Welcome to COE 205
 Assembly-, Machine-, and High-Level Languages
 Assembly Language Programming Tools
 Programmer’s View of a Computer System
 Basic Computer Organization
 Data Representation
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 52
Data Representation
 Binary Numbers
 Hexadecimal Numbers
 Base Conversions
 Integer Storage Sizes
 Binary and Hexadecimal Addition
 Signed Integers and 2's Complement Notation
 Binary and Hexadecimal subtraction
 Carry and Overflow
 Character Storage
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 53
Binary Numbers
 Digits are 1 and 0
 1 = true
 0 = false
 MSB – most significant bit
 LSB – least significant bit
 Bit numbering:
MSB
LSB
1011001010011100
15
Basic Concepts
0
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 54
Binary Numbers
 Each digit (bit) is either 1 or 0
1
1
1
1
1
1
1
1
 Each bit represents a power of 2:
27
26
25
24
23
22
21
20
Every binary
number is a
sum of powers
of 2
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 55
Converting Binary to Decimal
Weighted positional notation shows how to calculate the
decimal value of each binary bit:
Decimal = (dn-1  2n-1) + (dn-2  2n-2) + ... + (d1  21) + (d0  20)
d = binary digit
binary 00001001 = decimal 9:
(1  23) + (1  20) = 9
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 56
Convert Unsigned Decimal to Binary
 Repeatedly divide the decimal integer by 2. Each
remainder is a binary digit in the translated value:
least significant bit
most significant bit
37 = 100101
Basic Concepts
stop when
quotient is zero
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 57
Hexadecimal Integers
Binary values are represented in hexadecimal.
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 58
Converting Binary to Hexadecimal
• Each hexadecimal digit corresponds to 4 binary bits.
• Example: Translate the binary integer
000101101010011110010100 to hexadecimal:
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 59
Converting Hexadecimal to Decimal
 Multiply each digit by its corresponding power of 16:
Decimal = (d3  163) + (d2  162) + (d1  161) + (d0  160)
d = hexadecimal digit
 Examples:
 Hex 1234 = (1  163) + (2  162) + (3  161) + (4  160) =
Decimal 4,660
 Hex 3BA4 = (3  163) + (11 * 162) + (10  161) + (4  160) =
Decimal 15,268
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 60
Converting Decimal to Hexadecimal
 Repeatedly divide the decimal integer by 16. Each
remainder is a hex digit in the translated value:
least significant digit
most significant digit
stop when
quotient is zero
Decimal 422 = 1A6 hexadecimal
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 61
Integer Storage Sizes
byte
Standard sizes:
word
doubleword
quadword
8
16
32
64
What is the largest unsigned integer that may be stored in 20 bits?
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 62
Binary Addition
 Start with the least significant bit (rightmost bit)
 Add each pair of bits
 Include the carry in the addition, if present
carry:
0
0
0
0
0
1
0
0
(4)
0
0
0
0
0
1
1
1
(7)
0
0
0
0
1
0
1
1
(11)
bit position: 7
6
5
4
3
2
1
0
+
Basic Concepts
1
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 63
Hexadecimal Addition
 Divide the sum of two digits by the number base (16).
The quotient becomes the carry value, and the
remainder is the sum digit.
36
42
78
28
45
6D
1
1
28
58
80
6A
4B
B5
21 / 16 = 1, remainder 5
Important skill: Programmers frequently add and subtract the
addresses of variables and instructions.
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 64
Signed Integers
 Several ways to represent a signed number
 Sign-Magnitude
 1's complement
 2's complement
 Divide the range of values into 2 equal parts
 First part corresponds to the positive numbers (≥ 0)
 Second part correspond to the negative numbers (< 0)
 Focus will be on the 2's complement representation
 Has many advantages over other representations
 Used widely in processors to represent signed integers
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 65
Two's Complement Representation
 Positive numbers
 Signed value = Unsigned value
 Negative numbers
 Signed value = 2n – Unsigned value
 n = number of bits
 Negative weight for MSB
 Another way to obtain the signed
value is to assign a negative weight
to most-significant bit
1
0
-128 64
1
1
0
1
0
0
32
16
8
4
2
1
= -128 + 32 + 16 + 4 = -76
Basic Concepts
8-bit Binary Unsigned
value
value
Signed
value
00000000
0
0
00000001
1
+1
00000010
2
+2
...
...
...
01111110
126
+126
01111111
127
+127
10000000
128
-128
10000001
129
-127
...
...
...
11111110
254
-2
11111111
255
-1
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 66
Forming the Two's Complement
starting value
00100100 = +36
step1: reverse the bits (1's complement)
11011011
step 2: add 1 to the value from step 1
+
sum = 2's complement representation
11011100 = -36
1
Sum of an integer and its 2's complement must be zero:
00100100 + 11011100 = 00000000 (8-bit sum)  Ignore Carry
The easiest way to obtain the 2's complement of a
binary number is by starting at the LSB, leaving all the
0s unchanged, look for the first occurrence of a 1. Leave
this 1 unchanged and complement all the bits after it.
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 67
Sign Bit
Highest bit indicates the sign. 1 = negative, 0 = positive
sign bit
1
1
1
1
0
1
1
0
0
0
0
0
1
0
1
0
Negative
Positive
If highest digit of a hexadecimal is > 7, the value is negative
Examples: 8A and C5 are negative bytes
A21F and 9D03 are negative words
B1C42A00 is a negative double-word
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 68
Sign Extension
Step 1: Move the number into the lower-significant bits
Step 2: Fill all the remaining higher bits with the sign bit
 This will ensure that both magnitude and sign are correct
 Examples
 Sign-Extend 10110011 to 16 bits
10110011 = -77
11111111 10110011 = -77
 Sign-Extend 01100010 to 16 bits
01100010 = +98
00000000 01100010 = +98
 Infinite 0s can be added to the left of a positive number
 Infinite 1s can be added to the left of a negative number
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 69
Two's Complement of a Hexadecimal
 To form the two's complement of a hexadecimal
 Subtract each hexadecimal digit from 15
 Add 1
 Examples:
2's complement of 6A3D = 95C2 + 1 = 95C3
2's complement of 92F0 = 6D0F + 1 = 6D10
2's complement of FFFF = 0000 + 1 = 0001
 No need to convert hexadecimal to binary
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 70
Binary Subtraction
 When subtracting A – B, convert B to its 2's complement
 Add A to (–B)
–
00001100
00000010
00001010
00001100
+
11111110
(2's complement)
00001010
(same result)
 Carry is ignored, because
 Negative number is sign-extended with 1's
 You can imagine infinite 1's to the left of a negative number
 Adding the carry to the extended 1's produces extended zeros
Practice: Subtract 00100101 from 01101001.
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 71
Hexadecimal Subtraction
 When a borrow is required from the digit to the left,
add 16 (decimal) to the current digit's value
16 + 5 = 21
-1
-
11
C675
A247
242E
+
C675
5DB9
242E
(2's complement)
(same result)
 Last Carry is ignored
Practice: The address of var1 is 00400B20. The address of the next
variable after var1 is 0040A06C. How many bytes are used by var1?
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 72
Ranges of Signed Integers
The unsigned range is divided into two signed ranges for positive
and negative numbers
Practice: What is the range of signed values that may be stored in 20 bits?
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 73
Carry and Overflow
 Carry is important when …
 Adding or subtracting unsigned integers
 Indicates that the unsigned sum is out of range
 Either < 0 or >maximum unsigned n-bit value
 Overflow is important when …
 Adding or subtracting signed integers
 Indicates that the signed sum is out of range
 Overflow occurs when
 Adding two positive numbers and the sum is negative
 Adding two negative numbers and the sum is positive
 Can happen because of the fixed number of sum bits
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 74
Carry and Overflow Examples
 We can have carry without overflow and vice-versa
 Four cases are possible
1
0
0
0
0
1
1
1
1
1
15
+
1
1
1
1
0
0
0
0
1
1
1
1
15
+
0
0
0
0
1
0
0
0
8
1
1
1
1
1
0
0
0
245 (-8)
0
0
0
1
0
1
1
1
23
0
0
0
0
0
1
1
1
7
Carry = 0
Overflow = 0
Carry = 1
1
1
0
1
0
0
1
1
1
1
79
+
Overflow = 0
1
1
1
1
0
1
1
0
1
0 218 (-38)
+
0
1
0
0
0
0
0
0
64
1
0
0
1
1
1
0
1 157 (-99)
1
0
0
0
1
1
1
1
143
(-113)
0
1
1
1
0
1
1
1
Carry = 0
Basic Concepts
Overflow = 1
Carry = 1
119
Overflow = 1
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 75
Character Storage
 Character sets
 Standard ASCII: 7-bit character codes (0 – 127)
 Extended ASCII: 8-bit character codes (0 – 255)
 Unicode: 16-bit character codes (0 – 65,535)
 Unicode standard represents a universal character set
 Defines codes for characters used in all major languages
 Used in Windows-XP: each character is encoded as 16 bits
 UTF-8: variable-length encoding used in HTML
 Encodes all Unicode characters
 Uses 1 byte for ASCII, but multiple bytes for other characters
 Null-terminated String
 Array of characters followed by a NULL character
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 76
Printable ASCII Codes
0
1 2 3
! " #
4 5 6 7 8 9 A B C D E F
$ % & ' ( ) * + , - . /
3
0 1 2 3
4 5 6 7 8 9 : ; < = > ?
4
@ A B C
D E F G H I J K L M N O
5
P Q R S
T U V W X Y Z [ \ ] ^ _
6
` a b c
d e f g h i j k l m n o
7
p q r s
t u v w x y z { | } ~
2
space
DEL
 Examples:
 ASCII code for space character = 20 (hex) = 32 (decimal)
 ASCII code for 'L' = 4C (hex) = 76 (decimal)
 ASCII code for 'a' = 61 (hex) = 97 (decimal)
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 77
Control Characters
 The first 32 characters of ASCII table are used for control
 Control character codes = 00 to 1F (hex)
 Not shown in previous slide
 Examples of Control Characters
 Character 0 is the NULL character  used to terminate a string
 Character 9 is the Horizontal Tab (HT) character
 Character 0A (hex) = 10 (decimal) is the Line Feed (LF)
 Character 0D (hex) = 13 (decimal) is the Carriage Return (CR)
 The LF and CR characters are used together
 They advance the cursor to the beginning of next line
 One control character appears at end of ASCII table
 Character 7F (hex) is the Delete (DEL) character
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 78
Terminology for Data Representation
 Binary Integer
 Integer stored in memory in its binary format
 Ready to be used in binary calculations
 ASCII Digit String
 A string of ASCII digits, such as "123"
 ASCII binary
 String of binary digits: "01010101"
 ASCII decimal
 String of decimal digits: "6517"
 ASCII hexadecimal
 String of hexadecimal digits: "9C7B"
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 79
Summary
 Assembly language helps you learn how software is constructed at
the lowest levels
 Assembly language has a one-to-one relationship with machine
language
 An assembler is a program that converts assembly language
programs into machine language
 A linker combines individual files created by an assembler into a
single executable file
 A debugger provides a way for a programmer to trace the execution of
a program and examine the contents of memory and registers
 A computer system can be viewed as consisting of layers. Programs
at one layer are translated or interpreted by the next lower-level layer
 Binary and Hexadecimal numbers are essential for programmers
working at the machine level.
Basic Concepts
COE 205 – Computer Organization and Assembly Language – KFUPM
slide 80