Basic Concepts
COE 205
Computer Organization and Assembly Language
Computer Engineering Department
King Fahd University of Petroleum and Minerals
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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
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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
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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/
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Course ObjectivesAfter 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.
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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.
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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.
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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.
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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 lecturesAssembly language format. Directives vs. instructions. Constants and variables. I/O. INT 21H. Addressing modes.
8086 Assembly Language Programming: 19 lecturesRegister 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.
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Course Topics CPU Design: 12
lecturesRegister transfer. Data-path design. 1-bus, 2-bus and 3-bus 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 lectureFixed vs. variable instruction format. Examples of instruction formats.
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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
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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?
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A Hierarchy of Languages
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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
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Compiler and Assembler
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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.
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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.
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Assembly vs. Machine CodeInstruction Address Machine Code Assembly Instruction 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
Flash Movie
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Translating LanguagesEnglish: D is assigned the sum of A times B plus 10.
High-Level Language: D = A * B + 10
Intel Assembly Language:mov eax, Amul Badd eax, 10mov D, eax
Intel Machine Language:A1 00404000F7 25 0040400483 C0 0AA3 00404008
A statement in a high-level language is translated typically into several machine-level instructions
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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
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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
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Assembly vs. High-Level Languages
Some representative types of applications:
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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
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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)
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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
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Assemble and Link ProcessSource
File
SourceFile
SourceFile
AssemblerObject
File
AssemblerObject
File
AssemblerObject
File
Linker ExecutableFile
LinkLibraries
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
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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
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Editor Allows you to create assembly language source files Some editors provide syntax highlighting features and
can be customized as a programming environment
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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
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Programmer’s View of a Computer System
Application ProgramsHigh-Level Language
Assembly Language
Operating System
Instruction SetArchitecture
Microarchitecture
Digital Logic Level 0
Level 1
Level 2
Level 3
Level 4
Level 5Increased level of abstraction
Each level hides the
details of the level below it
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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)
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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
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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
Flash Movie
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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
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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 Address Bus Control Bus
Processor(CPU)
Memory
registers
ALU clock
I/ODevice
#1
I/ODevice
#2
data bus
control bus
address bus
CU
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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
Processor
Flash Movie
Flash Movie
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Synchronizes Processor and Bus operations
Clock cycle = Clock period = 1 / Clock rate
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
Clock
Cycle 1 Cycle 2 Cycle 3
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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
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Address Space
Address Space is the set of memory locations (bytes) that can be addressed
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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 Write request Done transfer
Memory
0123
2a – 1
. . .readwritedone
data bus
address busProcessor
d bits
a bitsAddress Register
Data Register
Bus Control
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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 cycle1. 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
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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
Data
Address
CLK
ADDR
RD
DATA
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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 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
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Processor-Memory Performance Gap
1980 – No cache in microprocessor 1995 – Two-level cache on microprocessor
CPU: 55% per year
DRAM: 7% per year1
10
100
100019
8019
81
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
1982
Processor-MemoryPerformance Gap:(grows 50% per year)
Per
form
ance
“Moore’s Law”
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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
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Typical Memory Hierarchy Registers are at the top of the hierarchy
Typical size < 1 KB Access time < 0.5 ns
Level 1 Cache (8 – 64 KB) Access time: 0.5 – 1 ns
L2 Cache (512KB – 8MB) Access time: 2 – 10 ns
Main Memory (1 – 2 GB) Access time: 50 – 70 ns
Disk Storage (> 200 GB) Access time: milliseconds
Microprocessor
Registers
L1 Cache
L2 Cache
Memory
Disk, Tape, etc
Memory Bus
I/O Bus
Fast
er
Big
ger
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Magnetic Disk Storage
Track 0Track 1
Sector
Recording area
Spindle
Direction of rotation
Platter
Read/write head
Actuator
Arm
Track 2
Disk Access Time = Seek Time + Rotation Latency + Transfer Time
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
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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 Rotation time in milliseconds Average rotational latency Time to transfer 32 sectors Average access time
= 7200/60 = 120 RPS
= 1000/120 = 8.33 ms= time of half rotation = 4.17 ms= (32/200) * 8.33 = 1.33 ms= 8 + 4.17 + 1.33 = 13.5 ms
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Input Devices
Logical arrangement of keys0 1 2 3
c d e f
8 9 a b
4 5 6 7
Mechanical switch
Spring
Key Cap
Contacts
Membrane switch
Conductor-coated membrane
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Output Devices
Laser printing
Rollers
Sheet of paper
Light from optical system
Toner
Rotating drum
Cleaning of excess toner
Charging
Heater
Fusing of toner
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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
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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
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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
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Binary Numbers
Digits are 1 and 0 1 = true 0 = false
MSB – most significant bit LSB – least significant bit Bit numbering:
015
1 0 1 1 0 0 1 0 1 0 0 1 1 1 0 0MSB LSB
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Binary Numbers Each digit (bit) is either 1 or 0 Each bit represents a power of 2:
Every binary number is a sum of powers of 2
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Converting Binary to DecimalWeighted 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
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Convert Unsigned Decimal to Binary
Repeatedly divide the decimal integer by 2. Each remainder is a binary digit in the translated value:
37 = 100101stop when
quotient is zero
least significant bit
most significant bit
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Hexadecimal IntegersBinary values are represented in hexadecimal.
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Converting Binary to Hexadecimal• Each hexadecimal digit corresponds to 4 binary bits.
• Example: Translate the binary integer 000101101010011110010100 to hexadecimal:
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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
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Converting Decimal to Hexadecimal
Decimal 422 = 1A6 hexadecimal
stop when quotient is zero
least significant digit
most significant digit
Repeatedly divide the decimal integer by 16. Each remainder is a hex digit in the translated value:
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Integer Storage Sizes
What is the largest unsigned integer that may be stored in 20 bits?
Standard sizes:
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Binary Addition Start with the least significant bit (rightmost bit) Add each pair of bits Include the carry in the addition, if present
0 0 0 0 0 1 1 1
0 0 0 0 0 1 0 0
+
0 0 0 0 1 0 1 1
1
(4)
(7)
(11)
carry:
01234bit position: 567
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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 28 28 6A42 45 58 4B78 6D 80 B5
11
21 / 16 = 1, remainder 5
Important skill: Programmers frequently add and subtract the addresses of variables and instructions.
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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
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Two's Complement Representation8-bit Binary
valueUnsigned
valueSignedvalue
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
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
= -128 + 32 + 16 + 4 = -76
1 0 1 1 0 1 0 0
-128 64 32 16 8 4 2 1
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Forming the Two's Complement
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.
starting value 00100100 = +36
step1: reverse the bits (1's complement) 11011011
step 2: add 1 to the value from step 1 + 1
sum = 2's complement representation 11011100 = -36
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Sign BitHighest bit indicates the sign. 1 = negative, 0 = 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
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Sign ExtensionStep 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
Sign-Extend 01100010 to 16 bits
Infinite 0s can be added to the left of a positive number Infinite 1s can be added to the left of a negative number
10110011 = -77 11111111 10110011 = -77
01100010 = +98 00000000 01100010 = +98
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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
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Binary Subtraction When subtracting A – B, convert B to its 2's complement Add A to (–B)
0 0 0 0 1 1 0 0 0 0 0 0 1 1 0 0 0 0 0 0 0 0 1 0 1 1 1 1 1 1 1 0 (2's complement)
0 0 0 0 1 0 1 0 0 0 0 0 1 0 1 0 (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.
– +
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Hexadecimal Subtraction When a borrow is required from the digit to the left,
add 16 (decimal) to the current digit's value
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?
C675A247242E
-1
-
16 + 5 = 21
C6755DB9 (2's complement)242E (same result)
1
+1
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Ranges of Signed IntegersThe 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?
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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 – KFUPMslide 75
0 1 0 0 0 0 0 0
0 1 0 0 1 1 1 1+
1 0 0 0 1 1 1 1
79
64
143(-113)
Carry = 0 Overflow = 1
1
1 0 0 1 1 1 0 1
1 1 0 1 1 0 1 0+
0 1 1 1 0 1 1 1
218 (-38)
157 (-99)
119
Carry = 1 Overflow = 1
111
Carry and Overflow Examples We can have carry without overflow and vice-versa Four cases are possible
1 1 1 1 1 0 0 0
0 0 0 0 1 1 1 1+
0 0 0 0 0 1 1 1
15
245 (-8)
7
Carry = 1 Overflow = 0
11111
0 0 0 0 1 0 0 0
0 0 0 0 1 1 1 1+
0 0 0 1 0 1 1 1
15
8
23
Carry = 0 Overflow = 0
1
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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 – KFUPMslide 77
Printable ASCII Codes0 1 2 3 4 5 6 7 8 9 A B C D E F
2 space ! " # $ % & ' ( ) * + , - . /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 O5 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 o7 p q r s t u v w x y z { | } ~ 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 – KFUPMslide 78
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 – KFUPMslide 79
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 – KFUPMslide 80
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.
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