What is Assembly Language?

1
Introduction

• Chapter 1
• What is Assembly Language?
• Data Representation

2
Table 1. Software Hierarchy Levels
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What is Assembly Language?

• A low-level processor-specific programming
  language design to match the processors machine
  instruction set
• each assembly language instruction matches
  exactly one machine language instruction
• we study here Intels 80x86 (and Pentiums)

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Why learn Assembly Language?

• To learn how high-level language code gets
  translated into machine language
• i.e. to learn the details hidden in HLL code
• To learn the computers hardware
• by direct access to memory, video controller,
  sound card, keyboard
• To speed up applications
• direct access to hardware (ex writing directly
  to I/O ports instead of doing a system call)
• good ASM code is faster and smaller rewrite in
  ASM the critical areas of code

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Assembly Language Applications

• Application programs are rarely written
  completely in assembly language
• only time-critical parts are written in ASM
• Ex an interface subroutine (called from HLL
  programs) is written in ASM for direct hardware
  access
• Ex2 device drivers (called from the OS)
• ASM often used for embedded systems (programs
  stored in PROM chips)
• computer cartridge games, microcontrollers
  (automobiles, industrial plants...),
  telecommunication equipment
• Very fast and compact but processor-specific

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Table 2. Comparison of Assembly Language and
High-Level Languages
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Machine Language

• An assembler is a program that converts ASM code
  into machine language code
• mov al,5 (Assembly Language)
• 1011000000000101 (Machine Language)
• most significant byte is the opcode for move
  into register AL
• the least significant byte is for the operand 5
• Directly programming in machine language offers
  no advantage (over Assembly)...

8
Binary Numbers/Storage Size

• are used to store both code and data
• On Intels x86
• byte 8 bits (smallest addressable unit)
• word 2 bytes
• doubleword 2 words
• quadword 2 doublewords

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Data Representation

• Even if we know that a block of memory contains
  data, to obtain its value we need to choose an
  interpretation
• Ex memory content 0100 0001 can either
  represent
• the number 26 1 65
• or the ASCII code of character A

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Data Representation

• Number Systems
• Binary/Octal/Decimal/Hexadecimal
• Converting between various number systems
• Signed/Unsigned Interpretation
• Twos Complement
• Addition/Subtraction
• Character Storage

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Number Systems

• A written number is meaningful only with respect
  to a base
• To tell the assembler which base we use
• Hexadecimal 25 is written as 25h
• Octal 25 is written as 25o or 25q
• Binary 1010 is written as 1010b
• Decimal 1010 is written as 1010 or 1010d
• You are supposed to know how to convert from one
  base to another (see appendix A)

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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

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Converting between various number systems

• Converting Binary to Decimal
• Converting Decimal to Binary
• Converting Binary to Hexadecimal
• Converting Hexadecimal to Decimal

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Signed and Unsigned Interpretation

• When a memory block contains a number, to obtain
  its value we must choose either
• the signed interpretation in that case the most
  significant bit (msb) represents the sign
• Positive number (or zero) if msb 0
• Negative number if msb 1
• the unsigned interpretation in that case all the
  bits are used to represent a magnitude (ie
  positive number, or zero)

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Signed Integers

• The highest bit indicates the sign. 1 negative,
  0 positive

If the highest digit of a hexadecimal integer is
gt 7, the value is negative. Examples 8A, C5, A2,
9D
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Twos Complement Notation

• Used to represent negative numbers
• The twos complement of a positive number X,
  denoted by NEG(X), is obtained by complementing
  all its bits and adding 1
• NEG(X) NOT(X) 1
• Ex NEG(10) NOT(10) 1
• NOT(0000 1010b) 1
• (1111 0101b) 1 1111 0110b NEG(10) -10
• It follows that X NEG(X) 0

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Forming the Two's Complement

• Negative numbers are stored in two's complement
  notation
• Represents the additive Inverse

Note that 00000001 11111111 00000000
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Binary Subtraction

• To perform the difference X - Y
• the machine executes the addition X NEG(Y)
• 0 0 0 0 1 1 0 0 0 0 0 0 1 1 0 0
• 0 0 0 0 0 0 1 1 1 1 1 1 1 1 0 1
• 0 0 0 0 1 0 0 1

Practice Subtract 0101 from 1001.
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Maximum and Minimum Values

• The msb of a signed number is used for its sign
• fewer bits are left for its magnitude
• Ex for a signed byte
• smallest positive 0000 0000b
• largest positive 0111 1111b 127
• largest negative -1 1111 1111b
• smallest negative 1000 0000b -128

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Ranges of Unsigned Integers
Standard sizes
What is the largest unsigned integer that may be
stored in 20 bits?
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Ranges of Signed Integers
The highest bit is reserved for the sign. This
limits the range
Practice What is the largest positive value that
may be stored in 20 bits?
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Signed/Unsigned Interpretation (again)

• To obtain the value of a number we need to chose
  an interpretation
• Ex memory content 1111 1111 can either
  represent
• -1 if a signed interpretation is used
• 255 if an unsigned interpretation is used
• Only the programmer can provide an interpretation
  of the content of memory

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Character Storage Systems

• Character sets
• Standard ASCII (0 127)
• Extended ASCII (0 255)
• ANSI (0 255)
• Unicode (0 65,535)
• Null-terminated String
• Array of characters followed by a null byte

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ASCII vs Extended ASCII

• The ASCII code (from 00h to 7Fh)
• Only codes from 20h to 7Eh represent printable
  characters. The rest are control codes (used for
  printing, transmission).
• Extended ASCII character set (codes 80h to FFh)
• Varies from one system to another
• MS-DOS usage for accentuated characters, Greek
  symbols and some graphic characters

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The ASCII character set

• CR carriage return (MSDOS move to beginning
  of line)
• LF line feed (MSDOS move directly one line
  below)
• SPC blank space

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Text Files

• These are files containing only ASCII characters
• But different conventions are used for indicating
  an end-of line
• MS-DOS ltCRgtltLFgt
• UNIX ltLFgt
• MAC ltCRgt
• This is at the origin of many problems
  encountered during transfers of text files from
  one system to another

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Strings and numbers

• A strings is stored as an array of characters
• A 1-byte ASCII code is stored for each char
• Hence, we can either store the number 123 in
  numerical form or as the string 123
• The string form is best for display
• The numerical form is best for computations