Essentials of 80x86 Assembly Language

1
Chapter 1 Data in a Computer
2
1.1 Binary and Hexadecimal Numbers
3
Decimal Numbers

• Base 10
• 4053 3 x 1 5 x 10 0 x 102 4 x 103

1s
10s
100s
1000s
4
Binary Numbers

• Base 2, using bits (binary digits) 0 and 1
• 10111 1 x 1 1 x 2 1 x 22 0 x 23 1 x 24

1s
101112 2310
2s
4s
8s
16s
5
Hexadecimal Numbers

• Base 16
• Digits
• 0-9 same as decimal
• A for 10
• B for 11
• C for 12
• D for 13
• E for 14
• F for 15

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Hexadecimal to Decimal

• Base 16
• 5CB 11 x 1 12 x 16 5 x 162

5CB16 148310
1s
16s
256s
7
Converting decimal to hex

• Use a calculator that does hex calculations, or
• Use this algorithm
• until DecimalNumber 0 loopdivide DecimalNumber
  by 16, getting Quotient and RemainderRemainder
  (in hex) is the next digit (right to
  left)DecimalNumber Quotient
• end until

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1.2 80x86 Memory
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80x86 Memory

• Collection of slots each of which holds one
  byte (8 bits)
• Each slot identified by a 32-bit physical address
• The flat memory model encodes 32 bit addresses in
  instructions

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Memory Units

• 1Kb 210 1,024 (about a thousand),
• 1Mb 220 1,048,576 (about a million)
• 1Gb 230 1,073,741,824 (about a billion)

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1.3 80x86 Registers
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80x86 Registers

• Internal to CPU rapidly accessed
• Used in arithmetic and other operations

13
General Registers

• EAX, EBX, ECX, EDX, each 32 bits long
• The low-order 16 bits can be referenced by AX,
  BX, CX and DX, respectively
• The low-order 8 bits can be referenced by AL, BL,
  CL and DL, respectively
• Bits 8-15 can be referenced by AH, BH, CH and CH,
  respectively
• EAX is called the accumulator and sometimes has
  special instruction formats associated with it

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Index Registers

• ESI, 32-bit source index used for
• Source address in string moves
• Array index
• General purposes
• EDI, 32-bit destination index used for
• Address of destination in string moves
• Array index
• General purposes

15
Stack Pointer, Base Pointer and Segment Registers

• ESP, 32-bit stack pointer
• Holds address of top of stack
• EBP, 32-bit base pointer
• Used in procedure calls to hold address of
  reference point in the stack
• Segment Registers used in segmented memory model
• CS, DS, ES, FS, SS, GS
• Each 16 bits long holding a segment selector
• Managed by the operating system with flat memory
  model programming

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Other Registers

• EIP, 32-bit instruction pointer
• Holds address of next instruction to be fetched
  for execution
• EFLAGS, 32-bit
• Collection of flags, or status bits
• Records information about many operations

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1.4 Character Codes
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Character Codes

• Letters, numerals, punctuation marks and other
  characters are represented in a computer by
  assigning a numeric value to each character
• The system commonly used with microcomputers is
  the American Standard Code for Information
  Interchange (ASCII)

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Examples of ASCII Codes

• Printable characters
• Uppercase M codes as 4D16 10011012
• Lowercase m codes as 6D16 11011012
• Numeral 5 codes as 3516
• Space codes as 2016
• Control characters
• Backspace codes as 0816

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1.5 2's Complement Representation for Signed
Integers
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Standard Number Lengths

• Byte 8 bits
• Word 16 bits
• Doubleword 32 bits
• Quadword 64 bits

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Unsigned Representation

• Just binary in one of the standard lengths
• E47A is the word-length unsigned representation
  for the decimal number 58490

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Signed Representation

• 2s complement representation used in 80x86
• One of the standard lengths
• High-order (leading) bit gives sign
• 0 for positive
• 1 for negative
• Must perform 2s complement operation to find
  corresponding positive number

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2s Complement Operation

• /- button on many calculators
• Manually by subtracting from 1000
• E47A represents a negative word-length signed
  number since E 1110
• 10000 E47A 1B86 704610,so E47A is the 2s
  complement signed representation for -7046

minus
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1.6 Integer Addition and Subtraction
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Addition

• Same for unsigned and 2s complement signed
  numbers, but the results may be interpreted
  differently
• The two numbers will be byte-size, word-size or
  doubleword-size
• Add the bits and store the sum in the same length
  as the operands, discarding an extra bit (if any)

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Carry

• If the sum of two numbers is one bit longer than
  the operands, the extra 1 is a carry (or carry
  out).
• A carry is recorded in the carry flag CF (bit 0
  in the EFLAGS register), not in the standard
  length result.
• For unsigned numbers, a carry means that the
  result is too large to be stored in the length of
  the operands.

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Carry In

• A 1 carried into the high-order (sign, leftmost)
  bit position during addition is called a carry
  in.
• Byte-length example

carry in
carry out
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Overflow

• Overflow occurs when there is a carry in but no
  carry out, or when there is a carry out but no
  carry in. It is recorded in OF (bit 11 of
  EFLAGS).
• If overflow occurs when adding two signed
  numbers, then the result will be incorrect.
• Word-length example
• Overflow, so AC99 incorrectif viewed as sum of
  signed numbers
• No carry, so AC99 correct ifviewed as sum of
  unsigned numbers

483F 645A AC99
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Subtraction

• Take the 2s complement of second number and add
  it to the first number
• Overflow occurs in subtraction if it does not
  occur in the corresponding addition
• CF is set for a borrow in subtraction when the
  second number is larger than the first as
  unsigned numbers