MachineLevel Representation of Programs II

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Machine-Level Representation of Programs II
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Outline

• Assembly code and object code
• Memory and Registers
• Suggested reading
• Chap 3.2, 3.3, 3.4

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Hardware Organization
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Virtual spaces

• A linear array of bytes
• each with its own unique address (array index)
  starting at zero

0xffffffff 0xfffffffe 0x2 0x1 0x0
contents
addresses
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C constructs

• Variable
• Different data types can be declared
• Operation
• Arithmetic expression evaluation
• control
• Loops
• Procedure calls and returns

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Code Examples
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Code Examples
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C Code

• Add two signed integers
• int t xy

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Assembly Code

• Operands
• x Register eax
• y Memory Mebp8
• t Register eax
• Instruction
• addl 8(ebp),eax
• Add 2 4-byte integers
• Similar to expression x y
• Return function value in eax

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Assembly Programmers View
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Programmer-Visible States

• Program Counter(eip)
• Address of the next instruction
• Register File
• Heavily used program data
• Integer and floating-point

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Programmer-Visible States

• Conditional code register
• Hold status information about the most recently
  executed instruction
• Implement conditional changes in the control flow

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Operands

• In high level languages
• Either constants
• Or variable
• Example
• A A 4

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Where are the variables? registers Memory
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Operands

• Counterparts in assembly languages
• Immediate ( constant )
• Register ( variable )
• Memory ( variable )
• Example
• movl 8(ebp), eax
• addl 4, eax

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Simple Addressing Mode

• Immediate
• represents a constant
• The format is imm (4, 0xffffffff)
• Registers
• The fastest storage units in computer systems
• Typically 32-bit long
• Register mode Ea
• The value stored in the register
• Noted as REa

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Virtual spaces

• A linear array of bytes
• each with its own unique address (array index)
  starting at zero

0xffffffff 0xfffffffe 0x2 0x1 0x0
contents
addresses
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Memory References

• The name of the array is annotated as M
• If addr is a memory address
• Maddr is the content of the memory starting at
  addr
• addr is used as an array index
• How many bytes are there in Maddr?
• It depends on the context

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Indexed Addressing Mode

• An expression for
• a memory address (or an array index)
• Most general form
• Imm(Eb, Ei, s)
• Constant displacement Imm 1, 2 or 4 bytes
• Base register Eb Any of 8 integer registers
• Index register Ei Any, except for esp
• S Scale 1, 2, 4, or 8

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Memory Addressing Mode

• The address represented by the above form
• imm REb REi s
• It gives the value
• Mimm REb REi s

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Addressing Mode
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Value
Operand
0x100
eax
0xFF
(eax)
0x108
0x108
0x13
0x108
(0x108)0x13
260(ecx,edx)
(0x10C)0x11
(eax,edx,4)
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Code Examples
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Code Examples
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Code Examples
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Object Code

• 3-byte instruction
• Stored at address 0x80483ba
• 0x80483ba 03 45 08

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Understanding Machine Execution

• Where the sequence of instructions are stored?
• In virtual memory
• Code area
• How the instructions are executed?
• eip stores an address of memory
• from the address, machine can read a whole
  instruction once
• then execute it
• eip is also called program counter (PC)
• increase eip

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Operations in Assembly Instructions

• Performs only a very elementary operation
• Normally one by one in sequential
• Operate data stored in registers
• Transfer data between memory and a register
• Conditionally branch to a new instruction address

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Data layout

• Object model in assembly
• A large, byte-addressable array
• No distinctions even between signed or unsigned
  integers
• Code, user data, OS data
• Run-time stack for managing procedure call and
  return
• Blocks of memory allocated by user

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Data Formats
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Data Formats

• Move data instruction
• mov (general)
• movb (move byte)
• movw (move word)
• movl (move double word)

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Move Instructions

• Format
• mov src, dest
• src and dest can only be one of the following
• Immediate
• Register
• Memory

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Move Instructions

• Format
• The only possible combinations of the (src, dest)
  are
• (immediate, register)
• (memory, register) load
• (register, register)
• (immediate, memory) store
• (register, memory) store

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Data Movement
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Data Movement Example

• movl 0x4050, eax immediate register
• movl ebp, esp register register
• movl (edx, ecx), eax memory register
• movl -17, (esp) immediate memory
• movl eax, -12(ebp) register memory

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Data Movement Example

• Initial value dh8d eax 98765432
• movb dh, al eax9876548d
• movsbl dh, eax eaxffffff8d
• 3 movzbl dh, eax eax0000008d

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Stack operations
Increasing address
0x108
esp
Stack top
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Stack operations
pushl eax
0x108
0x104
esp
Stack top
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Stack operations
popl edx
esp
0x108
0x104
Stack top
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Data Movement Example

• int exchange(int xp, int y)
• int x xp / operator performs deferencing
  /
• xp y
• return x
• int a 4
• int b exchange(a, 3) / address of operator
  creates a pointer /
• printf(a d, b d\n, a, b)

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Data Movement Example
1 pushl ebp 2 movl esp, ebp 3 movl
8(ebp), eax 4 movl 12(ebp), edx 5 movl
(eax), ecx 6 movl edx, (eax) 7 movl
ecx, eax 8 movl ebp, esp 9 popl ebp

• int exchange(int xp, int y)
• int x xp
• xp y
• return x

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Data Movement Example
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Data Movement Example
1 pushl ebp
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Data Movement Example
2 movl esp, ebp
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Data Movement Example
3 movl 8(ebp), eax 4 movl 12(ebp),
edx 5 movl (eax), ecx 6 movl
edx, (eax) 7 movl ecx, eax
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Data Movement Example
8 movl ebp, esp 9 popl ebp