Chapter 3 Loaders and Linkers

1
Chapter 3Loaders and Linkers
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Purpose and Function

• Places object program in memory
• Linking
• Combines 2 or more obj programs
• Relocation
• Allows loading at different locations
• Linkage Editor
• Provides linking without loading

3
Kinds of Loaders

• Absolute
• Single pass
• Checks for correct header record
• Checks for sufficient available memory
• Moves each text record to proper location
• Upon seeing END passes control to the pgm

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Kinds of loaders (cont.)

• bootstrap
• A special absolute loader
• ROM
• Loads the OS

5
Kinds of loaders (cont.)

• Relocating
• Modifies appropriate addresses
• Loads object program at a variety of locations
• May perform loading during execution (repeatedly)
• Allows for multiple programs (multiprocessing)
• System libraries require relocation

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Methods of Relocation

• Modification records
• Use absolute addressing and fixed format
• No modification records required
• Use same text records with flag (relocation bit)
• Relocation bits gathered into a mask
• If relocation bit is 1, add starting address to
  word

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Mask Method of Relocation

• HCOPY 000000001077A
• T0000001EFFC1400334810390000362800303000154810613C
  000300002A0C003900002D
• 
  
  
• FFC 111111111100 all 10 words need
  modification
• T 00001E 15 E00 0C0036 481061 080033 4C0000
  454F46 000003 000000
• 
  
  
• E00 111000000000 instructions 0,1,2 need load
  addresses
• T0000391EFFC0400300000030E0105D30103FD8105D2800303
  010575480392C105E38103F
• 
  
  
• T0010570A8001000364C0000F1001000
• The F1 fouls up alignment, thus a new text record
  has to be started.
• 
  FIGURE 3.7

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Program Linking

• Necessary for separate CSECTS
• External References
• External Definitions

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LOADING

• Forward references to external symbols common
• Use 2 pass
• Pass 1 assigns addr. to external symbols
• Provides a load map (info in symbol table)
• Pass 2 performs actual loading, relocation, and
  linking

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Data Structures for Loading

• ESTAB external symbol table Stores
• Names
• Addresses
• CSECT of external symbols
• PROGADDR program load address
• Provided by the OS
• CSADDR CSECT addr. of control sect. loaded