Chapter 3 Loaders and Linkers

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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

4
Kinds of loaders (cont.)

• Bootstrap
• A special absolute loader
• Typically single pass
• ROM
• Loads the OS

5
Kinds of loaders (cont.)

• Relocating
• Modifies appropriate addresses
• Two pass
• 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

• Our method - 6 types of records
• Masked used flags to indicate modification
• 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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Our OBJ Records - Header

• H header
• H PgmName Startaddr Length
• 1ch 6ch 6ch 6ch

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Our OBJ Records - Text

• T text
• T Startaddr Length records
• 1ch 6ch 2ch ???ch

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Our OBJ Records- Define

• D Define defined here, used elsewhere
• D Label addr Label addr Label addr .
• 1ch 6ch 6ch 6ch 6ch 6ch 6ch

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Our OBJ Records - Refer

• R Refer used here, defined elsewhere
• R Label Label Label .
• 1ch 6ch 6ch 6ch

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Our OBJ Records - Modification

• M Modification
• M addr len action
• 1ch 6ch 2ch /- label
• Addr location to modify
• Len number of bytes to modify
• Action how to modify

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Our OBJ Records - End

• E End
• E addr
• 1ch 6ch
• Addr is the starting execution location

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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 address 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

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Pass 1

• All external symbols from define records are
  stored and have destination addresses
• Provides load map containing
• Header records
• Define records
• Efficiency can be increased if a reference number
  is given to each external symbol. Ref number
  indexes an array removing the need for a hash
  function.

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Efficiency References

• HPROGA 000000 000063
• DLISTA 000040 ENDA 000054
• R 02LISTB 03ENDB 04LISTC 05ENDC (refer record)
• T 000020 0A 03201D 77100004 050014
• T 000054 0F 000014 FFFFF6 00003F 000014 FFFFC0
• M 000024 05 02 lt----- 02 references LISTB
• M 000054 06 04
• M 000057 06 05
• M 000057 06 - 04
• M 00005A 06 05
• M 00005A 06 - 04
• M 00005A 06 01
• M 00005D 06 - 03
• M 00005D 06 02
• M 000060 06 02
• M 000060 06 - 01
• E 000020
• Fig 3.12 Object program corresponding to Fig3.8
  using reference numbers for code modification
  (PROGA only, PROGB and PROGC are similar)

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Pass 2

• Loads text records
• Resolves addresses (relocating)
• Linking of CSECTS
• Starts execution at address of end record
• Uses last end record when each CSECT contains an
  END with an address

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Machine Independent Loader Features

• Include library routines -lm
• Specify options
• Load object program

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Automatic Library Search

• Library routines are external references
• Users can include routines to override library
  routines
• Library search is a search of the directory that
  contains addresses of the routines.

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Loader Options

• Exist as a separate command language
• OR
• As part of the compiled/assembled program

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Loader Options (cont.)

• Select alternate source
• Include program name
• Delete external symbols or entire CSECTS
• Change names

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Loader Options Example

• Fig2.15 is COPY using RDREC and WRREC. Suppose
  new
• routines READ and WRITE are to replace them,
  but we want to
• test READ and WRITE first. Without assembling
  we could give the
• loader
• INCLUDE READ(UTLIB)
• INCLUDE WRITE(UTLIB)
• DELETE RDREC, WRREC
• CHANGE RDREC, READ
• CHANGE WRREC, WRITE
• Now we have the new routines for execution
  without removing
• and reassembling the source code.

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Loader Options Libraries

• Specify alternative libraries to be searched.
  These are searched before system libraries,
  allowing user versions to replace system
  versions.
• LIBRARY MYLIB

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Loader Options Libraries

• Specify that library routines not be included.
  If, for example, statistics were normally done,
  but not done in this run.
• NOCALL STDDEV, PLOT, CORREL
• allows these references to be unresolved, but the
  assemble to succeed.

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Loader Options Libraries

• Specify no external references be resolved.
• Good for programs are linked but not executed
  immediately.
• Calls to external references, of course, will
  error.

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Loader Output

• Output from the loader can vary
• load map with the level of detail.
• CSECT only
• CSECT and addresses, external symbol address and
  cross reference table showing where each is used.

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Loader Design Options

• Linking loaders all linking and relocation at
  load time
• Linkage editors perform linking prior to load
  time
• Dynamic linking performed at execution time

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Linkage Editors

• Can replace one function without relinking.
  Similar to what make does for compiling
• INCLUDE PLANNER(PROGLIB)
• DELETE PROJECT (delete from
  existing planner)
• INCLUDE PROJECT(NEWLIB) (include new version)
• REPLACE PLANNER(PROGLIBK)

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Linkage Editors (cont.)

• Can be used to combine several library routines
  into a package so that they do not need to be
  recombined each time a program is run that uses
  those packages.
• INCLUDE READR(FTNLIB)
• INCLUDE WRITER(FTNLIB)
• INCLUE BLOCK(FTNLIB)
• .
• .
• .
• SAVE FTNIO(SUBLIB)
• Result is a much more efficient linking of
  functions.

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Linkage Editors (cont.)

• Can indicate that external references are not to
  be resolved by automatic library search
• Example suppose 100 programs use I/O routes, if
  all external references were resolved, there
  would be 100 copies of the library. Using
  commands to the linkage editor like those above,
  the user could specify not to include the
  library. A linking loader could be used to
  include the routines at run time. There would be
  a little more overhead since two linking
  operations would be done, one for user external
  references by the linkage editor and one for
  libraries by the linking loader.

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Dynamic Linking

• Perform the above operations but during load
  time.
• For example, a subroutine is loaded and linked to
  the rest of the program when it is first called.
• Used to allow several executing programs to share
  one copy of a subroutine or library. One copy of
  the function could be provided for all programs
  executing that use that function.

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Dynamic Linking (cont.)

• Used in Object Oriented Programming
• Allows the object to be shared by several
  programs.
• An implementation of an object can be changed
  without effecting the program making use of the
  object.

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Dynamic Linking (cont.)

• Enhanced efficiency (time and space)
• A subroutine is loaded only if it is needed,
  maybe an error handler routine would never be
  loaded if the error was never found.

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Dynamic Linking (cont.)

• Implementation
• During execution time the loader must be kept and
  invoked when the function is needed.
• In this case the loader can be thought of as part
  of the OS and thus an OS call occurs.
• The binding is at execution time rather than load
  time.
• Delayed binding gives more capabilities at higher
  cost.

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Bootstrap Loaders

• How is the loader loaded?
• Machine is idle and empty, thus no need for
  relocation.
• Some computers have a permanently resident in
  read-only memory (ROM) an absolute loader. Upon
  hardware signal occurring the machine executes
  this ROM program.