1. Loaders and Linkers: Features
CSCI/CMPE 3334
David Egle

2. Machine Independent Features
Most loaders/linkers include fewer different features than are found in a typical assembler
The following features are considered here:
Automatic Library Search
Loader Options

3. Automatic Library Search
Handles case where programmer does not provide the object programs defining the external references
A standard set of system libraries are automatically searched for those external references
Other libraries to search may be specified by control statements or by parameters to the linking loader

4. Automatic Library Search - 2
The linking loader keeps track of external symbols that are referred to, but not defined
these Refer record symbols are inserted in ESTAB and marked not defined
a programmer can override a standard library by including it in the primary input to the loader
At the end of pass 1, the undefined symbols represent unresolved external references
the loader searches the libraries specified
the routines fetched may contain external references
repeat this process until all references are resolved

5. Automatic Library Search - 3
A library (object programs) search technique
The linking loader could scan the Define records for all object programs
this would be inefficient
A special file structure is used for libraries – it contains a directory that specifies a routine name and a pointer to its address within the file
a search in the directory is followed by reading the object programs
the directory can be kept (by the OS) permanently in memory for faster search process

6. Loader Options Loader options modify its standard processing
Many loaders have a special command language
embedded in the primary input stream between object programs,
in a separate input file, or
included in the source program and retained in the object program

7. Loader Options – 2 Select alternate source of input
read program-name from library-name as a primary input
INCLUDE program-name (library-name)
Delete external symbols or control sections
DELETE csect-name
Rename external symbol
CHANGE name1, name2
Automatic inclusion of library routines
searched before the standard system libraries
LIBRARY MYLIB
Don’t resolve some external reference
routines that will not be performed in this execution
NOCALL STDDEV, PLOT, CORREL

8. Loader Options – 3 Don’t resolve any external reference
the program is linked but not executed
a linkage editor performs this function
Control the load map the loader outputs
a load map might contain control section names, its addresses, external symbol addresses, or cross-reference table
Other options
specify the location at which the execution begin
specify whether or not should execute the program if errors are detected

9. Loader Options Example
The main program COPY uses the subprograms RDREC and WRREC (control sections used only with COPY)
We want COPY to use the utility subroutines READ and WRITE that are available in UTLIB
INCLUDE READ(UTLIB)
INCLUDE WRITE(UTLIB)
DELETE RDREC, WRREC
CHANGE RDREC, READ
CHANGE WRREC, WRITE
How might the loader handle these commands?

10. Loader Design Options
A linking loader performs linking and relocations when loading the program into memory for execution (described in previous sections)
A linkage editor performs linking prior to load time
In dynamic linking the linking is performed at execution time

11. Linkage Editor
Produces a linked version of the program written to a file for later execution
often called a load module, or an executable image
Performs relocation of all control sections relative to the start of the linked program
resolves external references and library searching
Later, a one-pass relocating loader loads the program into memory for execution

12. Linkage Editor – 2
Reduces the overhead if the program to be executed many times
Information concerning external references is often retained in the linked program to allow replacing control sections, modifying external references, etc.
Can be used to build packages of control sections (libraries)
If the actual program load address is known in advance an exact image of the program is produced
Often has an option that allows the user to specify not to resolve external reference by automatic library search
only resolves user-written routines
requires a linking loader at execution time
saving in library space vs. overhead

13. Dynamic Linking
In dynamic linking, a subroutine is loaded and linked when it first called
also called dynamic-loading or load-on-call
The binding of the name to an actual address is delayed from load time until execution time

14. Dynamic Linking Advantages
Allows several executing programs to share one copy of a subroutine or library
Allows the implementation of a software “object” to be determined at the time the program is run
Allows the ability to load subroutines only when (and if) they are needed
substantial saving if a program contains large subroutines that are rarely called

15. Dynamic Linking Mechanism
Instead of executing JSUB instruction the program makes a load-and-call service request to the OS
the parameter to this request is the symbolic name of the subroutine
The OS examines its internal tables to determine whether or not the subroutine is already loaded
If necessary, the subroutine is loaded
Control is passed from the OS to the subroutine
When the called subroutine completes its processing, it returns to its caller (OS). The OS returns control to the program
the OS might release the subroutine memory or retain it for later calls

16. Implementation Examples
MS-DOS Linker
SunOS Linker
Cray MPP Linker

17. Problems 3.1 3.2 3.3 3.4 #1 is part of Phase 4 #7 (problems), 13 #3
#3, 4, 5, 6