Chapter 3: Loaders and Linkers

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1 Department of Electr rical Eng ineering, Chapter 3: Loaders and Linkers 王振傑 (Chen-Chieh Wang) ncku edu 3.1 Basic Loader Functions Design of an Absolute Loader A Simple Bootstrap Loader Outline 3.2 Machine-Dependent Loader Features Relocation Program Linking Algorithm and Data Structures for a Linking Loader 3.3 Machine-Independent Loader Features Automatic Library Search Loader Options 3.4 Loader Design Options Linkage Editors Dynamic Linking Bootstrap Loaders 2

C program Compiler Assembly language program Assembler Object: Machine language module Object: Library routine (machine language) Executable: e: Machine language program Memory 3 Loaders and Linkers

2 C program Compiler Assembly language program Assembler Object: Machine language module Object: Library routine (machine language) Executable: e: Machine language program Memory 3 Loaders and Linkers In Chapter 2, we discussions Loading: brings the object program into memory for execution. Relocation: modifies the object program so that it can be loaded at an address different from the location originally specified. Linking: combines two or more separate object programs and supplies the information needed to allow references between them. In Chapter 3, we will discussion A loader is a system program that performs the loading function. Many loaders also support relocation and linking. A linker (or linkage editor) performs the linking operations and a separate loader to handle relocation and loading. 4

3.1 Basic Loader Functions 3.1.1 Design of an Absolute Loader 3.1.2 A Simple Bootstrap Loader Outline 3.2 Machine-Dependent Loader Features 3.2.1 Relocation 3.2.2 Program Linking 3.2.3 Algorithm and Data Structures for a Linking Loader 3.

3 3.1 Basic Loader Functions Design of an Absolute Loader A Simple Bootstrap Loader Outline 3.2 Machine-Dependent Loader Features Relocation Program Linking Algorithm and Data Structures for a Linking Loader 3.3 Machine-Independent Loader Features Automatic Library Search Loader Options 3.4 Loader Design Options Linkage Editors Dynamic Linking Bootstrap Loaders 5 Depar rtment of Electrical Engineering, Feng-Chia Unive ersity Loading of an absolute program [ Figure 3.1 ] 6

4 Depar rtment of Electrical Engineering, Feng-Chia Unive ersity Algorithm for an absolute loader [ Figure 3.2 ] 7 Department of Electrical Engineering, Feng-Chia Unive ersity 8

5 3.1 Basic Loader Functions Design of an Absolute Loader A Simple Bootstrap Loader Outline 3.2 Machine-Dependent Loader Features Relocation Program Linking Algorithm and Data Structures for a Linking Loader 3.3 Machine-Independent Loader Features Automatic Library Search Loader Options 3.4 Loader Design Options Linkage Editors Dynamic Linking Bootstrap Loaders 9 Depar rtment of Electr rical Eng 0x00000 Bootstrap Loader 0x Registers A X L B S T F PC SW Bus Processing Unit Control Unit Fetch Unit Processor Loader ineering, Read-Only Memory (ROM) Operating System (O/S), Feng-Chia Unive ersity Memory (DRAM) Device "F1" ( Hard Disk / Flash) 10

6 Bootstrap Loader When a computer is first turned on or restarted, a special type of absolute loader, called a bootstrap loader, is executed. This bootstrap loads the first program to be run by the computer usually an operating system. Example ( Figure 3.3 ) Located at address 0 in memory Loads first program (e.g. OS) at address 0x Each byte of object code to be loaded is represented on device F1 as two hexadecimal digits After reading end-of-file (0x04), jumps to address 0x00080 and starts execution of first program 11 Depar rtment of Electrical Engineering, Feng-Chia Unive ersity Bootstrap loader for SIC/XE [ Figure 3.33 ] 12

7 3.1 Basic Loader Functions Design of an Absolute Loader A Simple Bootstrap Loader Outline 3.2 Machine-Dependent Loader Features Relocation Program Linking Algorithm and Data Structures for a Linking Loader 3.3 Machine-Independent Loader Features Automatic Library Search Loader Options 3.4 Loader Design Options Linkage Editors Dynamic Linking Bootstrap Loaders 13 Relocation Efficient sharing of the machine requires that we write relocatable programs instead of absolute ones. Writing absolute programs makes it difficult to use subroutine libraries efficiently. Two methods for specifying relocation as part of the object program. Modification Record (for SIC/XE) Relocation Bit (for SIC) 14

8 Depar rtment of Electrical Engineering, Feng-Chia Unive ersity Example of a SIC/XE program [ Figure 3.4 ] 15 Depar rtment of Electrical Engineering, Feng-Chia Unive ersity SUBROUTINE TO READ RECORD INTO BUFFER SUBROUTINE TO WRITE RECORD FROM BUFFER 16

9 Depar rtment of Electrical Engineering, Feng-Chia Unive ersity Object program with relocation by Modification i Records [ Figure 3.5 ] 17 Depar rtment of Electrical Engineering, Feng-Chia Unive ersity Relocatable program for a standard SIC machine [ Figure 3.6 ] 18

10 Depar rtment of Electrical Engineering, Feng-Chia Unive ersity SUBROUTINE TO READ RECORD INTO BUFFER SUBROUTINE TO WRITE RECORD FROM BUFFER 19 Depar rtment of Electrical Engineering, Object program with relocation by Bit Mask [ Figure 3.7 ] SIC Instruction Format 20

11 3.1 Basic Loader Functions Design of an Absolute Loader A Simple Bootstrap Loader Outline 3.2 Machine-Dependent Loader Features Relocation Program Linking Algorithm and Data Structures for a Linking Loader 3.3 Machine-Independent Loader Features Automatic Library Search Loader Options 3.4 Loader Design Options Linkage Editors Dynamic Linking Bootstrap Loaders 21 Program linking The basic concepts involved in program linking were introduced in Section In this section we consider more complex examples of external references between programs and examine the relationship between relocation and linking. The loader has no way of knowing (and no need to know) which control sections were assembled at the same time. Example Three (separately assembled) programs in Fig. 3.8, each of which consists of a single control section. 22

12 Depar rtment of Electrical Engineering, Feng-Chia Unive ersity Sample programs illustrating linking and relocation (PROGA) 23 Depar rtment of Electrical Engineering, Feng-Chia Unive ersity Object programs (PROGA) 24

13 Depar rtment of Electrical Engineering, Feng-Chia Unive ersity Sample programs illustrating linking and relocation (PROGB) 25 Depar rtment of Electrical Engineering, Feng-Chia Unive ersity Object programs (PROGB) 26

14 Depar rtment of Electrical Engineering, Feng-Chia Unive ersity Sample programs illustrating linking and relocation (PROGC) 27 Depar rtment of Electrical Engineering, Feng-Chia Unive ersity Object programs (PROGC) 28

15 Depar rtment of Electrical Engineering, Feng-Chia Unive ersity Program for Fig. 3.8 after linking and loading [ Figure 3.10(a) ] 29 Depar rtment of Electrical Engineering, Feng-Chia Unive ersity Relocation and linking operations performed on REF4 from PROGA [ Figure 3.10(b) ] 30

16 3.1 Basic Loader Functions Design of an Absolute Loader A Simple Bootstrap Loader Outline 3.2 Machine-Dependent Loader Features Relocation Program Linking Algorithm and Data Structures for a Linking Loader 3.3 Machine-Independent Loader Features Automatic Library Search Loader Options 3.4 Loader Design Options Linkage Editors Dynamic Linking Bootstrap Loaders 31 Algorithm and Data Structures for a Linking Loader The input to such a loader consists of a set of object programs (i.e., control sections) that are to be linked together. Thus a linking loader usually makes two passes over its input, just as an assembler does. Pass 1 : assigns addresses to all external symbols Pass 2 : performs the actual loading, relocation, and linking The main data structure needed for our linking loader is an external symbol table (ESTAB). 32

External Symbol Table (ESTAB) Store the name and address of each external symbol in the set of control sections being loaded. Also indicates in which control section the symbol is defined.

17 External Symbol Table (ESTAB) Store the name and address of each external symbol in the set of control sections being loaded. Also indicates in which control section the symbol is defined. Two other important variables: PROGADDR (program load address) is the beginning address in memory where the linked program is to be loaded. CSADDR (control section address) contains the starting address assigned to the control section currently being scanned by the loader. 33 Department of Electr rical Eng ineering, Feng-Chia Unive ersity Linking Loader Pass 1 Load Map 34

18 Depar rtment of Electrical Engineering, Feng-Chia Unive ersity Linking Loader Algorithm 35 Department of Electrical Engineering, Feng-Chia Unive ersity 36

19 3.1 Basic Loader Functions Design of an Absolute Loader A Simple Bootstrap Loader Outline 3.2 Machine-Dependent Loader Features Relocation Program Linking Algorithm and Data Structures for a Linking Loader 3.3 Machine-Independent Loader Features Automatic Library Search Loader Options 3.4 Loader Design Options Linkage Editors Dynamic Linking Bootstrap Loaders 37 C program Compiler Assembly language program Assembler Object: Machine language module Object: Library routine (machine language) Executable: e: Machine language program Memory 38

20 Automatic Library Search (1/3) Many linking loaders can automatically incorporate routines from a subprogram library into the program being loaded. In most cases there is a standard system library that is used in this way. Other libraries may be specified by control statements or by parameters to the loader. The subroutines called by the program being loaded are automatically fetched from the library, linked with the main program, and loaded. 39 Automatic Library Search (2/3) Linking loaders that support automatic library search must keep track of external symbols that are referred to, but not defined, in the primary input to the loader. At the end of Pass 1, the symbols in ESTAB that remain undefined represent unresolved external references. The loader searches the library or libraries specified for routines that contain the definitions of these symbols, and processes the subroutines found by this search exactly as if they had been part of the primary input stream. The process just described allows the programmer to override the standard subroutines in the library by supplying his or her own routines. 40

21 Feng-Chia Unive ersity Automatic Library Search (3/3) In most cases a special file structure is used for the libraries. This structure contains a directory that gives the name of each routine and a pointer to its address within the file. Some operating systems can keep the directory for commonly used libraries i permanently in memory. This can expedite the search process if a large number of external references are to be resolved Basic Loader Functions Design of an Absolute Loader A Simple Bootstrap Loader Outline 3.2 Machine-Dependent Loader Features Relocation Program Linking Algorithm and Data Structures for a Linking Loader 3.3 Machine-Independent Loader Features Automatic Library Search Loader Options 3.4 Loader Design Options Linkage Editors Dynamic Linking Bootstrap Loaders 42

22 Loader Options Many loaders allow the user to specify options that modify the standard processing. Many loaders have a special command language that is used to specify options. Sometimes there is a separate input file to the loader that contains such control statements. Sometimes these same statements can also be embedded in the primary input stream between object programs. On a few systems the programmer can even include loader control statements in the source program. On some systems options are specified as a part of the job control language that is processed by the O/S. 43 Command Language INCLUDE program-name (library-name) Include the designated objection program from a library as a part of input DELETE csect-name Delete the named control section from the set of programs being loaded CHANGE name1, name2 Replace the external symbol name1 with name2 LIBRARY library-name Specify alternative libraries to be searched NOCALL name1, name2 Instruct the loader that these external references are to remain unresolved 44

23 Example The source program in Fig 2.15 and the corresponding object program in Fig Three control sections: COPY, RDREC, WRREC Suppose now that a set of utility subroutines is made available on the computer system. Two of these, READ and WRITE, are designed to perform the same functions as RDREC and WRREC. A sequence of loader commands could be used to make this change without reassembling the program. INCLUDE INCLUDE DELETE CHANGE CHANGE READ(UTLIB) WRITE(UTLIB) RDREC, WRREC RDREC, READ WRREC, WRITE Basic Loader Functions Design of an Absolute Loader A Simple Bootstrap Loader Outline 3.2 Machine-Dependent Loader Features Relocation Program Linking Algorithm and Data Structures for a Linking Loader 3.3 Machine-Independent Loader Features Automatic Library Search Loader Options 3.4 Loader Design Options Linkage Editors Dynamic Linking Bootstrap Loaders 46

Depar rtment of Electrical Engineering, Feng-Chia Unive ersity The essential difference between a linking loader and a linkage editor [ Figure 3.

Linkage Editor Produces a linked version of the program (often called a load module or an executable image), which is written to a file or library for later execution.

24 Depar rtment of Electrical Engineering, Feng-Chia Unive ersity The essential difference between a linking loader and a linkage editor [ Figure 3.13 ] 47 Linking Loader Linkage Editors Performs all linking and relocation operations, including automatic library search if specified, and loads the linked program directly into memory for execution. Linkage Editor Produces a linked version of the program (often called a load module or an executable image), which is written to a file or library for later execution. When the user is ready to run the linked program, a simple relocating loader can be used to load the program into memory. This means that the loading can be accomplished in one pass with no external symbol table required. This approach reduces the overhead when the program is executed. All that is required at load time is a very simple form of relocation. 48

25 3.1 Basic Loader Functions Design of an Absolute Loader A Simple Bootstrap Loader Outline 3.2 Machine-Dependent Loader Features Relocation Program Linking Algorithm and Data Structures for a Linking Loader 3.3 Machine-Independent Loader Features Automatic Library Search Loader Options 3.4 Loader Design Options Linkage Editors Dynamic Linking Bootstrap Loaders 49 Linkage editor Dynamic Linking Perform linking operations before the program is loaded for execution Linking loader Perform linking operations at load time Dynamic linking (dynamic loading, load on call) Postpone the linking function until execution time A subroutine is loaded and linked to the rest of the program when it is first called 50

26 Feng-Chia Unive ersity Load-and-call ERRHANDL Dynamic Linking Dynamic Loader (part of the O/S) User Program 51 Dynamic Linking Dynamic Loader User Program ERRHANDL Library 52

27 Dynamic Linking Dynamic Loader User Program ERRHANDL Library 53 Dynamic Linking Dynamic Loader User Program ERRHANDL 54

28 Load-and-call ERRHANDL Dynamic Linking Dynamic Loader User Program ERRHANDL Library If a subroutine is still in memory, a second call to it may not require another load operation. 55 Dynamic Linking Dynamic Loader User Program ERRHANDL 56

29 3.1 Basic Loader Functions Design of an Absolute Loader A Simple Bootstrap Loader Outline 3.2 Machine-Dependent Loader Features Relocation Program Linking Algorithm and Data Structures for a Linking Loader 3.3 Machine-Independent Loader Features Automatic Library Search Loader Options 3.4 Loader Design Options Linkage Editors Dynamic Linking Bootstrap Loaders 57 Depar rtment of Electr rical Eng 0x00000 Bootstrap Loader 0x Registers A X L B S T F PC SW Bus Processing Unit Control Unit Fetch Unit Processor Loader ineering, Read-Only Memory (ROM) Operating System (O/S), Feng-Chia Unive ersity Memory (DRAM) Device "F1" ( Hard Disk / Flash) 58

Chapter 3 Loaders and Linkers

Chapter 3 Loaders and Linkers Three fundamental processes: Loading brings the object program into memory for execution. Relocation modifies the object program so that it can be loaded at an address different