LOADERS AND LINKERS 1. BASIC LOADER FUNCTIONS 2. DESIGN OF AN ABSOLUTE LOADER 3. A SIMPLE BOOTSTRAP LOADER 4. MACHINE DEPENDENT LOADER FEATURES

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1 UIT III LOADERS AD LIKERS 1. BASIC LOADER FUCTIOS 2. DESIG OF A ABSOLUTE LOADER 3. A SIMPLE BOOTSTRAP LOADER 4. MACHIE DEPEDET LOADER FEATURES 5. RELOCATIO 6. PROGRAM LIKIG 7. ALGORITHM AD DATA STRUCTURES FOR LIKIG LOADER 8. MACHIE-IDEPEDET LOADER FEATURES 9. AUTOMATIC LIBRAR SEARCH 10. LOADER OPTIOS 11. LOADER DESIG OPTIOS 12. LIKAGE EDITORS 13. DAMIC LIKIG 14. BOOTSTRAP LOADERS 15. IMPLEMETATIO EXAMPLE 16. MSDOS LIKER.

2 LIKERS AD LOADERS 1. Introduction Installing a code in memory is called loading. Memory Assembler Loader Obj. prog. Source program Object program Figure 1. Loading an object code into memory The task of integrating the program modules together is called linking. Module1 relocatable object modules Assembler Linker Loader Module2 Source modules Module 3 Linked Object modules Figure 2. Linking and loading as split processes

3 Linking and loading are frequently done at one time by a linking loader. Module 1 Assembler Linking Loader Module 2 relocatable Source object modules modules object Module 3 modules Figure 3. Linking loader Types of loaders: 1. Absolute loaders 2. Relocating loaders 3. Linking loaders 2. Absolute Loaders Assembler generates the code and writes instructions in a file together with their load addresses. The loader reads the file and places the code at the absolute address given in the file. Example 1: Assume that the assembler generates the following code: Address: instruction: 0100 F B F

4 The above code is written in the file as follows: 0100 location (2 bytes) 5 number of bytes F code 47 B location 4 number of bytes 05 F2 code EOF

5 read 2 bytes EOF marker return set LC to the byte read read a byte and set B to it read a byte place it into the memory location pointed to by LC Counter of Bytes B = B 1 LC = LC + 1 LC: Location B: umber B>0 Figure 4. Absolute loader

6 3. Relocating Loader independently assembled modules Assembler relocation information Relocating Loader Example 2: Assume that the following two relocatable codes and the associated DATs are generated by the assembler: Address: instruction: 0000 F B5 DAT F DAT 0002

7 The relocating loader adds the load-point of the code to all the references specified in DAT. If the load-points of the above programs are 500 and 700, they are placed in the memory as follows: Memory A B F

8 Get load-point LC=0 Read a byte EOF marker return LC is in DAT Read next byte Place the byte at memory location LC + Load Point Add load point Place the bytes at memory locations LC + Load Point and LC + Load Point + 1 LC = LC + 1 LC = LC + 2 Figure 5. Relocating loader

9 4. Linking Loader Linking loaders perform four functions: 1. Allocation: allocating space in memory for programs 2. Linking: resolving external references between modules 3. Relocation: adjusting all address references 4. Loading: physically placing machine instructions and data in memory Entry and External Points When a statement in one module refers to a symbol in another module, it is called an external reference (EXTREF). Any symbol in a module that can be referred to from another module is called an entry point (EXTDEF). Module A Module B EXTERAL ALPHA, BETA ETR ALPHA, BETA LDA ALPHA ALPHA: LXI BETA BETA:

10 Example 3: The assembler has generated the object codes of the following source programs where external references have not been resolved. These codes will be linked and loaded into memory by a linking-loader. Source programs Code generated by the assembler H R R R PROGA SIZE BUF SUM PROGA: START 0 address EXTREF SIZE, BUF, SUM LDA # STA SIZE C LDA # LDX # L1: STA BUF, X 000C 0F ADD #1 000F TIX SIZE C JEQ L B (placed by the assembler in pass 2) J L C 00 0C (placed by the assembler in pass 1) L2: JSUB SUM 001B 48 RSUB 001E 4F ED DAT 0016 DAT 0019 M SIZE 0004 M BUF 000D M SIZE 0013 M SUM 001B H PROGB D SUM 0000 R BUF R SIZE R TOT PROGB: START 0 EXTDEF SUM EXTREF SIZE, BUF, TOT SUM: LDA # LDX # L3: ADD BUF, X B TIX SIZE C JEQ L4 000C (placed by the assembler in pass 2) J L3 000F 3C (placed by the assembler in pass 1)

11 L4: STA TOT C RSUB F ED DAT 000D DAT 0010 M BUF 0007 M SIZE 000A M TOT 0013 H PROGC D SIZE 0000 D TOT 0003 D BUF 0006 PROGC: START 0 EXTDEF SIZE, BUF, TOT SIZE: RESW TOT: RESW BUF: RESW ED C Relocatable machine codes Assembler DATs Linker H/D/R/M Information The linker does two passes: Pass1: 1. Gets load points of all programs. 2. Creates ESTAB table, using the information in D/R table. Calculates the absolute address of each label in ESTAB as: load point + relative address. Pass 2: 1. Using the M information and the absolute addresses in ESTAB, resolves the external references in all programs. 2. Using the DAT information, adds load points to the local references.

12 ESTAB program load point label absolute address PROGA 1000 PROGB 2000 SUM 2000 PROGC 3000 SIZE 3000 TOT 3003 BUF 3006 Source programs Code generated Code generated by the assembler at the end of linking address address PROGA: START 0 EXTREF SIZE, BUF, SUM LDA # STA SIZE C C LDA # LDX # L1: STA BUF, X 000C 0F 000C 0F ADD #1 000F F TIX SIZE C C JEQ L B B J L C 00 0C C 10 0C L2: JSUB SUM 001B B RSUB 001E 4F E 4F ED PROGB: START 0 EXTDEF SUM EXTREF SIZE, BUF, TOT SUM: LDA # LDX # L3: ADD BUF, X B B TIX SIZE C C JEQ L4 000C C J L3 000F 3C F 3C L4: STA TOT C C RSUB F F ED

13 PROGC: START 0 EXTDEF SIZE, BUF, TOT SIZE: RESW TOT: RESW BUF: RESW ED C 000C Get Load Point Read program name and enter in ESTAB Read a record D record Enter symbol in ESTAB and calculate absolute address end of program end of all programs Pass 2 Figure 6. Pass 1 of the Linker

14 Get load point from ESTAB and set LC to the load point Read a record object code Write it in the memory location pointed by LC M record symbol in ESTAB pointed by Find absolute address of the and write it in memory location LC + relative location DAT record Add load point to the value in memory location pointed by LC + DAT entry more records Done Figure 7. Pass 2 of the Linker

15 Linked codes are combined into a single file in the following format: File format: File name File type umber of blocks Load point (2 bytes) umber of bytes Code Header Block 1 Checksum Block Checksum: To calculate checksum, add up all data in the block and take its low order byte. When writing the code to the file, calculate checksum and write it at the end of each block. When installing the code in memory, recompute checksum and compare it with the one read from the file. Figure 8. Absolute loader, which reads the linked codes from a file and loads them into the memory.

16 Read header B = block count Read load point LC = load point Read byte count BC = byte count Initialize checksum Read a byte Write the byte in memory at location pointed to by LC Add the byte to checksum LC = LC + 1 BC = BC 1 BC>0 Read checksum byte Checksum OK B = B 1 print error message B > 0 B: umber of blocks LC: Location counter BC: Byte count Return

17 5. Linking with Libraries Library contains the relocatable image of each module. User programs are assembled individually. The library routines used in user programs appear as external references. Library routines Relocatable machine code DAT D/R/M Information user Relocatable machine code programs DAT Linker D/R/M Information Most linkers execute a default transparent search of a set of libraries that contain most of the commonly called routines. Searches are performed at the end of Pass 1 Library routines may have global symbols to be resolved, linker performs iterative search until all symbols are resolved Library files are designed to expedite the search. It is not necessary to go through the whole file, instead, a library file header is reviewed, usually located at the beginning of a file. The header contains all information. Ex: Library file header Routine ame Global Symbols Location in the File Library routines are linked with user programs, between Pass 1 and Pass 2, while the linker is resolving external references.

18 Pass 2 more references Take an External reference Resolve it Entry found for it in user programs Check all libraries (or only those specified by the user) to find an Entry for it found print error message Calculate a load point for the library routine Configure direct addresses in the modules Append the library routine to the user file Merge the new D/R information to the ESTAB Figure 9. Linking the library routines with the user programs.

19 Dynamic Address Resolution In dynamic linking, a subroutine is loaded and linked to the other programs when it is first called during the execution. Dynamic linking allows several executing programs to share the same copy of a subroutine. For example, a single copy of the routines in a dynamic linking library (run-time library) can be loaded into the memory, and all currently executing programs can be linked to this copy. Dynamic linking provides the ability to load the routines only when they are needed. For example, error correction routines are not need to be linked and loaded if no errors occur during the program. Dynamic linking also avoids the necessity of loading the entire library for each execution. For example, a program may be calling a different routine depending on the input data. Dynamic linking and loading has the following steps: 1. The user program makes a Load-and-Call request to the operating system (dynamic loader). 2. The operating system examines its internal tables to determine whether or not the routine is already loaded. If not, loads it into the memory. Then it transfers control to the routine. 3. After the subroutine is processed, it returns control to the operating system. 4. The operating system returns control to the program that issued the request. 5. The subroutine may be retained in the memory for later use. Machine Independent Loader Features Automatic Library Search Automatic library call The programmer does not need to take any action beyond mentioning the subroutine names as external references Solution 1 Enter the symbols from each Refer record into ESTAB 2 When the definition is encountered (Define record), the address is assigned 3 At the end of Pass 1, the symbols in ESTAB that remain undefined represent unresolved external references 4 The loader searches the specified (or standard) libraries for undefined symbols or subroutines The library search process may be repeated Since the subroutines fetched from a library may themselves contain external references Programmer defined subroutines have higher priority The programmer can override the standard subroutines in the library by supplying their own routines Library structures Assembled or compiled versions of the subroutines in a library can be

20 structured using a directory that gives the name of each routine and a pointer to its address within the library Loader Options Many loaders have a special command language that is used to specify options. The commands may be: In a separate input file In the source program Embedded in the primary input stream between programs Command Language Specifying alternative sources of input ICLUDE program-name(library-name) Changing or deleting external reference DELETE name CHAGE symbol1, symbol2 Controlling the automatic library search LIBRAR MLIB Specify that some references not be resolved OCALL name Specify the location at which execution is to begin Example If we would like to evaluate the use of READ and WRITE instead of RDREC and WRREC, for a temporary measure, we use the following loader commands ICLUDE READ(UTLIB) ICLUDE WRITE(UTLIB) DELETE RDREC, WRREC CHAGE RDREC, READ CHAGE WRREC, WRITE If it is know that the statistical analysis is not to be performed in an execution OCALL STDDEV, PLOT, CORREL Loader Design Options Linkage Editors Definition A 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 a library for later execution Procedure A linkage editor performs relocation of all control sections relative to the start of the linked program, resolves all external reference, and output a relocatable module for later execution A simple relocating loader can be used to load the program into memory (one-pass without external symbol table) Linking Loader vs. Linkage Editors Comparison Linking Loader: performs all linking and relocation operations, including library search if specified, and loads the linked program directly into memory for execution Linkage Editors: produces a linked version of the program (often called a load module or an executable image), which is written onto a file or library for later execution Resolution of external reference and library searching are only performed once for

21 linkage editor If a program is to be executed many times without being reassembled, the use of a linkage editor substantially reduces the overhead required. If a program is under development or is used infrequently, the use of a linking loader outperforms a linkage editor Dynamic Linking Comparison Linkage editors perform linking operations before the program is loaded for execution Linking loaders perform linking operations at load time Dynamic linking (dynamic loading, load on call) perform linking at execution time Delayed Binding Avoid the necessity of loading the entire library for each execution, i.e. load the routines only when they are needed Allow several executing programs to share one copy of a subroutine or library (Dynamic Link Library, DLL) Via an OS Dynamic loader is one part of the OS Instead of executing a JSUB instruction that refers to an external symbol, the program makes a load-and-call service request to the OS

22 Pass of control User program OS OS: load the subroutine OS Subroutine Subroutine OS OS User program

23 Bootstrap loaders When a computer is first turned on or restarted, bootstrap loader is executed. Bootstrap loader is a simple absolute loader. Its function is to load the first system program to be run by the computer, which is the operating system or a more complex loader that loads the rest of the system. Bootstrap loader is coded as a fixed-length record and added to the beginning of the system programs that are to be loaded into an empty system. A built-in hardware or a very simple program in ROM reads this record into the memory and transfers control to it. When it is executed, it loads the following program, which is either the operating system itself or other system programs to be run without the operating system. MS DOS linker object file (.OBJ) generated by assembler (or compiler) format THEADR name of this object module PUBDEF external symbols defined in this module EXTDEF external symbols used here TPDEF data types for pubdef and extdef SEGDEF describes segments in this module GRPDEF segment grouping LAMES name list indexed by segdef and grpdef LEDATA binary image of code LIDATA repeated data FIXUPP modification record MODED end LIK pass 1:» allocates segments defined in SEGDEF» resolve external symbols pass 2:» prepare memory image if needed, disk space is also used» expand LIDATA» relocations within segment» write.exe file

3.3 Machine-Independent Loader Features

3.3 Machine-Independent Loader Features Loading and linking are often thought of as operating system service functions. Machine independent loader features: 3.3.1 Automatic Library Search 3.3.2 Loader