Prepared By : Ms. Sanchari Saha ( Asst. Professor) Department : Computer Science & Engineering

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1 Subject Name: System Software Subject Code: 10CS52 Prepared By : Ms. Sanchari Saha ( Asst. Professor) Department : Computer Science & Engineering Date :

2 UNIT-4 LOADERS & LINKERS Engineered for Tomorrow Prepared by- Sanchari Saha, Assistant Professor, Dept of CSE, MVJCE

3 Topic Details 1) Basic Loader Functions - Design of an Absolute Loader 2) A Simple Bootstrap Loader 3) Machine-Dependent Loader Features Relocation, Program Linking, Algorithm and Data Structures for a Linking Loader 4) Machine-Independent Loader Features - Automatic Library Search, Loader Options, 5) Loader Design Options - Linkage Editor, Dynamic Linkage, Bootstrap Loaders, 6) Implementation Examples - MS-DOS Linker. 7) Chapter Summary 8) Self-check exercise

4 Learning Outcome: Students will understand:- The basic functionality of loaders & linkers Relationship between linking & relocation Working principle of Bootstrap loader, absolute loader Implementation example-ms-dos linker

5 1. Basic Loader Functions - Design of an Absolute Loader Engineered for Tomorrow

6 1.1 Three Working Items Loading: loading an object program into memory for execution. Relocation: modify the object program so that it can be loaded at an address from the location originally specified. Linking: combines two or more separate object programs and supplies the information needed to allow references between them. A loader is a system program that performs the loading function. Many loaders also support relocation and linking. Some systems have a linker to perform the linking and a separate loader to handle relocation and loading.

7 1.2 Design of an Absolute Loader An object program is loaded at the address specified on the START directive. No relocation or linking is needed, thus is very simple

9 1.3 Absolute loader implementation 14 occupies two bytes if it is represented in char form. When loaded into memory, 14 should occupy only one byte.

10 2. A Simple Bootstrap Loader Engineered for Tomorrow

11 Bootstrap Loader: When a computer is first turned on or restarted, a special type of absolute loader must be executed (stored in ROM on a PC). The bootstrap loader loads the first program to be run by the computer usually the operating system, from the boot disk (e.g., a hard disk or a floppy disk) It then jumps to the just loaded program to execute it. Normally, the just loaded program is very small (e.g., a disk sector s size, 512 bytes) and is a loader itself. The just loaded loader will continue to load another larger loader and jump to it. This process repeats another the entire large operating system is loaded. The just loaded loader will continue to load another larger loader and jump to it. This process repeats another the entire large operating system is loaded

12 Bootstrap Loader Example Convert 14 in char form to 14 in byte form

13 Bootstrap Loader Example Convert 1 in char form to 1 in its ASCII code

14 3. Machine Dependent Loader Features Engineered for Tomorrow

15 3.1 Relocating Loader Two methods to describe where in the object program to modify the address (add the program starting address) Use modification records Suitable for a small number of changes Use relocation bit mask Suitable for a large number of changes

16 Program Written in SIC/XE

18 This program is written in SIC/XE instructions. Program counterrelative and base-relative addressing are extensively used to avoid the need for many address modification records.

19 The Object Program Only lines 15, 35, and 65 need to be modified.

20 The Same Program Written in SIC

22 This program is written in SIC instructions. Only direct addressing can be used. As such, we need many modification records. This not only makes the object program bigger, it also slows down the loading process.

23 Relocation Bit Mask If an object needs too many modification records, it would be more efficient to use a relocation bit mask to indicate where in the object program should be modified when the object program is loaded. A relocation bit is associated with each word of object code. Since all SIC instructions occupy one word, this means that there is one relocation bit for each possible instruction. If the relocation bit corresponding to a word of object code is set to 1, the program s starting address will be added to this word when the program is relocated.

24 Relocation Bit Mask Example This one-byte F1 makes the LDX instruction on line 210 begins a new text record. This is because each relocation bit should be associated with a three-byte word. However, this data item occupies only one byte, which violates the Alignment rule.

25 Program Linking A program may be composed of many control sections. These control sections may be assembled separately. These control sections may be loaded at different addresses in memory. External references to symbol defined in other control sections can only be resolved (calculating their addresses in memory) after these control sections are loaded into memory.

26 Program Linking Example

29 Object Program Example

32 After Loading into Memory Suppose that program A is loaded at , program B at , and program C at 0040E2. Notice that REF4, REF5, REF6, and REF7 in all of these three programs have the same values.

34 Algorithm & Data Structure A linking loader makes two passes over its input In pass 1: assign addresses to external references In pass 2: perform the actually loading, relocation, and linking Very similar to what a two-pass assembler does.

35 Data Structures External symbol tables (ESTAB) Like SYMTAB, store the name and address of each external symbol in the set of control sections being loaded. It needs to indicate in which control section the symbol is defined. PROGADDR The beginning address in memory where the linked program is to be loaded. (given by the OS) CSADDR It contains the starting address assigned to the control section currently being scanned by the loader. This value is added to all relative addresses within the control sections.

38 Reference Number The linking loader algorithm can be made more efficient if we assign a reference number to each external symbol referred to in a control section. This reference number is used (instead of the symbol name) in modification record. This simple technique avoid multiple searches of ESTAB for the same symbol during the loading of a control section. After the first search for a symbol (the REFER records), we put the found entries into an array. Later in the same control section, we can just use the reference number as an index into the array to quickly fetch a symbol s value.

39 Reference Number Example Reference number 01 is reserved for the current control section name. All other reference numbers start from 02.

40 Reference Number Example

41 Reference Number Example

42 Reference Number Example

43 4. Machine Independent Features Engineered for Tomorrow

44 Automatic Library Search Many linking loaders can automatically incorporate routines from a subprogram library into the program being loaded. (E.g., the standard C library) The subroutines called by the program are automatically fetched from the library, linked with the main program, and loaded. The programmer does not need to take any action beyond mentioning the subroutine names as external references in the source program

45 Linking loader 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 for routines that contain the definitions of these symbols, and processes the subroutines found by this search process exactly as if they had been part of the primary input stream.

46 The subroutines fetched from a library in this way may themselves contain external references. It is necessary to repeat the library search process until all references are resolved. If unresolved references remain after the library search is completed, they are treated as errors. If a symbol (or a subroutine name) is defined both in the source program and in the library, the one in the source program is used first. A programmer can make his own library easily on UNIX by using the ar command.

47 Loader Options Many loaders allow the user to specify options that modify the standard processing. For example: Include program-name (library name) Direct the loader to read the designated object program from a library Delete csect-name Instruct the loader to delete the named control sections from the set of programs being loaded Change name1, name2 Cause the external symbol name1 to be changed to name2 wherever it appears in the program

48 Loader Option Application In the COPY program, we write two subroutines RDREC and WRREC to perform read records and write records. Suppose that the computer system provides READ and WRITE subroutines which has similar but advanced functions. Without modifying the source program and reassembling it, we can use the following loader options to make the COPY object program use READ rather than RDREC and WRITE rather than WRREC. Include READ (Util) Include WRITE (Util) Delete RDREC, WRREC Change RDREC, READ Change WRREC, WRITE

49 Loader Design Options Linkage Editor The difference between a linkage editor and a linking loader: A linking loader performs all linking and relocation operations, including automatic library search, and loads the linked program into memory for execution. A linkage editor produces a linked version of the program, which is normally written to a file for later execution.

50 When the user is ready to run the linked program, a simple relocating loader can be used to load the program into memory. The only object code modification necessary is the addition of an actual address to relative values within the program. The linkage editor performs relocation of all control sections relative to the start of the linked program.

51 All items that need to be modified at load time have values that are relative to the start of the linked program. This means that the loading can be accomplished in one pass with no external symbol table required. Thus, if a program is to be executed many times without being reassembled, the use of a linkage editor can substantially reduces the overhead required. Resolution of external references and library searching are only performed once.

52 Dynamic Linking Linkage editors perform linking before the program is loaded for execution. Linking loaders perform these same operations at load time. Dynamic linking postpones the linking function until execution time. A subroutine is loaded and linked to the test of the program when it is first called.

53 Dynamic Linking Application Dynamic linking is often used to allow several executing programs to share one copy of a subroutine or library. For example, a single copy of the standard C library can be loaded into memory. All C programs currently in execution can be linked to this one copy, instead of linking a separate copy into each object program.

54 Dynamic Linking Application In an object-oriented system, dynamic linking is often used for references to software object. This allows the implementation of the object and its method to be determined at the time the program is run. (e.g., C++) The implementation can be changed at any time, without affecting the program that makes use of the object.

55 Dynamic Linking Advantage The subroutines that diagnose errors may never need to be called at all. However, without using dynamic linking, these subroutines must be loaded and linked every time the program is run. Using dynamic linking can save both space for storing the object program on disk and in memory, and time for loading the bigger object program.

56 On PC Windows or UNIX operating systems, normally you are using (e.g., ld) a linkage editor to generate an executable program.

57 Dynamic Linking Implementation A subroutine that is to be dynamically loaded must be called via an operating system service request. This method can also be thought of as a request to a part of the loader that is kept in memory during execution of the program Instead of executing a JSUB instruction to an external symbol, the program makes a load-and-call service request to the OS. The parameter of this request is the symbolic name of the routine to be called..

58 The OS examines its internal tables to determines whether the subroutine is already loaded. If needed, the subroutine is loaded from the library. Then control is passed from the OS to the subroutine being called. When the called subroutine completes its processing, it returns to its caller (operating system). The OS then returns control to the program that issues the request. After the subroutine is completed, the memory that was allocated to it may be released.

59 However, often this is not done immediately. If the subroutine is retained in memory, it can be used by later calls to the same subroutine without loading the same subroutine multiple times. Control can simply pass from the dynamic loader to the called routine directly.

60 Issue a load-andcall service request Load the called subroutine into memory

62 Bootstrap Loaders Given an idle computer with no program in memory, how do we get things started? With the machine empty and idle, there is no need for program relocation Some early computers required the operator to enter into memory the object code for an absolute loader, using switches on the computer console One some computer, an absolute loader program is permanently resident in a ROM A built-in hardware function that reads a fixed-length record form some device into memory at a fixed location

63 When a computer is first turned on or restarted, a special type of absolute loader must be executed (stored in ROM on a PC). The bootstrap loader loads the first program to be run by the computer usually the operating system, from the boot disk (e.g., a hard disk or a floppy disk) It then jumps to the just loaded program to execute it. Normally, the just loaded program is very small (e.g., a disk sector's size, 512 bytes) and is a loader itself. The just loaded loader will continue to load another larger loader and jump to it. This process repeats another the entire large operating system is loaded

65 Engineered for Tomorrow 5. Self Check Exercise

66 1) Write a sequence of instructions to set ALPHA=BETA*GAMMA 2) Write a sequence of instruction to set ALPHA to the integer portion of BETA/GAMMA 3) Write a sequence of instructions to set ALPHA=GAMMA*BETA-9 4) Write a sequence of instructions to set ALPHA=4*BETA-9 5) Write a sequence of instructions to set ALPHA=BETA+DECREMENT+1 6) Write SIC instructions to swap the values of ALPHA & BETA 7) Write a sequence of instructions in SIC/XE to add 2 arrays of 100 integers. 8) Write sequence of instructions for SIC/XE to divide BETA by GAMMA setting ALPHA to the integer portion of quotient & DELTA to the remainder 9) Write a sequence of instructions for SIC to clear 20 bytes of string to all blanks 10) Write a sequence of instructions for SIC & SIC/XE to copy the string SYSTEM SOFTWARE to another string. 11) Write a sequence of instructions for SIC to clear 20 bytes of string to all blanks using register to register instruction & immediate addressing. 12) Suppose that ALPHA is an array of 100 words. Write a sequence of instructions for SIC to set all 100 elements of the array to 0. 13) Bring out the difference between system software & application software 14) Explain the following w.r.to both SIC & SIC/XE Instruction format, addressing mode, data formats, Register organization 15) Give the target address generated for the following machine instructions: H, 03C300H, 0310C303 If B=006000, PC=003000, X=000090

67 Engineered for Tomorrow 6. Summary

68 1) Write a sequence of instructions to set ALPHA=BETA*GAMMA 2) Write a sequence of instruction to set ALPHA to the integer portion of BETA/GAMMA 3) Write a sequence of instructions to set ALPHA=GAMMA*BETA-9 4) Write a sequence of instructions to set ALPHA=4*BETA-9 5) Write a sequence of instructions to set ALPHA=BETA+DECREMENT+1 6) Write SIC instructions to swap the values of ALPHA & BETA 7) Write a sequence of instructions in SIC/XE to add 2 arrays of 100 integers. 8) Write sequence of instructions for SIC/XE to divide BETA by GAMMA setting ALPHA to the integer portion of quotient & DELTA to the remainder 9) Write a sequence of instructions for SIC to clear 20 bytes of string to all blanks 10) Write a sequence of instructions for SIC & SIC/XE to copy the string SYSTEM SOFTWARE to another string. 11) Write a sequence of instructions for SIC to clear 20 bytes of string to all blanks using register to register instruction & immediate addressing. 12) Suppose that ALPHA is an array of 100 words. Write a sequence of instructions for SIC to set all 100 elements of the array to 0. 13) Bring out the difference between system software & application software 14) Explain the following w.r.to both SIC & SIC/XE Instruction format, addressing mode, data formats, Register organization 15) Give the target address generated for the following machine instructions: H, 03C300H, 0310C303 If B=006000, PC=003000, X=000090

69 Engineered for Tomorrow END of UNIT-4

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