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Loader. Role of Loader Source Program Translator Object Program Loader Object program ready for execution Memory Translator – Assembler/Compiler.

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Presentation on theme: "Loader. Role of Loader Source Program Translator Object Program Loader Object program ready for execution Memory Translator – Assembler/Compiler."— Presentation transcript:

1 Loader

2 Role of Loader Source Program Translator Object Program Loader Object program ready for execution Memory Translator – Assembler/Compiler

3 Role of Loader and Linker Source Program Assembler Object Program Linker Executable Code Loader Object program ready for execution Memory

4 Loader is a system s/w that makes a program ready for execution, by copying the file from secondary memory to main memory for execution. It is usually part of Operating system. The main task of the loader is to bring binary executable image into main memory and bind the relocatable addresses into absolute addresses. The binary image of a program consists of following parts :- Header – it shows the type of file (executable or library file) Text – it shows the actual code of the program List of shared libraries - libraries that have been used in the object file

5 Design of The absolute loader – An absolute object file consists of three part- 1)The start address of the program 2)The object instructions 3)The address of the first executable instruction. This is placed in the object file by assembler in response to the END directive. It is either the address specified by the END or, in the absence of such an address is identical to the first address of the program. The loader reads the first item and loads the rest of object file into successive memory locations.

6 Algorithm for absolute loader:- Begin Read Header record Verify program name and length Read first text record While record type is not ‘E’ do begin if object code is in character form { convert into internal representation } read next object program record end Jump to the address specified in End Record end

7 Role of Absolute Loader Object Program Absolute Loader Object program ready for execution Memory 1000 2000

8 Object Program H COPY 001000 00107A T 001000 1E 141033 482039 001036 281030 301015 482061... T 00101E 15 0C1036 482061 081044 4C0000 454F46 000003 000000 T 002039 1E 041030 001030 E0205D 30203F D8205D 281030 … T 002057 1C 101036 4C0000 F1 001000 041030 E02079 302064 … T 002073 07 382064 4C0000 05 E 001000


10 Absolute Loader Advantage - Simple and efficient Disadvantage - the need for programmer to specify the actual address - difficult to use subroutine libraries

11 Bootstrap Loader

12 A Simple Bootstrap Loader When a computer is first tuned on or restarted, a special type of absolute loader, called bootstrap loader is executed This bootstrap loads the first program to be run by the computer -- usually an operating system It is permanently stored in ROM. So it can not be erased.

13 Example (SIC bootstrap loader) The bootstrap itself begins at address 0 It loads the OS at starting address 0x80 No header record or control information, the object code is consecutive bytes of memory

14 After load the OS, the control is transferred to the instruction at address 80.

15 Algorithm of bootstrap loader Begin X=0x80 (the address of the next memory location to be loaded) Loop A  GETC ( and convert it from the ASCII character code to the value of the hexadecimal digit ) save the value in the high-order 4 bits of S A  GETC combine the value to form one byte A  (A+S) store the value (in A) to the address in register X X  X+1 End

16 GETC reads one character and convert it into hexadecimal digit. For example, the ASCII code for ‘1’ through ‘9’ are converted to the numeric values 1 through 9. and codes for ‘A’ through ‘F’ ( 41 to 46 Hex) are converted to the values 10 through 15. this is accomplished by subtracting 48 ( 30 in Hex) from the character codes of 0-9 and by subtracting 55 (37 in Hex) from the codes of A-F ASCII value of 0~9 : 0x30~39 A~F : 0x41~46

17 Machine Dependent Loader Features Relocation Program Linking

18 Example of Relocation LineLoc Source statementObject code 50000COPY START 0 100000FIRST STL RETADR17202D 120003 LDB #LENGTH69202D 13 BASE LENGTH 150006CLOOP+JSUB RDREC4B101036 20000A LDA LENGTH032026 25000D COMP#0290000 300010 JEQ ENDFIL332007 350013+JSUB WRREC4B10105D 400017 J CLOOP3F2FEC 45001AENDFIL LDA EOF032010 50001D STA BUFFER0F2016 550020 LDA#3010003 600023 STA LENGTH0F200D 650026+JSUB WRREC4B10105D 70002A J@RETADR3E2003 80002DEOF BYTE C’EOF’454F46 950030RETADR RESW 1 1000033LENGTH RESW 1 1050036BUFFER RESB 4096

19 1251036RDREC CLEAR XB410 130 132 133 135 140 145 150 155 160 165 170 175 180 185 210105DWRREC CLEAR XB410 212 215 220 225 230 235...(omitted)

20 Instructions at line no 15,35 & 65 are the only items whose values are affected by relocation. We use modification records for the relocation purpose. Each modification record specifies the starting address and the length of the field whose value to be altered. It then describes the modification to be performed. Modification record col 1: M col 2-7: relocation address col 8-9: length (halfbyte) col 10: flag (+/-) col 11-17: segment name

21 Modification Record H  COPY  000000 001077 T  000000  1D  17202D  69202D  48101036  …  4B105D  3F2FEC  032010 T  00001D  13  0F2016  010003  0F200D  4B10105D  3E2003  454F46 T  001035  1D  B410  B400  B440  75101000  …  332008  57C003  B850 T  001053  1D  3B2FEA  134000  4F0000  F1 ..  53C003  DF2008  B850 T  00070  07  3B2FEF  4F0000  05 M  000007  05+COPY M  000014  05+COPY M  000027  05+COPY E  000000

22 Begin Get PROGADDR from OS While not end of input do { read next record while record type != ‘E’ do { read next input record while record type = ‘T’ do {move object code from record to location ADDR + specified address } while record type = ‘M’ add PROGADDR at the location PROGADDR + specified address } end Relocation Loader Algorithm

23 Modification record scheme is not well suited for use with all machine architectures and if many instructions require relocation then the number of modification record will be very large. To solve this problem we can use relocation bit method. If the relocation bit corresponding to a word of object code is set to 1, the program’s starting address is to be added to this word when the program is relocated. Relocation bit - 0: no modification is necessary - 1: modification is needed Twelve-bit mask is used in each Text record Since each text record contains < 12 words Unused words are set to 0

24 Text record format Text record col 1: T col 2-7: starting address col 8-9: length (byte) col 10-12: relocation bits col 13-72: object code

25 Relocatable Program for SIC LineLoc Source statementObject code 50000COPYSTART0 100000FIRSTSTLRETADR140033 150003CLOOPJSUBRDREC481039 200006LDALENGTH000036 250009COMPZERO280030 30000CJEQENDFIL300015 35000FJSUBWRREC481061 400012JCLOOP3C0003 450015ENDFILLDAEOF00002A 500018STABUFFER0C0039 55001BLDATHREE00002D 60001ESTALENGTH0C0036 650021JSUBWRREC481061 700024LDLRETADR080033 750027RSUB4C0000 80002AEOFBYTEC’EOF’454F46 85002DTHREEWORD3000003 900030ZEROWORD0000000 950033RETADRRESW1 1000036LENGTHRESW1 1050039BUFFERRESB4096

26 1251039RDRECLDXZERO040030 130103CLDAZERO000030 135103FRLOOPTDINPUTE0105D 1401042JEQRLOOP30103D 1451045RDINPUT D8105D 1501048COMPZERO280030 155104BJEQEXIT301057 160104ESTCHBUFFER,X548039 1651051TIXMAXLEN2C105E 1701054JLTRLOOP38103F 1751057EXITSTXLENGTH100036 180105ARSUB4C0000 185105DINPUTBYTEX’F1’F1 190105EMAXLENWORD4096001000 2101061WRRECLDXZERO040030 2151064WLOOPTDOUTPUTE01079 2201067JEQWLOOP301064 225106ALDCHBUFFER,X508039 230106DWDOUTPUTDC1079 2351070TIXLENGTH2C0036 2401073JLTWLOOP381064 2451076RSUB4C0000 2501079OUTPUTBYTEX’05’05 255ENDFIRST

27 Relocation by Bit Mask H COPY 000000 00107A T 000000 1E FFC 140033 481039 000036 280030 300015 481061... T 00001E 15 E00 0C0036 481061 080044 4C0000 454F46 000003 … T 001039 1E FFC 040030 000030 E0105D 30103F D8105D … T 001057 1C 800 100036 4C0000 F1 001000 T 001061 19 FE0 040030 E01079 301064 508039 DC1079 2C0036 … E 000000 Here the bit mask FFC (representing the bit string 111111111100) In the first text record specifies that all 10 words of object code are to be modified during relocation. Similarly bit mask E00 represents that the first 3 words are to be modified.

28 Machine Independent Loader Features Automatic library search Loader options

29 Automatic library search :- This feature allows a programmer to use standard subroutines without explicitly including them in the program to be loaded. So programmer can use any subroutine by just mentioning their names. The subroutines are automatically fetched from the library, linked with the main program and loaded. Loader maintains an External Symbol Table (ESTAB) for these external symbols. The loader searches the library specified for routines that contain the definitions of these symbols, and process the subroutines found by this search. In most cases a special file structure is used for libraries. This structure contains a directory that gives the name of each routine and a pointer to its address within the file. NamePointer

30 Loader Options :- Many loader allow user to specify options that modify the standard processing. Loaders have a special command language that is used to specify the options. Options:- 1)INCLUDE 2)DELETE 3)CHANGE 4)LIBRARY

31 1)INCLUDE:- this loader option allows the selection of alternative sources of input. For example, the command INCLUDE program-name (library name) Might direct the loader to read the designated object program from a library and treat it as the primary loader input. 2) DELETE:- it allows the user to delete external symbols or entire control sections. For example, the command DELETE csect-name might instruct the loader to delete the name control section from the set of programs being loaded. 3) CHANGE :- it allows the user to change name1 to name 2 in the program. For example CHAMGE name1, name 2 Might cause the external symbol name1 to be changed to name 2 wherever it appears in the object programs

32 LineLoc Source statementObject code 50000COPY START 0 10 12 13 150006CLOOP+JSUB RDREC4B101036 30 350013+JSUB WRREC4B10105D 40 650026+JSUB WRREC4B10105D 70 105 1251036RDREC CLEAR XB410 130 180 185 210105DWRREC CLEAR XB410 212 230 235...(omitted) Example of Relocation INCLUDE READ(ULIB) INCLUDE WRITE(ULIB) DELETE RDREC,WRREC CHANGE RDREC, READ CHANGE WRREC, WRITE

33 These command would direct the loader to include control sections READ & WRITE from the library ULIB, and to delete the control sections RDREC and WRREC from the load. The first CHANGE command would cause all external references to symbol RDREC to be changed to refer to symbol.

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