loader.ppt
- 1. 1 Loaders and Linkers Chapter 3 System Software An introduction to systems programming Leland L. Beck
- 2. 2 Introduction To execute an object program, we needs » Relocation, which modifies the object program so that it can be loaded at an address different from the location originally specified » Linking, which combines two or more separate object programs and supplies the information needed to allow references between them » Loading and Allocation, which allocates memory location and brings the object program into memory for execution
- 3. 3 Overview of Chapter 3 Type of loaders » assemble-and-go loader » absolute loader (bootstrap loader) » relocating loader (relative loader) » direct linking loader Design options » linkage editors » dynamic linking » bootstrap loaders
- 4. 4 Assemble-and-go Loader Characteristic » the object code is stored in memory after assembly » single JUMP instruction Advantage » simple, developing environment Disadvantage » whenever the assembly program is to be executed, it has to be assembled again » programs have to be coded in the same language
- 5. 5 Design of an Absolute Loader Absolute Program » Advantage – Simple and efficient » Disadvantage – the need for programmer to specify the actual address – difficult to use subroutine libraries Program Logic
- 6. 6 Fig. 3.2 Algorithm for an 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} move object code to specified location in memory read next object program record end jump to address specified in End record end
- 7. 7 Object Code Representation Figure 3.1 (a) » each byte of assembled code is given using its hexadecimal representation in character form » easy to read by human beings In general » each byte of object code is stored as a single byte » most machine store object programs in a binary form » we must be sure that our file and device conventions do not cause some of the program bytes to be interpreted as control characters
- 8. 8 A Simple Bootstrap Loader 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 Example (SIC bootstrap loader) » The bootstrap itself begins at address 0 » It loads the OS starting address 0x80 » No header record or control information, the object code is consecutive bytes of memory
- 9. 9 Fig. 3.3 SIC Bootstrap Loader Logic Begin X=0x80 (the address of the next memory location to be loaded Loop AGETC (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 AGETC combine the value to form one byte A (A+S) store the value (in A) to the address in register X XX+1 End 0~9 : 48 A~F : 65 GETC Aread one character if A=0x04 then jump to 0x80 if A<48 then GETC A A-48 (0x30) if A<10 then return A A-7 (48+7=55) return
- 10. 10 Relocating Loaders Motivation » efficient sharing of the machine with larger memory and when several independent programs are to be run together » support the use of subroutine libraries efficiently Two methods for specifying relocation » modification record (Fig. 3.4, 3.5) » relocation bit (Fig. 3.6, 3.7) – each instruction is associated with one relocation bit – these relocation bits in a Text record is gathered into bit masks
- 11. 11 Modification Record For complex machines Also called RLD specification » Relocation and Linkage Directory Modification record col 1: M col 2-7: relocation address col 8-9: length (halfbyte) col 10: flag (+/-) col 11-17: segment name
- 12. 12 Relocation Bit For simple machines 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 less than 12 words » unused words are set to 0 » any value that is to be modified during relocation must coincide with one of these 3-byte segments – e.g. line 210 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
- 13. 13 Program Linking Goal » Resolve the problems with EXTREF and EXTDEF from different control sections Linking » 1. User, 2. Assembler, 3. Linking loader Example » Program in Fig. 3.8 and object code in Fig. 3.9 » Use modification records for both relocation and linking – address constant – external reference
- 14. 14 Program Linking Example Program A Program B Program C Label Expression LISTA, ENDA LISTB, ENDB LISTC, ENDC REF1 LISTA local, R, PC external external REF2 LISTB+4 external local, R, PC external REF3 ENDA-LISTA local, A external external REF4 ENDA-LISTA+LISTC local, A external local, R REF5 ENDC-LISTC-10 external external local, A REF6 ENDC-LISTC+LISTA-1 local, R external local, A REF7 ENDA-LISTA-(ENDB-LISTB) local, A local, A external REF8 LISTB-LISTA local, R local, R external
- 15. 15 Program Linking Example Fig. 3.10 Load address for control sections » PROGA 004000 63 » PROGB 004063 7F » PROGC 0040E2 51 Load address for symbols » LISTA: PROGA+0040=4040 » LISTB: PROGB+0060=40C3 » LISTC: PROGC+0030=4112 REF4 in PROGA » ENDA-LISTA+LISTC=14+4112=4126 » T0000540F000014FFFFF600003F000014FFFFC0 » M00005406+LISTC
- 16. 16 Program Logic and Data Structure Two Passes Logic » Pass 1: assign addresses to all external symbols » Pass 2: perform the actual loading, relocation, and linking ESTAB (external symbol table) Control section Symbol Address Length Progam A 4000 63 LISTA 4040 ENDA 4054 Program B 4063 7F LISTB 40C3 ENDB 40D3 Program C 40E2 51 LISTC 4112 ENDC 4124
- 17. 17 Pass 1 Program Logic Pass 1: » assign addresses to all external symbols Variables » PROGADDR (program load address) from OS » CSADDR (control section address) » CSLTH (control section length) » ESTAB Fig. 3.11(a) » Process Define Record
- 18. 18 Pass 2 Program Logic Pass 1: » perform the actual loading, relocation, and linking Modification record » lookup the symbol in ESTAB End record for a main program » transfer address Fig. 3.11(b) » Process Text record and Modification record
- 19. 19 Improve Efficiency Use local searching instead of multiple searches of ESTAB for the same symbol » assign a reference number to each external symbol » the reference number is used in Modification records Implementation » 01: control section name » other: external reference symbols Example » Fig. 3.12
- 20. 20 Figure 3.12 Ref No. Symbol Address 1 PROGA 4000 2 LISTB 40C3 3 ENDB 40D3 4 LISTC 4112 5 ENDC 4124 Ref No. Symbol Address 1 PROGB 4063 2 LISTA 4040 3 ENDA 4054 4 LISTC 4112 5 ENDC 4124 Ref No. Symbol Address 1 PROGC 4063 2 LISTA 4040 3 ENDA 4054 4 LISTB 40C3 5 ENDB 40D3 PROGA PROGB PROGC