Linker and Loaders

Linkers and Loader • Loader is program which accepts the object program and prepares it for executions (Loads into Memory) • Function of Loader 1. Allocation  Allocates space in Memory for Program 2. Linking  Resolves symbolic references between objects codes 3. Relocation  Adjust all the address dependent locations 4. Loading  Physically place M/C instructions and data into memory

Linkers and Loaders Loading Schemes to be discussed  Compile / Assemble and go loader  Absolute Loader  Relocating Loaders  Direct- Linking Loaders

Assemble and Go Loader • Assembler runs in memory and loaded assembled M/C instructions and data as they are assembled. • Directly placed into memory at assigned location. • At end, assembler executes transfer instruction to transfer control of executions at first line of M/C code.

Assemble and Go Loader • Advantages – Simple to design – Only one transfer instruction as a part of loader

• Disadvantages – Memory wasted for holding assembler – Necessary to re-translate code every time – Source program need to be in one language only.

Assemble and Go Loader

General Loader Scheme • Object program is already created • Object code can be loaded whenever it is to be executed. • System program known as loader must be there in the memory • It accepts all object programs and places them in memory for execution. • Loader is smaller than assembler in size.

General Loader Scheme

General Loader Scheme • Advantages – No need of re-assembling of program at every execution – Assembler need not be in memory – Subroutines can be written in several languages

Absolute Loader • Simplest type of General Loader Scheme • The assembled code is stored on Secondary storage • The absolute loader simply accepts the object code and places it in the main memory at the location predescribed by assembler. • More memory is available to user as assembler is not in memory

Absolute Loader • Advantage – Simple to implement

• Dis-advantage – Programmer has to specify the physical address to assembler where object code is to be loaded. – Programmer should remember address of each subroutine – Programmer should use those addresses explicitly to call those functions. – Programmer should take care of non-conflicting subroutine address.

Absolute Loader

Absolute Loader

Absolute Loader • Four loader function accomplished by – – – –

Allocation  programmer Linking  Programmer Relocation  Assembler Loading  :Loader

Subroutine Linkages • Main program A wants to transfer control to subroutine B. • Assembler does not understand call to B as B is not defined in program A. • To resolve such linking problems (symbolic references) pseudo opcodes EXTRN and ENTRY are used Relocating / DLLs loaders.

Subroutine Linkages • EXTRN indicates assembler that these symbols are referenced in this program but defined in some other program. EXTRN list of symbols

• ENTRY informs the assembler that these symbols are defined here but may be referenced in other programs. ENTRY list of symbols

Subroutine Linkages • Example: MAIN START EXTRN SUBROUT ------------------L 15, =A(SUBROUT) BALR 14, 15 ----------END

Subroutine Linkages

SUBROUT START USING *,15 ---------BR 14 END

Subroutine Linkages • Example ENTRY A START ENTRY B1, B2, B3,….Bn (entry points in subroutine) ----------B1 ------B2 ------END

Relocating Loaders • Purpose:

– To reduce efforts of programmer to allocate and link the object codes. – To avoid re-assembling of all subroutines even if one is modified.

• Assembler assembles every routine independently. • Assembler has to provide object code and other information to the loader to support the loader functionalities. • Binary Symbolic Subroutine is simple example of relocating loader. • It is used with system having fixed-length direct address instruction • BSS allows many procedure segments and one data segment.

Relocating Loaders

Relocating Loaders

Relocating Loaders • The Relocating assembler has to provide object code and relocating and linking information to the Loader. – – – –

Length of program for Allocation. Transfer vector for Linking Relocation Bits for Relocation Object codes for Loading.

• Allocation: – Program Length, Transfer Vector length is determined by Assembler. – All subroutines referenced in program must be allocated space in memory.

Relocating Loaders • Linking: – Transfer vector is part of object code generated by assembler. – It contains symbolic name/references to all subroutines referenced in the program.

• How linking is done? – Call to function becomes a transfer instruction that passes control to Transfer vector. – Transfer vector contains actual address of subroutine and control is transferred to that physical memory location.

• For eg.- SQRT function is called in MAIN

Relocating Loaders • Relocation: – It is done with the help of RELOCATION BITS. – If the relocations bit = 1 then instruction or part of instruction need to be relocated. – Relocation bits included as a part of object code for every instruction. – In example for every half word the relocation bit is present.

• Check that EXTRN says that SQRT and ERR are external symbols and for every external symbol transfer vector entry is generated.

Relocating Loaders • Advantages: – All four functions are performed by Loader.

• Disadvantages: – Transfer Vector can be used only for linking not for storage of external data. – Transfer vector increases the size of object code. – Common data segment so every subroutine has to access it in synchronization.

Direct Linking Loader

• A general relocatable loader. • Multiple procedure and data segments. Every segment can be accessed by other segment. • Independent translation of program. • We are studying hypothetical loader supporting IBM 370 Model

Direct Linking Loader • Following information assembler must provide to the loader for every segment: – Length of segment. – List of symbols in segment referenced by other segments. – A list of all symbols not defined in segment but referenced in segment. – Information about address dependent values: their location, and how to revise their addresses. – Translated code for every segment and their relative addresses.

Direct Linking Loader

Direct Linking Loader • Some Observations: – POINTER contains value 28 ie relative address of TABLE. – ASUM = A(SUM). SUM is not defined in segment so absolute address of ASUM can not be mentioned immediately. – JOHN, RESULT may be referenced by other procedures so must be informed to the loader.

Direct Linking Loader • In this scheme assembler produces four types of cards as object deck (input to the direct-linking loader) : – ESD (External Symbol Dictionary) – TXT (Object code for every segment) – RLD (Relocation and Linkage Directory) – END (Specifies end of object code)

Direct Linking Loader -ESD • ESD stores all symbols defined in program may be referenced somewhere else and all symbols referenced in program but defined elsewhere. • Types of symbols: – SD: Segment Definition. – LD: Local Definition. – ER: External Reference. ER symbols are used in conjunction with RLD cards for resolving symbolic references

Direct Linking Loader -ESD

Direct Linking Loader - TXT

Direct Linking Loader - RLD • RLD is Relocation and Linkage Directory. • Contains information about contents that depends on address in core. (Relocation and linking). • RLD contains following information: – Location of each constant that needs to be changed due to relocation. (relative address of that instruction) – By what value it has to be changed.(segment symbol) – The operation to be performed ( add/subtract)

• Adjusting address constants of internal symbol is relocation. • Adjusting address constants of external symbol is Linking.

Direct Linking Loader - RLD

Other Loading Schemes – Dynamic Loading

Design of Absolute Loader • Allocation, Linking and Relocation performed by Programmer and Assembler. • Absolute Loader has to read object card and load it into the memory. • Assembler conveys through two cards: – Object deck for where to load and what to load in memory. – Entry point for execution of code.

• Card types are: – Text card (type = 0) for instruction and data – Transfer card ( type = 1) to hold entry point to program.

Design of Absolute Loader • Text Card: Card column 1 2 3-5 6-7 8-72 73-80

Contents Card type = 0 No. Of bytes of information. Address at which data on card to put. Empty Instruction and data to be loaded. Card sequence number

• Transfer Card: Card column 1 2 3-5 6-72 73-80

Contents Card type = 1 Count = 0 Address of entry point Empty Card sequence number

Design of Absolute Loader

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