COP 3402 Systems Programming

Dr. Ali Orooji

School of EECSUniversity of Central Florida

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Chapter 3 - LOADERS AND LINKERSobject program contains translated instructions and

data values from the source program, and specifies

addresses in memory where these items are to be loaded

Three Functions

Loading brings the object program into memory for execution

Relocation modifies the object program when the program is loaded at an address different from the location originally specified

Linking combines two or more separate object programs

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Absolute Loader

- no relocation - no linking

Relocation (Program Relocation)

two methods

- modification records - bit mask (relocation bit) (if direct addressing and fixed instruction format)

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Linking (Program Linking) Figure 3.8 on pages 136-137 of text

PROGA ...

REF1 LDA LISTA REF2 +LDT LISTB+4 REF3 LDX #ENDA-LISTA ... LISTA EQU * ... ENDA EQU * REF4 WORD ENDA-LISTA+LISTC REF5 WORD ENDC-LISTC-10 REF6 WORD ENDC-LISTC+LISTA-1 REF7 WORD ENDA-LISTA-(ENDB-LISTB) REF8 WORD LISTB-LISTA

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PROGB…

LISTB EQU * …

ENDB EQU * …

PROGC…

LISTC EQU * …

ENDC EQU * …

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Linking and Relocating Loader

Let's assume control section 1 has external references to control section 2. Control section 2 comes after control section 1; so when loading control section 1 we don't know the addresses for variables in control section 2

- two passes

Pass 1 assigns addresses to all external symbols

Pass 2 performs loading, relocation, and linking

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Loader Data Structures

external symbol table (ESTAB - ESTBL) name and address of each external symbol; and in which control section the symbol is defined

program load address (PROGADDR) beginning address in memory where the linked program is to be loaded

control section address (CSADDR) starting address assigned to the control section currently being scanned by the loader; starting address of the control section currently being loaded

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Loader Logic

Pass 1 - Page 143 of text

Pass 2 - Page 144 of text

Figure 3.11(a) Algorithm for Pass 1 of a linking loader

Figure 3.11(b) Algorithm for Pass 2 of a linking loader

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Section 3.3: Machine-Independent Loader Features

Automatic Library Search

standard system library other libraries

- put external references in ESTBL

- after processing the primary input to loader if unresolved external references search the libraries for the routines

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Section 3.4: Loader Design Options

Linking (and Relocating) Loaders perform all linking and relocation at load time

Linkage Editors perform linking prior to load time

Dynamic Linking linking is performed at execution time

Bootstrap Loaders - used to load the operating system or the loader itself into memory - used to run stand-alone programs independent of the operating system or the system loader

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Linkage EditorsFigure 3.13 on page 153 of text

object program

linking and relocating loader

Memory

object program

linkage editor

Linked program

relocating loader

Memory

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Advantage avoids repeated linking

Disadvantage requires extra writing to disk

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Dynamic Linking

postpones the linking function until execution time: a subroutine is loaded and linked to the rest of the program when it is first called

Advantage avoids unnecessary linking

Figure 3.14 on page 157 of textdynamicloader

userprogram

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Bootstrap Loaders

How do we get the first program into memory?

- operator enters into memory the object code for an absolute loader, using switches on the computer console

- have absolute loader program in a read-only memory (ROM); when the operator presses "system start" button, the machine begins to execute this program

- when pressing a button, a record is read from some device into memory at a fixed location; this record contains instructions for an absolute loader