This document discusses linker scripts and their purpose. It explains that linker scripts control how sections from input files are mapped into the output file and define the memory layout. It describes several keywords used in linker scripts like ENTRY, OUTPUT_FORMAT, MEMORY, and SECTIONS. The SECTIONS keyword is used to describe the memory layout by assigning sections like .text, .data, and .bss to addresses.

1. Linker scripts

2. Introduction

Linker script is written in the linker command language.

Every link is controlled by a linker script.

The main purpose of the linker script is to describe how
the sections in the input files should be mapped into the
output file.

Also used to control the memory layout of the output file.

The linker will use a default script that is compiled into the
linker executable.

We may supply our own linker script by using the `-T'
command line option.
ld -o prog -T my_script.lds main.o

3. Keywords used in linker scripts

ENTRY

OUTPUT_FORMAT

STARTUP

SEARCH_DIR

INPUT

OUTPUT

MEMORY

SECTIONS

4. ENTRY

ENTRY takes one argument.
ENTRY(main)
ENTRY(Multi bootEntry)

That is the symbol name for the entry point of the
linked program/kernel.

This can be "start" or "__main", but this will be the
very first byte of your loaded program.

(or) the first byte of the .text section in ELF and PE
binaries.

5. OUTPUT_FORMAT

OUTPUT_FORMAT also takes one argument.

It specifies the output format of our executable.
OUTPUT_FORMAT(elf64-x86-64)
OUTPUT_FORMAT("pe-i386")
The more common formats are

binary --This is just a flat binary with no formatting at all.

elf32-i386 --This is just the ELF format, usually little endian too.

elf64-x86-64 --This is the ELF format for 64bit, usually little endian.

pe-i386 --The PE format.

6. STARTUP

STARTUP takes one argument.
STARTUP(Boot.o)
STARTUP(crt0.o)

i.e. which file you want to be linked to the beginning of the
executable.
SEARCH_DIR
 It treats linker script specified search directories as standard
directories.
 This will add a path to your library search directory.
SEARCH_DIR(Directory)

7. INPUT

INPUT is a 'in-linker script' replacement for adding object
files to the command line.
INPUT(File1.o File2.o File3.o ...)
INPUT
(
File1.o
File2.o
File3.o
...
)

8. OUTPUT

OUTPUT specifies the file that has to be output by
the linker.

This is the name of the executable.
OUTPUT(Kernel.bin)

OUTPUT(filename) in the linker script is exactly
like using `-o filename' on the command line.

You can use the OUTPUT command to define a
default name for the output file other than the usual
default of a.out.

9. MEMORY

MEMORY declares one or more memory regions with
attributes specifying whether the region can be written to,
read from or executed.

This is mostly used in embedded systems where different
regions of address space may contain different access
permissions.

For example:
MEMORY
{
ROM (rx) : ORIGIN = 0, LENGTH = 256k
RAM (wx) : org = 0x00100000, len = 1M
}

10. This script tells the linker that there are two
memory regions.
"ROM" starts at address 0x00000000, is 256kB in
length, can be read and executed.
"RAM" starts at address 0x00100000, is 1MB in
length, can be written, read and executed.

11. SECTIONS

We will use the SECTIONS command to describe
the memory layout of the output file.
SECTIONS
{
. = 0x10000;
.text : { *(.text) }
. = 0x8000000;
.data : { *(.data) }
.bss : { *(.bss) }
}

12. 
We will write the SECTIONS command as the keyword
SECTIONS, followed by a series of symbol assignments and output
section descriptions enclosed in curly braces.

code will be loaded at address 0x10000, and data will start at address
0x8000000.

Dot(.) is the location counter.

If we do not specify the address of an output section, the address is
set from the current value of the location counter.

The location counter is then incremented by the size of the output
section.

13. 
At the start of the SECTIONS command, the location counter has
the value 0.

The * is a wildcard which matches any file name.

The expression *(.text) means all .text input sections in all input
files.

After the linker places the .data output section, the value of the
location counter will be 0x8000000 plus the size of the .data output
section.

The linker will place the .bss output section immediately after the
.data output section in memory.