Lesson 1 - Hello, world!
First, some background...
Assembly language is bare-bones. The only interface a programmer has above the
actual hardware is the kernel itself. In order to build useful programs in
assembly we need to use the linux system calls provided by the kernel. These
system calls are a library built into the operating system to provide functions
such as reading input from a keyboard and writing output to the screen.
When you invoke a system call the kernel will immediately suspend execution of
your program. It will then contact the necessary drivers needed to perform the
task you requested on the hardware and then return control back to your
program.
Note: Drivers are called drivers because the kernel literally uses them to
drive the hardware.
We can accomplish this all in assembly by loading EAX with the function number
operation code OPCODE we want to execute and filling the remaining registers
with the arguments we want to pass to the system call. A software interrupt is
requested with the INT instruction and the kernel takes over and calls the
function from the library with our arguments. Simple.
For example requesting an interrupt when EAX1 will call sysexit and
requesting an interrupt when EAX4 will call syswrite instead. EBX, ECX EDX
will be passed as arguments if the function requires them. Click here to view
an example of a Linux System Call Table and its corresponding OPCODES.
Writing our program...
Firstly we create a variable msg in our .data section and assign it the
string we want to output in this case Hello, world!. In our .text section we
tell the kernel where to begin execution by providing it with a global
label start: to denote the programs entry point.
We will be using the system call syswrite to output our message to the console
window. This function is assigned OPCODE 4 in the Linux System Call Table. The
function also takes 3 arguments which are sequentially loaded into EDX, ECX and
EBX before requesting a software interrupt which will perform the task.
The arguments passed are as follows:
EDX will be loaded with the length in bytes of the our string.
ECX will be loaded with the address of our variable created in the .data
section.
EBX will be loaded with the file we want to write to in this case STDOUT.
The datatype and meaning of the arguments passed can be found in the functions
definition.
We compile, link and run the program using the commands below.
helloworld.asm
Hello World Program - asmtutor.com
Compile with: nasm -f elf helloworld.asm
Link with 64 bit systems require elfi386 option: ld -m elfi386
helloworld.o -o helloworld
Run with: ./helloworld
SECTION .data
msg db Hello World!, 0Ah assign msg variable with your message
string
SECTION .text
global start
start:
mov edx, 13 number of bytes to write - one for each letter plus
0Ah line feed character
mov ecx, msg move the memory address of our message string into
ecx
mov ebx, 1 write to the STDOUT file
mov eax, 4 invoke SYSWRITE kernel opcode 4
int 80h
Compile it with NASM as follows:
nasm -f elf helloworld.asm
ld -m elfi386 helloworld.o -o helloworld
./helloworld
Hello World!
Segmentation fault
The segmentation fault, right now is normal... more in lesson 2
First, some background...
Assembly language is bare-bones. The only interface a programmer has above the
actual hardware is the kernel itself. In order to build useful programs in
assembly we need to use the linux system calls provided by the kernel. These
system calls are a library built into the operating system to provide functions
such as reading input from a keyboard and writing output to the screen.
When you invoke a system call the kernel will immediately suspend execution of
your program. It will then contact the necessary drivers needed to perform the
task you requested on the hardware and then return control back to your
program.
Note: Drivers are called drivers because the kernel literally uses them to
drive the hardware.
We can accomplish this all in assembly by loading EAX with the function number
operation code OPCODE we want to execute and filling the remaining registers
with the arguments we want to pass to the system call. A software interrupt is
requested with the INT instruction and the kernel takes over and calls the
function from the library with our arguments. Simple.
For example requesting an interrupt when EAX1 will call sysexit and
requesting an interrupt when EAX4 will call syswrite instead. EBX, ECX EDX
will be passed as arguments if the function requires them. Click here to view
an example of a Linux System Call Table and its corresponding OPCODES.
Writing our program...
Firstly we create a variable msg in our .data section and assign it the
string we want to output in this case Hello, world!. In our .text section we
tell the kernel where to begin execution by providing it with a global
label start: to denote the programs entry point.
We will be using the system call syswrite to output our message to the console
window. This function is assigned OPCODE 4 in the Linux System Call Table. The
function also takes 3 arguments which are sequentially loaded into EDX, ECX and
EBX before requesting a software interrupt which will perform the task.
The arguments passed are as follows:
EDX will be loaded with the length in bytes of the our string.
ECX will be loaded with the address of our variable created in the .data
section.
EBX will be loaded with the file we want to write to in this case STDOUT.
The datatype and meaning of the arguments passed can be found in the functions
definition.
We compile, link and run the program using the commands below.
helloworld.asm
Hello World Program - asmtutor.com
Compile with: nasm -f elf helloworld.asm
Link with 64 bit systems require elfi386 option: ld -m elfi386
helloworld.o -o helloworld
Run with: ./helloworld
SECTION .data
msg db Hello World!, 0Ah assign msg variable with your message
string
SECTION .text
global start
start:
mov edx, 13 number of bytes to write - one for each letter plus
0Ah line feed character
mov ecx, msg move the memory address of our message string into
ecx
mov ebx, 1 write to the STDOUT file
mov eax, 4 invoke SYSWRITE kernel opcode 4
int 80h
Compile it with NASM as follows:
nasm -f elf helloworld.asm
ld -m elfi386 helloworld.o -o helloworld
./helloworld
Hello World!
Segmentation fault
The segmentation fault, right now is normal... more in lesson 2
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