assembly_tutorial

Apart from the DS, CS and SS registers, there are other extra segment registers - ES (extra segment), FS and
GS, which provides additional segments for storing data.
In assembly programming, a program needs to access the memory locations. All memory locations within a
segment are relative to the starting address of the segment. A segment begins in an address evenly disable by 16
or hexadecimal 10. So all the rightmost hex digit in all such memory addresses is 0, which is not generally stored
in the segment registers.
The segment registers stores the starting addresses of a segment. To get the exact location of data or instruction
within a segment, an offset value (or displacement) is required. To reference any memory location in a segment,
the processor combines the segment address in the segment register with the offset value of the location.
Example:
Look at the following simple program to understand the use of registers in assembly programming. This program
displays 9 stars on the screen along with a simple message:
section .text
global main ;must be declared for linker (gcc)
main: ;tell linker entry point
mov edx,len ;message length
mov ecx,msg ;message to write
mov ebx,1 ;file descriptor (stdout)
mov eax,4 ;system call number (sys_write)
int 0x80 ;call kernel
mov edx,9 ;message length
mov ecx,s2 ;message to write
mov ebx,1 ;file descriptor (stdout)
mov eax,4 ;system call number (sys_write)
int 0x80 ;call kernel
mov eax,1 ;system call number (sys_exit)
int 0x80 ;call kernel
section .data
msg db 'Displaying 9 stars',0xa ;a message
len equ $ - msg
;length of message
s2 times 9 db '*'
When the above code is compiled and executed, it produces following result:
Displaying 9 stars
*********
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