We have already discussed three
sections of an assembly program. These sections represent various memory
segments as well.
Interestingly, if you replace the
section keyword with segment, you will get the same result. Try the following
code:
segment .text ;code segment
global _start ;must be
declared for linker
_start: ;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 eax,1 ;system call number (sys_exit)
int 0x80 ;call
kernel
segment
.data ;data segment
msg db Hello, world!',0xa ;our dear string
len equ $
- msg ;length of our dear string
When the above code is compiled and
executed, it produces following result:
Hello,
world!
Memory
Segments
A segmented memory model divides the
system memory into groups of independent segments, referenced by pointers
located in the segment registers. Each segment is used to contain a specific
type of data. One segment is used to contain instruction codes, another segment
stores the data elements, and a third segment keeps the program stack.
In the light of the above
discussion, we can specify various memory segments as:
- Data segment - it is represented by .data section and the .bss. The .data section is used to declare the memory region where data elements are stored for the program. This section cannot be expanded after the data elements are declared, and it remains static throughout the program.
The .bss
section is also a static memory section that contains buffers for data to be
declared later in the program. This buffer memory is zero-filled.
- Code segment - it is represented by .text section. This defines an area in memory that stores the instruction codes. This is also a fixed area.
- Stack - this segment contains data values passed to functions and procedures within the program.
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