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>6.2. Hello, world!</H1
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>6.2.1. Program layout</H2
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>Linux is 32-bit, runs in protected mode, has flat memory model,
and uses the ELF format for binaries.</P
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>A program can be divided into sections:
<TT
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>.text</TT
> for your code (read-only),
<TT
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>.data</TT
> for your data (read-write),
<TT
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>.bss</TT
> for uninitialized data (read-write);
there can actually be a few other standard sections,
as well as some user-defined sections,
but there's rare need to use them and they are out of our interest here.
A program must have at least <TT
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>.text</TT
> section.</P
><P
>Now we will write our first program. Here is sample code:</P
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>6.2.2. NASM (hello.asm)</H2
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>section .text ;section declaration
;we must export the entry point to the ELF linker or
global _start ;loader. They conventionally recognize _start as their
;entry point. Use ld -e foo to override the default.
_start:
;write our string to stdout
mov edx,len ;third argument: message length
mov ecx,msg ;second argument: pointer to message to write
mov ebx,1 ;first argument: file handle (stdout)
mov eax,4 ;system call number (sys_write)
int 0x80 ;call kernel
;and exit
mov ebx,0 ;first syscall argument: exit code
mov eax,1 ;system call number (sys_exit)
int 0x80 ;call kernel
section .data ;section declaration
msg db "Hello, world!",0xa ;our dear string
len equ $ - msg ;length of our dear string </PRE
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>6.2.3. GAS (hello.S)</H2
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>.text # section declaration
# we must export the entry point to the ELF linker or
.global _start # loader. They conventionally recognize _start as their
# entry point. Use ld -e foo to override the default.
_start:
# write our string to stdout
movl $len,%edx # third argument: message length
movl $msg,%ecx # second argument: pointer to message to write
movl $1,%ebx # first argument: file handle (stdout)
movl $4,%eax # system call number (sys_write)
int $0x80 # call kernel
# and exit
movl $0,%ebx # first argument: exit code
movl $1,%eax # system call number (sys_exit)
int $0x80 # call kernel
.data # section declaration
msg:
.ascii "Hello, world!\n" # our dear string
len = . - msg # length of our dear string </PRE
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