Graphics Programs Reference
In-Depth Information
#define __NR_stime 25
#define __NR_ptrace 26
#define __NR_alarm 27
#define __NR_oldfstat 28
#define __NR_pause 29
#define __NR_utime 30
#define __NR_stty 31
#define __NR_gtty 32
#define __NR_access 33
#define __NR_nice 34
#define __NR_ftime 35
#define __NR_sync 36
#define __NR_kill 37
#define __NR_rename 38
#define __NR_mkdir 39
...
For our rewrite of helloworld.c in assembly, we will make a system call to
the
write()
function for the output and then a second system call to
exit()
so the process quits cleanly. This can be done in
x
86 assembly using just two
assembly instructions:
mov
and
int
.
Assembly instructions for the
x
86 processor have one, two, three, or no
operands. The operands to an instruction can be numerical values, memory
addresses, or processor registers. The
x
86 processor has several 32-bit registers
that can be viewed as hardware variables. The registers EAX, EBX, ECX, EDX,
ESI, EDI, EBP, and ESP can all be used as operands, while the EIP register
(execution pointer) cannot.
The
mov
instruction copies a value between its two operands. Using Intel
assembly syntax, the first operand is the destination and the second is the
source. The
int
instruction sends an interrupt signal to the kernel, defined
by its single operand. With the Linux kernel, interrupt
0x80
is used to tell
the kernel to make a system call. When the
int 0x80
instruction is executed, the
kernel will make a system call based on the first four registers. The EAX register
is used to specify which system call to make, while the EBX, ECX, and EDX
registers are used to hold the first, second, and third arguments to the system
call. All of these registers can be set using the
mov
instruction.
In the following assembly code listing, the memory segments are simply
declared. The string
"Hello, world!"
with a newline character (
0x0a
) is in the
data segment, and the actual assembly instructions are in the text segment.
This follows proper memory segmentation practices.
helloworld.asm
section .data ; Data segment
msg db "Hello, world!", 0x0a ; The string and newline char
section .text ; Text segment
global _start ; Default entry point for ELF linking
_start:
