摘要:The following is designed to be a Linux equivalent to Developing Assembly Language Programs on a PC by Douglas V Hall This tutorial requires the following:
Introduction
The following is designed to be a Linux equivalent to Developing Assembly Language Programs on a PC by Douglas V Hall This tutorial requires the following:
an i family PC running Linux
as the GNU assembler (included with any gcc installation) ld the GNU linker (also included with gcc) gdb the GNU debugger The tutorial was developed on a Redhat Linux installation running a version kernel and the version and C language libraries with ELF file format But I have tried to make the tutorial as general possible with respect to Linux systems I highly recommend working through this tutorial with as and gdb documentation close at hand
Overview
The process of developing an assembly program under linux is somewhat different from development under NT In order to accommodate object oriented languages which require the compiler to create constructor and destructor methods which execute before and after the execution of main the GNU development model embeds user code within a wrapper of system code In other words the users main is treated as a function call An advantage of this is that user is not required to initialize segment registers though user code must obey some function requirements
The Code
The following is the Linux version of the average temperature program It will be referred to as averages Note: Assembly language programs should use the s suffix
/* linux version of AVTEMPASM CS fall */
data /* beginning of data segment */
/* hi_temp data item */
type hi_temp@object /* declare as data object */
size hi_temp /* declare size in bytes */
hi_temp:
byte x /* set value */
/* lo_temp data item */
type lo_temp@object
size lo_temp
lo_temp:
byte x
/* av_temp data item */
type av_temp@object
size av_temp
av_temp:
byte
/* segment registers set up by linked code */
/* beginning of text(code) segment */
text
align /* set doubleword alignment */
globl main /* make main global for linker */
type main@function /* declare main as a function */
main:
pushl %ebp /* function requirement */
movl %esp%ebp /* function requirement */
movb hi_temp%al
addb lo_temp%al
movb $%ah
adcb $%ah
movb $%bl
idivb %bl
movb %alav_temp
leave /* function requirement */
ret /* function requirement */
assembly instructions
This code may be assembled with the following command:
as a gstabs o averageo averages
The a option prints a memory listing during assembly This output gives the location variables and code with respect to the beginnings of the data and code segments gstabs places debugging information in the executable (used by gdb) o specifies averageo as the output file name (the default is aout which is confusing since the file is not executable)
The object file (averageo) can then be linked to the Linux wrapper code in order to create an executable These files are crto crtio and crtno crto and crtio provide initialization code and crtno does cleanup These should all be located in /usr/lib be may be elsewere on some systems They and their source might be located by executing the following find command:
find / name crt* print
The link command is the following:
ld m elf_i static /usr/lib/crto /usr/lib/crtio
lc averageo /usr/lib/crtno
m elf_i instructs the linker to use the ELF file format static cause static rather than dynamic linking to occur And lc links in the standard c libraries (libca) It might be necessary to include I/libdirectory in the invocation for ld to find the c library
It will be necessary to change the mode of the resulting object file with chmod +x /aout
It should now be possible to execute the file But of course there will be no output
I recommend placing the above commands in a makefile
debugging
The gstabs option given to the assembler allows the assembly program to be debugged under gdb The first step is to invoke gdb:
gdb /aout
gdb should start with the following message:
[bjorn@pomade src]$ gdb /aout
GNU gdb
Copyright Free Software Foundation IncGDB is free software covered by the GNU General Public License and you are
welcome to change it and/or distribute copies of it under certain conditionsType show copying to see the conditions
There is absolutely no warranty for GDB Type show warranty for details
This GDB was configured as iredhatlinux
(gdb)
The l command will list the program sourcecode
(gdb) l
/* linux version of AVTEMPASM CS fall */
data /* beginning of data segment */
/* hi_temp data item */
type hi_temp@object /* declare as data object */
size hi_temp /* declare size in bytes */
hi_temp:
byte x /* set value */
/* lo_temp data item */
(gdb)
The first thing to do is set a breakpoint so it will be possible to step through the code
(gdb) break main
Breakpoint at xf
(gdb)
This sets a breakpoint at the beginning of main Now run the program
(gdb) run
Starting program: /home/bjorn/src//aout
Breakpoint main () at averages:
movb hi_temp%al
Current language: auto; currently asm
(gdb)
values in registers can be checked with either info registers
(gdb) info registers
eax x
ecx xbffffd
edx x
ebx xbf
esp xbffffdd xbffffdd
ebp xbffffdd xbffffdd
esi x
edi x
eip xf xf
eflags x
cs x
ss xb
ds xb
es xb
fs xb
gs xb
(gdb)
or p/x $eax which prints the value in the EAX register in hex The ein front of the register name indicates a bit register The Intel x family has included extended bit registers since the These E registers are to the X registers as the L and H are to the X registersLinux also uses a flat and protected memory model rather that segmentationthus the EIP stores the entire current address
(gdb
