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 i386 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 5.1 Redhat Linux installation running a 2.0.34 version kernel and the version 5 and 6 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 user's "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 "average.s". Note: Assembly language programs should use the ".s" suffix.
/* linux version of AVTEMP.ASM CS 200, fall 1998 */
.data /* beginning of data segment */
/* hi_temp data item */
.type hi_temp,@object /* declare as data object */
.size hi_temp,1 /* declare size in bytes */
hi_temp:
.byte 0x92 /* set value */
/* lo_temp data item */
.type lo_temp,@object
.size lo_temp,1
lo_temp:
.byte 0x52
/* av_temp data item */
.type av_temp,@object
.size av_temp,1
av_temp:
.byte 0
/* segment registers set up by linked code */
/* beginning of text(code) segment */
.text
.align 4 /* set 4 double-word 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 $0,%ah
adcb $0,%ah
movb $2,%bl
idivb %bl
movb %al,av_temp
leave /* function requirement */
ret /* function requirement */
assembly instructions
This code may be assembled with the following command:
as -a --gstabs -o average.o average.s
The "-a" option prints a memory listing during assembly. This output gives the location variables and code with respect to the beginnings of the data and code segments. "--gstabs" places debugging information in the executable (used by gdb). "-o" specifies average.o as the output file name (the default is a.out, which is confusing since the file is not executable.)
The object file (average.o) can then be linked to the Linux wrapper code in order to create an executable. These files are crt1.o, crti.o and crtn.o. crt1.o and crti.o provide initialization code and crtn.o does cleanup. These should all be located in "/usr/lib" be may be elsewere on some systems. They, and their source, might be located by executing the following find command:
find / -name "crt*" -print
The link command is the following:
ld -m elf_i386 -static /usr/lib/crt1.o /usr/lib/crti.o
-lc average.o /usr/lib/crtn.o
"-m elf_i386" instructs the linker to use the ELF file format. "-static" cause static rather than dynamic linking to occur. And "-lc" links in the standard c libraries (libc.a). It might be necessary to include "-I/libdirectory" in the invocation for ld to find the c library.
It will be necessary to change the mode of the resulting object file with "chmod +x ./a.out".
It should now be possible to execute the file. But, of course, there will be no output.
I recommend placing the above commands in a makefile .
debugging
The "--gstabs" option given to the assembler allows the assembly program to be debugged under gdb. The first step is to invoke gdb:
gdb ./a.out
gdb should start with the following message:
[bjorn@pomade src]$ gdb ./a.out
GNU gdb 4.17
Copyright 1998 Free Software Foundation, Inc.GDB 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 conditions.Type "show copying" to see the conditions.
There is absolutely no warranty for GDB. Type "show warranty" for details.
This GDB was configured as "i386-redhat-linux"...
(gdb)
The "l" command will list the program sourcecode.
(gdb) l
1 /* linux version of AVTEMP.ASM CS 200, fall 1998 */
2 .data /* beginning of data segment */
3
4 /* hi_temp data item */
5 .type hi_temp,@object /* declare as data object */
6 .size hi_temp,1 /* declare size in bytes */
7 hi_temp:
8 .byte 0x92 /* set value */
9
10 /* 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 1 at 0x80480f7
(gdb)
This sets a breakpoint at the beginning of main. Now run the program.
(gdb) run
Starting program: /home/bjorn/src/./a.out
Breakpoint 1, main () at average.s:31
31 movb hi_temp,%al
Current language: auto; currently asm
(gdb)
values in registers can be checked with either "info registers"
(gdb) info registers
eax 0x8059200 134582784
ecx 0xbffffd94 -1073742444
edx 0x0 0
ebx 0x8097bf0 134839280
esp 0xbffffdd8 0xbffffdd8
ebp 0xbffffdd8 0xbffffdd8
esi 0x1 1
edi 0x8097088 134836360
eip 0x80480f7 0x80480f7
eflags 0x246 582
cs 0x23 35
ss 0x2b 43
ds 0x2b 43
es 0x2b 43
fs 0x2b 43
gs 0x2b 43
(gdb)
...or "p/x $eax" which prints the value in the EAX register in hex. The "e"in front of the register name indicates a 32 bit register. The Intel x86 family has included "extended" 32 bit registers since the 80386. These E registers are to the X registers as the L and H are to the X registers.Linux also uses a "flat" and protected memory model rather that segmentation,thus the EIP stores the entire current address.
(gdb
