gas多文件工程
摘自：Programming from Groundup Chapter 6
主要目的是了解如何组织多文件工程（形式）
文件record-def.s:
.equ RECORD_FIRSTNAME, 0
.equ RECORD_LASTNAME, 40
.equ RECORD_ADDRESS, 80
.equ RECORD_AGE, 320
.equ RECORD_SIZE, 324
文件linux.s:
#Common Linux Definitions
#System Call Numbers
.equ SYS_EXIT, 1
.equ SYS_READ, 3
.equ SYS_WRITE, 4
.equ SYS_OPEN, 5
.equ SYS_CLOSE, 6
.equ SYS_BRK, 45
#System Call Interrupt Number
.equ LINUX_SYSCALL, 0x80
#Standard File Descriptors
.equ STDIN, 0
.equ STDOUT, 1
.equ STDERR, 2
#Common Status Codes
.equ END_OF_FILE, 0
第一个程序Writing Records
文件：write-record.s
.include "linux.s"
.include "record-def.s"
#PURPOSE: This function writes a record to
# the given file descriptor
#
#INPUT: The file descriptor and a buffer
#
#OUTPUT: This function produces a status code
#
#STACK LOCAL VARIABLES
.equ ST_WRITE_BUFFER, 8
.equ ST_FILEDES, 12
.section .text
.globl write_record
.type write_record, @function
write_record:
pushl %ebp
movl %esp, %ebp
pushl %ebx
movl $SYS_WRITE, %eax
movl ST_FILEDES(%ebp), %ebx
movl ST_WRITE_BUFFER(%ebp), %ecx
movl $RECORD_SIZE, %edx
int $LINUX_SYSCALL
#NOTE - %eax has the return value, which we will
# give back to our calling program
popl %ebx
movl %ebp, %esp
popl %ebp
ret
文件write-records.s：
.include "linux.s"
.include "record-def.s"
.section .data
#Constant data of the records we want to write
#Each text data item is padded to the proper
#length with null (i.e. 0) bytes.
#.rept is used to pad each item. .rept tells
#the assembler to repeat the section between
#.rept and .endr the number of times specified.
#This is used in this program to add extra null
#characters at the end of each field to fill
#it up
record1:
.ascii "Fredrick\0"
.rept 31 #Padding to 40 bytes
.byte 0
.endr
.ascii "Bartlett\0"
.rept 31 #Padding to 40 bytes
.byte 0
.endr
.ascii "4242 S Prairie\nTulsa, OK 55555\0"
.rept 209 #Padding to 240 bytes
.byte 0
.endr
.long 45
record2:
.ascii "Marilyn\0"
.rept 32 #Padding to 40 bytes
.byte 0
.endr
.ascii "Taylor\0"
.rept 33 #Padding to 40 bytes
.byte 0
.endr
.ascii "2224 S Johannan St\nChicago, IL 12345\0"
.rept 203 #Padding to 240 bytes
.byte 0
.endr
.long 29
record3:
.ascii "Derrick\0"
.rept 32 #Padding to 40 bytes
.byte 0
.endr
.ascii "McIntire\0"
.rept 31 #Padding to 40 bytes
.byte 0
.endr
.ascii "500 W Oakland\nSan Diego, CA 54321\0"
.rept 206 #Padding to 240 bytes
.byte 0
.endr
.long 36
#This is the name of the file we will write to
file_name:
.ascii "test.dat\0"
.equ ST_FILE_DESCRIPTOR, -4
.globl _start
_start:
#Copy the stack pointer to %ebp
movl %esp, %ebp
#Allocate space to hold the file descriptor
subl $4, %esp
#Open the file
movl $SYS_OPEN, %eax
movl $file_name, %ebx
movl $0101, %ecx #This says to create if it
#doesn’t exist, and open for
#writing
movl $0666, %edx
int $LINUX_SYSCALL
#Store the file descriptor away
movl %eax, ST_FILE_DESCRIPTOR(%ebp)
#Write the first record
pushl ST_FILE_DESCRIPTOR(%ebp)
pushl $record1
call write_record
addl $8, %esp
#Write the second record
pushl ST_FILE_DESCRIPTOR(%ebp)
pushl $record2
call write_record
addl $8, %esp
#Write the third record
pushl ST_FILE_DESCRIPTOR(%ebp)
pushl $record3
call write_record
addl $8, %esp
#Close the file descriptor
movl $SYS_CLOSE, %eax
movl ST_FILE_DESCRIPTOR(%ebp), %ebx
int $LINUX_SYSCALL
#Exit the program
movl $SYS_EXIT, %eax
movl $0, %ebx
int $LINUX_SYSCALL
To build the application, run the commands:
as write-records.s -o write-record.o
as write-record.s -o write-record.o
ld write-record.o write-records.o -o write-records
第二个程序Reading Records
文件read-record.s：
.include "record-def.s"
.include "linux.s"
#PURPOSE: This function reads a record from the file
# descriptor
#
#INPUT: The file descriptor and a buffer
#
#OUTPUT: This function writes the data to the buffer
# and returns a status code.
#
#STACK LOCAL VARIABLES
.equ ST_READ_BUFFER, 8
.equ ST_FILEDES, 12
.section .text
.globl read_record
.type read_record, @function
read_record:
pushl %ebp
movl %esp, %ebp
pushl %ebx
movl ST_FILEDES(%ebp), %ebx
movl ST_READ_BUFFER(%ebp), %ecx
movl $RECORD_SIZE, %edx
movl $SYS_READ, %eax
int $LINUX_SYSCALL
#NOTE - %eax has the return value, which we will
# give back to our calling program
popl %ebx
movl %ebp, %esp
popl %ebp
ret
文件count-chars.s：
#PURPOSE: Count the characters until a null byte is reached.
#
#INPUT: The address of the character string
#
#OUTPUT: Returns the count in %eax
#
#PROCESS:
# Registers used:
# %ecx - character count
# %al - current character
# %edx - current character address
.type count_chars, @function
.globl count_chars
#This is where our one parameter is on the stack
.equ ST_STRING_START_ADDRESS, 8
count_chars:
pushl %ebp
movl %esp, %ebp
#Counter starts at zero
movl $0, %ecx
#Starting address of data
movl ST_STRING_START_ADDRESS(%ebp), %edx
count_loop_begin:
#Grab the current character
movb (%edx), %al
#Is it null?
cmpb $0, %al
#If yes, we’re done
je count_loop_end
#Otherwise, increment the counter and the pointer
incl %ecx
incl %edx
#Go back to the beginning of the loop
jmp count_loop_begin
count_loop_end:
#We’re done. Move the count into %eax
#and return.
movl %ecx, %eax
popl %ebp
ret
文件write-newline.s：
.include "linux.s"
.globl write_newline
.type write_newline, @function
.section .data
newline:
.ascii "\n"
.section .text
.equ ST_FILEDES, 8
write_newline:
pushl %ebp
movl %esp, %ebp
movl $SYS_WRITE, %eax
movl ST_FILEDES(%ebp), %ebx
movl $newline, %ecx
movl $1, %edx
int $LINUX_SYSCALL
movl %ebp, %esp
popl %ebp
ret
文件read-records.s：
.include "linux.s"
.include "record-def.s"
.section .data
file_name:
.ascii "test.dat\0"
.section .bss
.lcomm record_buffer, RECORD_SIZE
.section .text
#Main program
.globl _start
_start:
#These are the locations on the stack where
#we will store the input and output descriptors
#(FYI - we could have used memory addresses in
#a .data section instead)
.equ ST_INPUT_DESCRIPTOR, -4
.equ ST_OUTPUT_DESCRIPTOR, -8
#Copy the stack pointer to %ebp
movl %esp, %ebp
#Allocate space to hold the file descriptors
subl $8, %esp
#Open the file
movl $SYS_OPEN, %eax
movl $file_name, %ebx
movl $0, %ecx #This says to open read-only
movl $0666, %edx
int $LINUX_SYSCALL
#Save file descriptor
movl %eax, ST_INPUT_DESCRIPTOR(%ebp)
#Even though it’s a constant, we are
#saving the output file descriptor in
#a local variable so that if we later
#decide that it isn’t always going to
#be STDOUT, we can change it easily.
movl $STDOUT, ST_OUTPUT_DESCRIPTOR(%ebp)
record_read_loop:
pushl ST_INPUT_DESCRIPTOR(%ebp)
pushl $record_buffer
call read_record
addl $8, %esp
#Returns the number of bytes read.
#If it isn’t the same number we
#requested, then it’s either an
#end-of-file, or an error, so we’re
#quitting
cmpl $RECORD_SIZE, %eax
jne finished_reading
#Otherwise, print out the first name
#but first, we must know it’s size
pushl $RECORD_FIRSTNAME + record_buffer
call count_chars
addl $4, %esp
movl %eax, %edx
movl ST_OUTPUT_DESCRIPTOR(%ebp), %ebx
movl $SYS_WRITE, %eax
movl $RECORD_FIRSTNAME + record_buffer, %ecx
int $LINUX_SYSCALL
pushl ST_OUTPUT_DESCRIPTOR(%ebp)
call write_newline
addl $4, %esp
jmp record_read_loop
finished_reading:
movl $SYS_EXIT, %eax
movl $0, %ebx
int $LINUX_SYSCALL
To build this program, we need to assemble all of the parts and link them together:
as read-record.s -o read-record.o
as count-chars.s -o count-chars.o
as write-newline.s -o write-newline.o
as read-records.s -o read-records.o
ld read-record.o count-chars.o write-newline.o \
read-records.o -o read-records
第三个程序Modifying the Records
文件add-year.s：
.include "linux.s"
.include "record-def.s"
.section .data
input_file_name:
.ascii "test.dat\0"
output_file_name:
.ascii "testout.dat\0"
.section .bss
.lcomm record_buffer, RECORD_SIZE
#Stack offsets of local variables
.equ ST_INPUT_DESCRIPTOR, -4
.equ ST_OUTPUT_DESCRIPTOR, -8
.section .text
.globl _start
_start:
#Copy stack pointer and make room for local variables
movl %esp, %ebp
subl $8, %esp
#Open file for reading
movl $SYS_OPEN, %eax
movl $input_file_name, %ebx
movl $0, %ecx
movl $0666, %edx
int $LINUX_SYSCALL
movl %eax, ST_INPUT_DESCRIPTOR(%ebp)
#Open file for writing
movl $SYS_OPEN, %eax
movl $output_file_name, %ebx
movl $0101, %ecx
movl $0666, %edx
int $LINUX_SYSCALL
movl %eax, ST_OUTPUT_DESCRIPTOR(%ebp)
loop_begin:
pushl ST_INPUT_DESCRIPTOR(%ebp)
pushl $record_buffer
call read_record
addl $8, %esp
#Returns the number of bytes read.
#If it isn’t the same number we
#requested, then it’s either an
#end-of-file, or an error, so we’re
#quitting
cmpl $RECORD_SIZE, %eax
jne loop_end
#Increment the age
incl record_buffer + RECORD_AGE
#Write the record out
pushl ST_OUTPUT_DESCRIPTOR(%ebp)
pushl $record_buffer
call write_record
addl $8, %esp
jmp loop_begin
loop_end:
movl $SYS_EXIT, %eax
movl $0, %ebx
int $LINUX_SYSCALL
To build it, type the following4:
as add-year.s -o add-year.o
ld add-year.o read-record.o write-record.o -o add-year