I solving exercise section from this book.
My question is regarding the toupper example from the book.
Question:
Rewrite the program so that it uses storage in the .bss section rather than the stack to store the file descriptors.
The default code can be found on the website.
My code:
#Rewrite the program so that it uses storage in the .bss section rather than the
#stack to store the file descriptors.
#PURPOSE: This program converts an input file
# to an output file with all letters
# converted to uppercase.
#
#PROCESSING: 1) Open the input file
# 2) Open the output file
# 3) While we're not at the end of the input file
# a) read part of file into our memory buffer
# b) go through each byte of memory
# if the byte is a lower-case letter,
# convert it to uppercase
# c) write the memory buffer to output file
.section .data
#######CONSTANTS########
#system call numbers
.equ SYS_OPEN, 5
.equ SYS_WRITE, 4
.equ SYS_READ, 3
.equ SYS_CLOSE, 6
.equ SYS_EXIT, 1
#options for open (look at
#/usr/include/asm/fcntl.h for
#various values. You can combine them
#by adding them or ORing them)
#This is discussed at greater length
#in "Counting Like a Computer"
.equ O_RDONLY, 0
.equ O_CREAT_WRONLY_TRUNC, 03101
#standard file descriptors
.equ STDIN, 0
.equ STDOUT, 1
.equ STDERR, 2
#system call interrupt
.equ LINUX_SYSCALL, 0x80
.equ END_OF_FILE, 0 #This is the return value
#of read which means we've
#hit the end of the file
.equ NUMBER_ARGUMENTS, 2
.section .bss
#Buffer - this is where the data is loaded into
# from the data file and written from
# into the output file. This should
# never exceed 16,000 for various
# reasons.
.equ BUFFER_SIZE, 500
.lcomm BUFFER_DATA, BUFFER_SIZE
.equ ST_FD_IN, 10
.equ ST_FD_OUT, 10
.section .text
#STACK POSITIONS
.equ ST_SIZE_RESERVE, 8
#.equ ST_FD_IN, -4
#.equ ST_FD_OUT, -8
.equ ST_ARGC, 0 #Number of arguments
.equ ST_ARGV_0, 4 #Name of program
.equ ST_ARGV_1, 8 #Input file name
.equ ST_ARGV_2, 12 #Output file name
.globl _start
_start:
###INITIALIZE PROGRAM###
#save the stack pointer
movl %esp, %ebp
#Allocate space for our file descriptors
#on the stack
subl $ST_SIZE_RESERVE, %esp
open_files:
open_fd_in:
###OPEN INPUT FILE###
#open syscall
movl $SYS_OPEN, %eax
#input filename into %ebx
movl ST_ARGV_1(%ebp), %ebx
#read-only flag
movl $O_RDONLY, %ecx
#this doesn't really matter for reading
movl $0666, %edx
#call Linux
int $LINUX_SYSCALL
store_fd_in:
#save the given file descriptor
movl %eax, ST_FD_IN(%ebp)
open_fd_out:
###OPEN OUTPUT FILE###
#open the file
movl $SYS_OPEN, %eax
#output filename into %ebx
movl ST_ARGV_2(%ebp), %ebx
#flags for writing to the file
movl $O_CREAT_WRONLY_TRUNC, %ecx
#permission set for new file (if it's created)
movl $0666, %edx
#call Linux
int $LINUX_SYSCALL
store_fd_out:
#store the file descriptor here
movl %eax, ST_FD_OUT
###BEGIN MAIN LOOP###
read_loop_begin:
###READ IN A BLOCK FROM THE INPUT FILE###
movl $SYS_READ, %eax
#get the input file descriptor
movl ST_FD_IN, %ebx
#the location to read into
movl $BUFFER_DATA, %ecx
#the size of the buffer
movl $BUFFER_SIZE, %edx
#Size of buffer read is returned in %eax
int $LINUX_SYSCALL
###EXIT IF WE'VE REACHED THE END###
#check for end of file marker
cmpl $END_OF_FILE, %eax
#if found or on error, go to the end
jle end_loop
continue_read_loop:
###CONVERT THE BLOCK TO UPPER CASE###
pushl $BUFFER_DATA #location of buffer
pushl %eax #size of the buffer
call convert_to_upper
popl %eax #get the size back
addl $4, %esp #restore %esp
###WRITE THE BLOCK OUT TO THE OUTPUT FILE###
#size of the buffer
movl %eax, %edx
movl $SYS_WRITE, %eax
#file to use
movl ST_FD_OUT, %ebx
#location of the buffer
movl $BUFFER_DATA, %ecx
int $LINUX_SYSCALL
###CONTINUE THE LOOP###
jmp read_loop_begin
end_loop:
###CLOSE THE FILES###
#NOTE - we don't need to do error checking
# on these, because error conditions
# don't signify anything special here
movl $SYS_CLOSE, %eax
movl ST_FD_OUT, %ebx
int $LINUX_SYSCALL
movl $SYS_CLOSE, %eax
movl ST_FD_IN, %ebx
int $LINUX_SYSCALL
###EXIT###
movl $SYS_EXIT, %eax
movl $0, %ebx
int $LINUX_SYSCALL
###CONSTANTS##
#The lower boundary of our search
.equ LOWERCASE_A, 'a'
#The upper boundary of our search
.equ LOWERCASE_Z, 'z'
#Conversion between upper and lower case
.equ UPPER_CONVERSION, 'A' - 'a'
###STACK STUFF###
.equ ST_BUFFER_LEN, 8 #Length of buffer
.equ ST_BUFFER, 12 #actual buffer
convert_to_upper:
pushl %ebp
movl %esp, %ebp
###SET UP VARIABLES###
movl ST_BUFFER(%ebp), %eax
movl ST_BUFFER_LEN(%ebp), %ebx
movl $0, %edi
#if a buffer with zero length was given
#to us, just leave
cmpl $0, %ebx
je end_convert_loop
convert_loop:
#get the current byte
movb (%eax,%edi, 1), %cl
#go to the next byte unless it is between
#'a' and 'z'
cmpb $LOWERCASE_A, %cl
jl next_byte
cmpb $LOWERCASE_Z, %cl
jg next_byte
#otherwise convert the byte to uppercase
addb $UPPER_CONVERSION, %cl
#and store it back
movb %cl, (%eax,%edi,1)
next_byte:
incl %edi #next byte
cmpl %edi, %ebx #continue unless
#we've reached the
#end
jne convert_loop
end_convert_loop:
#no return value, just leave
movl %ebp, %esp
popl %ebp
ret
I tried to define (.equ) the file descriptors in the .bss section (i.e. reserved them the space of 10) and then use them in the code. But, despite of trying different approaches I was not able to get it working.
.equ does not reserve space. Rather, it assigns a value to a label. To reserve space, use the .space or .lcomm directive.