[Pgubook-readers] Chapter 5, toupper-program: understanding the read loop

[david helfgott]

Hi there !

I got some serious problems with understanding the read loop in the toupper programm.

Where does the computer know to start again from, after the "jmp read_loop begin" ?

Why doesn't the computer always reads the same first 500 bytes into the buffer ?

Where does he knows from, where he has to continue ?

after the "jmp read_loop begin" he starts again at

movl $SYS_READ, %eax
movl $ST_FD_IN(%ebp), %ebx
movl $BUFFER_DATA, %ecx
movl $BUFFER_SIZE, %edx
int $LINUX_SYSCALL

that's exactly the same information he got in the first place.

So if I have a file conatining 700 bytes.
Where does he know from to start reading at byte No. 501 in the secon run, and not just read the first 500 bytes again ?

Where's the extra information, that tells him, he already red the first 500 bytes and has to read the next 500 byte ?





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read_loop_begin:

###READ IN A BLOCK FROM THE INPUT FILE###

movl $SYS_READ, %eax


#get the input file descriptor

movl ST_FD_IN(%ebp), %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(%ebp), %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(%ebp), %ebx
int $LINUX_SYSCALL
movl $SYS_CLOSE, %eax
movl ST_FD_IN(%ebp), %ebx
int $LINUX_SYSCALL
###EXIT###
movl $SYS_EXIT, %eax
movl $0, %ebx
int $LINUX_SYSCALL

#######################

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