Hello, If I understand you correctly, you are asking why isn't the same N (buffer size) bytes of the file read over an over again, because there is no explicit "move the file position pointer forward" in the program.
This is handled "under the covers" by the Linux system call "read." .equ SYS_READ, 3 From the documentation: *read()* attempts to read up to *count* bytes from file descriptor *fd* into the buffer starting at *buf*. " On success, the number of bytes read is returned (zero indicates end of file), and the file position is advanced by this number." http://linux.about.com/od/commands/l/blcmdl2_read.htm Justis On Sat, May 17, 2008 at 6:10 AM, david helfgott <[EMAIL PROTECTED]> wrote: > 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 ? > > > > > > #################### > > 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 > > ####################### > > ------------------------------ > Windows Live Messenger - Schreiben. Sehen. Hören. Wie im echten Leben. Windows > Live > Messenger!<http://imagine-msn.com/messenger/launch80/default.aspx?locale=de-de&source=joinmsncom/messenger> > > _______________________________________________ > Pgubook-readers mailing list > [email protected] > http://lists.nongnu.org/mailman/listinfo/pgubook-readers > >
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