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

[roger]

I believe this is a known bug for this part of the book.

It's simple to fix once you understand the bug. ;-)

If you check the archives, you'll find the answer.  Maybe another poster
remembers the specifics for this program.


On Sat, 2008-05-17 at 13:10 +0000, david helfgott 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 
> 
> #######################
> 
> 
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-- 
Roger
http://www.eskimo.com/~roger/index.html

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