We can improve the efficiency of our code by buffering our input and output. We create an input buffer and read a whole sequence of bytes at one time. Then we fetch them one by one from the buffer.
We also create an output buffer. We store our output in it until it is full. At that time we ask the kernel to write the contents of the buffer to stdout.
The program ends when there is no more input. But we still need to ask the kernel to write the contents of our output buffer to stdout one last time, otherwise some of our output would make it to the output buffer, but never be sent out. Do not forget that, or you will be wondering why some of your output is missing.
%include 'system.inc'
%define BUFSIZE 2048
section .data
hex db '0123456789ABCDEF'
section .bss
ibuffer resb BUFSIZE
obuffer resb BUFSIZE
section .text
global _start
_start:
sub eax, eax
sub ebx, ebx
sub ecx, ecx
mov edi, obuffer
.loop:
; read a byte from stdin
call getchar
; convert it to hex
mov dl, al
shr al, 4
mov al, [hex+eax]
call putchar
mov al, dl
and al, 0Fh
mov al, [hex+eax]
call putchar
mov al, ' '
cmp dl, 0Ah
jne .put
mov al, dl
.put:
call putchar
jmp short .loop
align 4
getchar:
or ebx, ebx
jne .fetch
call read
.fetch:
lodsb
dec ebx
ret
read:
push dword BUFSIZE
mov esi, ibuffer
push esi
push dword stdin
sys.read
add esp, byte 12
mov ebx, eax
or eax, eax
je .done
sub eax, eax
ret
align 4
.done:
call write ; flush output buffer
push dword 0
sys.exit
align 4
putchar:
stosb
inc ecx
cmp ecx, BUFSIZE
je write
ret
align 4
write:
sub edi, ecx ; start of buffer
push ecx
push edi
push dword stdout
sys.write
add esp, byte 12
sub eax, eax
sub ecx, ecx ; buffer is empty now
ret
We now have a third section in the source code, named .bss. This section is not included in our executable file, and,
therefore, cannot be initialized. We use resb instead of
db. It simply reserves the requested size of uninitialized
memory for our use.
We take advantage of the fact that the system does not modify the registers: We use
registers for what, otherwise, would have to be global variables stored in the .data section. This is also why the UNIX® convention of passing parameters to system calls on
the stack is superior to the Microsoft convention of passing them in the registers: We
can keep the registers for our own use.
We use EDI and ESI as
pointers to the next byte to be read from or written to. We use EBX and ECX to keep count of the
number of bytes in the two buffers, so we know when to dump the output to, or read more
input from, the system.
Let us see how it works now:
% nasm -f elf hex.asm % ld -s -o hex hex.o % ./hex Hello, World! Here I come! 48 65 6C 6C 6F 2C 20 57 6F 72 6C 64 21 0A 48 65 72 65 20 49 20 63 6F 6D 65 21 0A ^D %
Not what you expected? The program did not print the output until we pressed ^D. That is easy to fix by inserting three lines of code to write
the output every time we have converted a new line to 0A. I
have marked the three lines with > (do not copy the > in your hex.asm).
%include 'system.inc'
%define BUFSIZE 2048
section .data
hex db '0123456789ABCDEF'
section .bss
ibuffer resb BUFSIZE
obuffer resb BUFSIZE
section .text
global _start
_start:
sub eax, eax
sub ebx, ebx
sub ecx, ecx
mov edi, obuffer
.loop:
; read a byte from stdin
call getchar
; convert it to hex
mov dl, al
shr al, 4
mov al, [hex+eax]
call putchar
mov al, dl
and al, 0Fh
mov al, [hex+eax]
call putchar
mov al, ' '
cmp dl, 0Ah
jne .put
mov al, dl
.put:
call putchar
> cmp al, 0Ah
> jne .loop
> call write
jmp short .loop
align 4
getchar:
or ebx, ebx
jne .fetch
call read
.fetch:
lodsb
dec ebx
ret
read:
push dword BUFSIZE
mov esi, ibuffer
push esi
push dword stdin
sys.read
add esp, byte 12
mov ebx, eax
or eax, eax
je .done
sub eax, eax
ret
align 4
.done:
call write ; flush output buffer
push dword 0
sys.exit
align 4
putchar:
stosb
inc ecx
cmp ecx, BUFSIZE
je write
ret
align 4
write:
sub edi, ecx ; start of buffer
push ecx
push edi
push dword stdout
sys.write
add esp, byte 12
sub eax, eax
sub ecx, ecx ; buffer is empty now
ret
Now, let us see how it works:
% nasm -f elf hex.asm % ld -s -o hex hex.o % ./hex Hello, World! 48 65 6C 6C 6F 2C 20 57 6F 72 6C 64 21 0A Here I come! 48 65 72 65 20 49 20 63 6F 6D 65 21 0A ^D %
Not bad for a 644-byte executable, is it!
Note: This approach to buffered input/output still contains a hidden danger. I will discuss--and fix--it later, when I talk about the dark side of buffering.
Warning: This may be a somewhat advanced topic, mostly of interest to programmers familiar with the theory of compilers. If you wish, you may skip to the next section, and perhaps read this later.
While our sample program does not require it, more sophisticated filters often need to look ahead. In other words, they may need to see what the next character is (or even several characters). If the next character is of a certain value, it is part of the token currently being processed. Otherwise, it is not.
For example, you may be parsing the input stream for a textual string (e.g., when implementing a language compiler): If a character is followed by another character, or perhaps a digit, it is part of the token you are processing. If it is followed by white space, or some other value, then it is not part of the current token.
This presents an interesting problem: How to return the next character back to the input stream, so it can be read again later?
One possible solution is to store it in a character variable, then set a flag. We can
modify getchar to check the flag, and if it is set, fetch
the byte from that variable instead of the input buffer, and reset the flag. But, of
course, that slows us down.
The C language has an ungetc() function, just for that
purpose. Is there a quick way to implement it in our code? I would like you to scroll
back up and take a look at the getchar procedure and see if
you can find a nice and fast solution before reading the next paragraph. Then come back
here and see my own solution.
The key to returning a character back to the stream is in how we are getting the characters to start with:
First we check if the buffer is empty by testing the value of EBX. If it is zero, we call the read
procedure.
If we do have a character available, we use lodsb, then
decrease the value of EBX. The lodsb instruction is effectively identical to:
mov al, [esi]
inc esi
The byte we have fetched remains in the buffer until the next time read is called. We do not know when that happens, but we do know
it will not happen until the next call to getchar. Hence,
to "return" the last-read byte back to the stream, all we have to do is decrease the
value of ESI and increase the value of EBX:
ungetc:
dec esi
inc ebx
ret
But, be careful! We are perfectly safe doing this if our look-ahead is at most one
character at a time. If we are examining more than one upcoming character and call ungetc several times in a row, it will work most of the time, but
not all the time (and will be tough to debug). Why?
Because as long as getchar does not have to call read, all of the pre-read bytes are still in the buffer, and our
ungetc works without a glitch. But the moment getchar calls read, the contents of
the buffer change.
We can always rely on ungetc working properly on the
last character we have read with getchar, but not on
anything we have read before that.
If your program reads more than one byte ahead, you have at least two choices:
If possible, modify the program so it only reads one byte ahead. This is the simplest solution.
If that option is not available, first of all determine the maximum number of
characters your program needs to return to the input stream at one time. Increase that
number slightly, just to be sure, preferably to a multiple of 16--so it aligns nicely.
Then modify the .bss section of your code, and create a
small "spare" buffer right before your input buffer, something like this:
section .bss
resb 16 ; or whatever the value you came up with
ibuffer resb BUFSIZE
obuffer resb BUFSIZE
You also need to modify your ungetc to pass the value of
the byte to unget in AL:
ungetc:
dec esi
inc ebx
mov [esi], al
ret
With this modification, you can call ungetc up to 17
times in a row safely (the first call will still be within the buffer, the remaining 16
may be either within the buffer or within the "spare").
This, and other documents, can be downloaded from ftp://ftp.FreeBSD.org/pub/FreeBSD/doc/.
For questions about FreeBSD, read the documentation before contacting <[email protected]>.
For questions about this documentation, e-mail <[email protected]>.