upper() and lower()

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Hayden

Joined: 06 Oct 2005
Posts: 132

Hayden 16 Jun 2007, 16:49

this may sound stupid but I was wondering if anyone had some optimized code for upper/lower string conversions... this is the algo i usualy use... pesuedo code...

Code:

TO_LOWER = 'a' - 'A'

; convert string to lower case
    = x + (x >= 'A' and x <= 'Z') * TO_LOWER

; convert string to upper case
    = x - (x >= 'a' and x <= 'z') * TO_LOWER

I want to check for 1 of 3 possible ascii key presses, I'm useing int 16h and need to be capslock aware... I hav'nt seen upper/lower functions in asm before so i'm looking for a few pointers.

_________________
New User.. Hayden McKay.

16 Jun 2007, 16:49

LocoDelAssembly
Your code has a bug

Joined: 06 May 2005
Posts: 4624
Location: Argentina

LocoDelAssembly 16 Jun 2007, 18:46

Here my branchless version (which doesn't means that it's the most optimized in existence):

Code:

upper:; AL = char to convert
  mov  dl, al

  sub  al, 'a'
  cmp  al, 'z'-'a'+1
  salc

  and  al, 'A'-'a'
  add  al, dl

  ret


lower:; AL = char to convert
  mov  dl, al

  sub  al, 'A'
  cmp  al, 'Z'-'A'+1
  salc

  and  al, 'a'-'A'
  add  al, dl

  ret

[edit]

Code:

upperLetter: ; AL = letter to convert
  and al, not $20
  
  ret

lowerLetter: ; AL = letter to convert
  or  al, $20

  ret

Added Goplat's suggestion. Check below for remarks.[/edit]

Last edited by LocoDelAssembly on 16 Jun 2007, 22:49; edited 1 time in total

16 Jun 2007, 18:46

Goplat

Joined: 15 Sep 2006
Posts: 181

Goplat 16 Jun 2007, 21:51

If all you're doing is checking for specific letters, rather than actually converting to lower/uppercase, you don't need to do real case mapping, a simple bit flip will do the trick. Lowercase letters have the same codes as uppercase with bit 5 set, so for example

Code:

        or      al,20h
      cmp     al,'q'
    je      q_pressed

will jump on either 'Q' or 'q'. Of course, punctation and such will get changed - eg, '@' becomes '`' - but none of them become letters, so the check is still safe.

16 Jun 2007, 21:51

LocoDelAssembly
Your code has a bug

Joined: 06 May 2005
Posts: 4624
Location: Argentina

LocoDelAssembly 16 Jun 2007, 22:56

I can't understand why I thought that trick was unsafe (without previous checkings) even on these cases :S

Well, maybe because I saw it about 6 years ago when I started with programming and in all these years I forgot to think again about it Embarassed

Thanks for pointing out this Goplat!!

16 Jun 2007, 22:56

Hayden

Joined: 06 Oct 2005
Posts: 132

Hayden 19 Jun 2007, 14:51

thanks guys

_________________
New User.. Hayden McKay.

19 Jun 2007, 14:51

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