Re: A83: Just Curious...

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Re: A83: Just Curious...


Another fun thing to do, is to do calculations in 10 complement (just to annoy
people :)

like this.  say we want to calculate 98-43

we take 43, invert it to 56  (when using our ordinary numbers we have 10 of them
instead of two.  so 0 inverted is 9, 1 inv. is 8 and so on.)
add 1, we have now 57

       98
      +57
     ----
      155
      A
      |
      +-- "overflow" we only use 2 digit numbers, ignore.. :)

voila! we have the answer! 55!

neat... isn't it? :)
//Olle



Olle Hedman wrote:
> 
> no.. it is not _only_ bit 7. you can see on bit 7 if the number is negative but
> it is _not_ like this:
> 
> %00000111  ; 7
> %10000111  ; This is _not_ -7
> 
> the way of representing negative numbers that is commonly used in all computer
> hardware of today is 2 complement. that is:
> 
> %00000111  ; The number 7
> 
> %11111000  ; first invert.
> %11111001  ; add 1.
> 
> %11111001  ; This is -7.
> 
> When you represent a negative number like this, you can use the same algoritm to
> both add and subtract. and you get rid of the problem of having two zeros
> %10000000 and %00000000 would both be zero if you used the first method..
> (not very effective.)
> 
> //Olle
> 
> Scott Dial wrote:
> >
> > I thought of that, but a negative number in our respect is simply
> > denoted by bit 7's value. So, how would you "add" the 0-6 as if they
> > were negative then??? It is the same as if they were positive.
> >
> > Olle Hedman wrote:
> > >
> > > A subtraction is normally done by adding a negative number.
> > >
> > > //Olle
> > >
> > > Scott Dial wrote:
> > > >
> > > > I was playing around the other day and I was thinking about how a byte
> > > > is added to another byte, and I wrote some z80 code that used bit-level
> > > > commands to add a byte to another (Basically, doesn't use add). I got to
> > > > trying to figure out subtracting and I got stumped... This really as no
> > > > point, but I was just wondering how a subtraction is performed. And,
> > > > don't bother telling me how "unoptimized" it is and I don't need some
> > > > smart-ass to say, "Why don't you just say 'add a,b'?"!
> > > >
> > > >  ld a,1
> > > >  ld b,1
> > > > AddAtoB:
> > > >  push bc
> > > >  push af
> > > >  xor b          ;XOR A and B
> > > >  pop de
> > > >  pop bc
> > > >  push af
> > > >  push de
> > > >  pop af
> > > >  and b          ;AND A and B
> > > >  rs             ;RS (AND A and B)
> > > >  pop bc         ;    |
> > > >  or b           ;OR -/ and B
> > > >  ret            ;A = A + B
> > > >
> > > > --
> > > > Scott "_Wrath_" Dial
> > > > homosapian@geocities.com
> > > > ICQ#3608935
> > > > http://www.geocities.com/~homosapian/
> >
> > --
> > Scott "_Wrath_" Dial
> > homosapian@geocities.com
> > ICQ#3608935
> > http://www.geocities.com/~homosapian/
> > ________________________________________________________
> > NetZero - We believe in a FREE Internet.  Shouldn't you?
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