Math [divs]

[LeCaravage]

Hi,
Some problem I've got with divs instructions.

I'd like to divide 2 numbers in order to add them sequentially, a kind of linear interpolation. I wrote this code :

move.w #101,d0
move.w #100,d1
ext.l d
asl.l #8,d0
asl.l #6,d1
divs d1,d0
clr.w d0
asr.l #2,d0

The amplitude of D0-D1 = 352, it's an absolute value (D0 and D1 are signed).
I need to have result of iteration in 16:16 format so I could use a simple SWAP within interpolation loop instead of arithmetic shifts.

So my question is, why I lose precision ? for instance the result in D0 = $00010000 which means in my choice of decimal precision D0 = 1.
the decimal part should be around $28F ($0001028F).
The strange thing is also when I calculate the inverse of this divsion we get a worse approximation.
I could get a better precision avoiding the ASL of D1 but I expercience overflow errors more easily.
I suspect I'm using bad precision choices but I'm not sure.
Any hints you Amiga coders ?

[a/b]

The problem is you that are nuking the relevant part with clr.w d0. For a more detailed insight, look up fixed point math.
In general... For example, say you are using format 24:8: 24 bits int, 8 bits fraction (so, all numbers are multiplied by 256, precision is 1/256). Every arithmetic operation will require *normalization* of the result:
1. add/sub: auto-normalized, nothing do to
2. mul: result is 16:16 so you have to divide it by 256 (right-shift 8 times)
3. div: pre/post-normalization is required because after the div it's too late (fraction is nuked), mul the dividend by 256 (left-shift 8 times)

  move.l #100<<8,d0  ; 24:8
  move.l #101<<8,d1  ; 24:8
; add/sub
  move.l d0,d2
  add.l d1,d2  ; d2 is 24:8, done
; mul
  muls.w d1,d0  ; d0 is 16:16
  asr.l #8,d0  ; d0 is 24:8, normalized
; div
  asl.l #8,d0  ; d0 is 16:16
  divs.w d1,d0  ; d0 is 8:8
  ext.l d0  ; d0 is 24:8

[LeCaravage]

Well, according to my 68000 book, the upper 16 bits of divs result is the rest. I understand it like a divs is a succession of subtractions (like in decimal) and when it's too small this amount is placed in upper word.

My bad, I just checked, I transcibed with a missing swap before the clr.w.

I forgot this ext.l, wasn't a problem for the moment until I use negatives numbers.

Anyway many thanks for taking time to write your example, I'll investigated further using your code.

Edit : "I forgot this ext.l, wasn't a problem for the moment until I use negatives numbers." : There, I was speaking about the ext.l right after the divs

[LeCaravage]

@a/b : You' da man ! It works. By analysing my mistakes it seems it was a pb of precision, I missed the additional asl.l #8,d0 you did. Now I have quite accurate result
Also I had to reduce divisor precision, 24:8 was too much.
Cheers,