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flat assembler > Main > Integer Division

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CandyMan



Joined: 04 Sep 2009
Posts: 285
Location: film "CandyMan" directed through Bernard Rose OR Candy Shop
how to check whether the quotient will fit in AL before integer division (TheWord>0xFF)?
Code:
mov ax,[TheWord]
idiv cl    

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Post 10 Mar 2018, 17:14
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DimonSoft



Joined: 03 Mar 2010
Posts: 422
Location: Belarus
CandyMan wrote:
how to check whether the quotient will fit in AL before integer division (TheWord>0xFF)?
Code:
mov ax,[TheWord]
idiv cl    

You’d better perform division for double-sized values.

Code:
mov ax, [TheWord]
cwd
movsx cx, cl
idiv cx    


You’re not in control of #DE anyway and wouldn’t like to be. So just prevent it.
Post 10 Mar 2018, 19:06
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CandyMan



Joined: 04 Sep 2009
Posts: 285
Location: film "CandyMan" directed through Bernard Rose OR Candy Shop
Thanks. This works with byte integer division. How to prevent exception in this case:
Code:
mov edx,[Hi]
mov eax,[Lo]
idiv ecx    

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Post 10 Mar 2018, 20:39
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Tomasz Grysztar
Assembly Artist


Joined: 16 Jun 2003
Posts: 6966
Location: Kraków, Poland
You may already be aware that for unsigned division this is very simple:
Code:
mov edx,[Hi]
mov eax,[Lo]
cmp edx,ecx
jae overflow
div ecx    
This is because the range of EDX:EAX that does not cause an overflow is from 0 to (1 shl 32)*ECX (not inclusive), and you can test if EDX:EAX fits in this range simply by comparing EDX with ECX.

The signed case is much more complex. You need EDX:EAX to be larger than -(1 shl 31 + 1)*ECX and smaller than (1 shl 31)*ECX, this could be tested with some shifts, but would not be as nice and simple.
Post 10 Mar 2018, 21:38
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CandyMan



Joined: 04 Sep 2009
Posts: 285
Location: film "CandyMan" directed through Bernard Rose OR Candy Shop
I am thanking everyone for help.
Post 11 Mar 2018, 12:33
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