MOD vs divide

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me239

Joined: 06 Jan 2011
Posts: 200

me239 29 May 2011, 06:13

Ok, this may sound crazy, but I just really wanted to know If I'm the only one who sees this. Here is the code\equation I've seen on http://www.brokenthorn.com/Resources/OSDev6.html
It says that "absolute sector = (logical sector / sectors per track) + 1", but the code goes as

Code:

LBACHS:
          xor     dx, dx                              ; prepare dx:ax for operation
          div     WORD [bpbSectorsPerTrack]           ; divide by sectors per track
          inc     dl                                  ; add 1 (obsolute sector formula)
          mov     BYTE [absoluteSector], dl

Isn't this code MODing the sectorspertrack rather than just dividing?

29 May 2011, 06:13

LocoDelAssembly
Your code has a bug

Joined: 06 May 2005
Posts: 4624
Location: Argentina

LocoDelAssembly 29 May 2011, 06:43

Yes it is. The "/" in the equation should be read MOD. Also, if by "absolute sector" the documentation really means CHS sector (normally [1..63]), notice how wrong it would be a division there as it could easily exceed 63.

29 May 2011, 06:43

sinsi

Joined: 10 Aug 2007
Posts: 794
Location: Adelaide

sinsi 29 May 2011, 08:19

If they used AL then the mod result would apply but they are using DL which is the remainder of the div.

29 May 2011, 08:19

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