Decrementing a 16 bit number involves two steps:

decrement the lower byte
If the lower byte rolled over (0 to 255) then decrement the higher byte.

The easiest way to find out if the lower byte rolled over is to see if
it is zero before you decrement it.  Now it's two steps, but in reverse:

if lower byte is 0, then decrement higher byte
decrement lower byte

The easiest way to find out if the lower byte is zero is by using "MOVF
lowbyte, F" (in the 16 series - as noted the 18 series has a command
just for this kind of problem).  As you'll note in the instruction
description the MOVF instruction modifies the ZERO flag in the status
register based on the data it is working on.  So if you execute "MOVF
lowbyte, F" and F is 0, then the ZERO flag will be set to one.  You then
use a bit test instruction to determine if the flag is set - if it isn't
set, then skip the decrement high byte instruction.

-Adam

Thomas N wrote:

> Thank you for your help!  What's the purpose of "MOVF lowbyte,F".  Please
> help me to understand.
>
> Thank again!
> Tom
>
>
>> From: "M. Adam Davis" <adampic@UBASICS.COM>
>> Reply-To: pic microcontroller discussion list <PICLIST@MITVMA.MIT.EDU>
>> To: PICLIST@MITVMA.MIT.EDU
>> Subject: Re: [PIC]  Subtract 16-bit number by 1
>> Date: Sat, 1 Jun 2002 17:10:48 -0400
>>
>> This can be reduced by one instruction if you're using an 18 series
>> processor:
>>
>> TSTFSZ lowbyte
>> DECF highbyte, F
>> DECF lowbyte, F
>>
>> -Adam
>>
>> Mike Harrison wrote:
>>
>>> On Sat, 1 Jun 2002 15:18:35 -0400, you wrote:
>>>
>>>
>>>
>>>> Four instructions:
>>>>
>>>> MOVF lowbyte, W
>>>>
>>>>
>>> ..should be MOVF lowbyte,F - that way you don't overwrite W
>>>
>>>
>>>> BTFSC zeroflag
>>>> DECF highbyte, F
>>>> DECF lowbyte, F
>>>>
>>>> -Adam
>>>>
>>>> Thomas N wrote:
>>>>
>>>>
>>>>
>>>>> Hi all
>>>>>
>>>>> Is there an easy way to subtract 16-bit number by 1?  How?
>>>>> Thanks!
>>>>> Thomas
>>>>>
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