From: paul pigott 
Subject: Re: 'CF' & Interrupt List
 
Marc van den Dikkenberg wrote:
>
> I'm using Ralf Brown's interrupt list to do some low-level thing with
> PowerBasic... So far,e verything works the way it should.
>
> I'm wondering about one thing, though: in a lot of entries, a 'CF' is
> mentioned, mostly for detecting whether or not the operation was succesful.
> 'CF set on error' / 'CF clear if succesful'
>
> Are they talking about CX, part of CX, or something totally different?
> As far as I'm aware, there is no 'CF' register... Or is there?
> --
> Marc van den Dikkenberg
> -----------------------
> The PowerBasic Archives
> http://www.xs4all.nl/~excel/pb.html
 
The CF stands for "CARRY FLAG".  It's actually one particular bit in the
FLAGS register (which one I don't offhand recall).  Its basic use, as
you might surmise, is in arithmetic, indicating whether a mathematical
carry (as in "carry the one") or borrow has occurred.  However, in a
number of interrupt operations, it's used to indicate the success, or
lack thereof, of a particular operation.  There are a two assembler
mnemonics to test the carry flag, JC and JNC to be precise:  "Jump If
Carry" and "Jump If Not Carry".
 
Hope this helps.
 
Paul Pigott
paul.pigott@worldnet.att.net
 
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