Legacy apps in Windows/OS X was Re: Old MS-DOS & Win Software

Chuck Guzis cclist at sydex.com
Thu Dec 8 17:19:31 CST 2005


On 12/8/2005 at 4:24 PM Jim Leonard wrote:

>Is this "half-carry" flag addressable, or is it just something internal? 
>A 
>quick gander at 808x flags doesn't show a "half-carry" flag, unless I'm
>missing 
>something obvious.

Okay, on the 8080, when you do a PUSH PSW, the flags are stored in the
following arrangement:

	SZ-H-P-C

If you had a NEC 8080A, the flags are similar:

	SZsH-P-C

with the "s" being the subtract flag.  The Z80 used a similar, but not
identical arrangement:

	SZ-H-PsC

Where "'s" is still the subtract flag, but the parity flag "p" now serves
as both parity and overlow indicator.

The 8086 has a similar flag arrangement to the 8080 if you do a LAHF
instruction and look at what's in AH:

	SZ-H-P-C

The big difference in the half-carry with the Z80 and NEC 8080A from the
Intel 8080 is that one can DAA after a subtract and get a meaningful result
(which is why a "subtract" flag is kept around).  The 8086 drops the
problem in the user's lap and adds another instruction, DAS, to do the same
thing.

Cheers,
Chuck





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