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

[Sean 'Captain Napalm' Conner]

It was thus said that the Great Patrick Finnegan once stated:
> 
> Chuck Guzis declared on Wednesday 07 December 2005 20:26:
> > keeping things like the DAA instruction intact and supporting a bunch
> > of do-nothing instructions like MOV DL,DL.
> 
> FYI, that most certainly doesn't just "do nothing."  It updates the flags 
> based on the contents of the DL register, and you could do a conditional 
> jump based on the results.
> 
> > Consider on Windows XP that you can still run the following code:
> >
> > 	mov	cl,9
> > 	mov	dx,offset HWMes
> > 	call	5
> > 	mov	cl,0
> > 	call	5
> 
> Huh?  Do you mean this?
> 
> MOV  AH, 9
> MOV  DX, HWMes
> INT  21h
> MOV  AH, 0
> INT  21h

  No, he meant what he wrote.  It's the CP/M compatibility layer built into
MS-DOS.  Address 5 of the PSP (Program Segment Prefix) contains a long jump
to MS-DOS, and uses CL for the function code instead of AH.  

  -spc (@#$@#!# thought I forgot my MS-DOS arcana ... )