Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!gatech!ncar!ico!nbires!maa
From: maa@nbires.nbi.com (Mark Armbrust)
Newsgroups: comp.sys.ibm.pc
Subject: Re: MS-DOS puzzle #1
Summary: JFN table is version dependant!
Message-ID: <425@nbires.nbi.com>
Date: 14 Jul 89 18:07:50 GMT
References: <8167@bsu-cs.bsu.edu> <1171@calvin.EE.CORNELL.EDU> <5486@pt.cs.cmu.edu> <5565@tekgvs.LABS.TEK.COM>
Reply-To: maa@nbires.UUCP (Mark Armbrust)
Distribution: comp
Organization: NBI Inc, Boulder CO
Lines: 31

In article <5565@tekgvs.LABS.TEK.COM> toma@tekgvs.LABS.TEK.COM (Tom Almy) writes:
>In article <5486@pt.cs.cmu.edu> ralf@b.gp.cs.cmu.edu (Ralf Brown) writes:
>>What can be done, however, (and this works for DOS 2 and up), is to copy the
>>appropriate entries in the Process File Table in the PSP, zap them to FFh
>
>1. Save entire PFT (it's not that big!)
>2. Set entries above the std* ones to FFh (effectively temprarily closes
>
>Was that easy enough?  Much simpler than the contortions in UNIX with
>those DUP2s! There certainly is no reason COMMAND.COM couldn't
>do it.  And fork() has nothing to do with it.

I prefer to use the dos calls to do the job since there is a dos version
dependancy when mucking directly with the jfn_table in the pdb.  

Starting with some version of dos between 2.11 and 3.2 (I only have these two 
handy to look at), jfn_table_length and jfn_table_pointer have been added to 
the pdb.  

pdb + 32h:	jfn_table_len	dw	?	; length of jfn table
pdb + 34h:	jfn_table_ptr	dd	?	; FAR pointer to jfn table

These are initialized to point to the jfn_table in the pdb, but can be changed
by an application to point to a larger jfn_table.  Starting in dos 3.3, there
is a documented dos call (int 21h, ah=67h) to do this!

-- 

Mark Armbrust
maa@nbires.nbi.com
maa@nbires.UUCP