Xref: utzoo comp.sys.amiga.tech:7568 comp.sys.amiga:41586
Path: utzoo!utgpu!watmath!watdragon!rose!ccplumb
From: ccplumb@rose.waterloo.edu (Colin Plumb)
Newsgroups: comp.sys.amiga.tech,comp.sys.amiga
Subject: Re: More dos questions
Keywords: dos,sod,dso,osd
Message-ID: <17200@watdragon.waterloo.edu>
Date: 13 Oct 89 17:15:51 GMT
References: <5599@ucdavis.ucdavis.edu>
Sender: daemon@watdragon.waterloo.edu
Reply-To: ccplumb@rose.waterloo.edu (Colin Plumb)
Organization: U. of Waterloo, Ontario
Lines: 18

In article <5599@ucdavis.ucdavis.edu> rogers@iris.ucdavis.edu (Brewski Rogers)
writes:

>1.	How exactly is Seek() implemented? If I Seek 200K forward, will dos
>read through all the data until it gets there, or does it have some way of 
>figuring out where to move the head to? If it does have a way of knowing
>where to move the head to, is the information easily found, or does it
>have to move the head to find it?

In the current FS's, a file header block contains pointers to the first 72
data blocks in a file, and a pointer to an extension block, which holds
another 72 pointers and another next pointer.  So you have to follow
the chain of extension blocks until you find the correct one, then you
have all the pointers at your disposal.  If you Seek() forward, you can
start from the "current" extension block, but if you go backwards, you
have to go back to the start and scan forwards again.
-- 
	-Colin