Path: utzoo!attcan!uunet!lll-winken!lll-tis!helios.ee.lbl.gov!pasteur!agate!labrea!rutgers!bellcore!tness7!tness1!sugar!peter
From: peter@sugar.uu.net (Peter da Silva)
Newsgroups: comp.sys.amiga.tech
Subject: Re: Having your cake (Re: Address Space on the Amiga (was Re: Need info on exceptions))
Message-ID: <2600@sugar.uu.net>
Date: 9 Sep 88 00:39:58 GMT
References: <8809021811.AA13386@cory.Berkeley.EDU>
Organization: Sugar Land Unix - Houston, TX
Lines: 34

In article <8809021811.AA13386@cory.Berkeley.EDU>, dillon@CORY.BERKELEY.EDU
	(Matt Dillon) writes:
> :First thing to note... drivers already do a copy in the Amiga because (1)
> :the system calls don't require block alignment, and (2) the driver uses the
> :blitter to generate and decode MFM, so data outside MEMF_CHIP has to be copied
> :anyway.

> 	No, copying is not required.  For instance, the FFS will pass along
> data pointers to whichever trackdisk device it is mounted on whenever possible,
> And MY microbotics StarDrive does NOT copy data to any buffers when reading/
> writing but transfers it from the request to the SCSI bus directly.

That takes care of problem 2.

But the trackdisk.device or SCSI.device still has to take this nonaligned
chunk of data and merge it with the actual blocks.

Say you are doing a 512 byte write to a device with 1K blocks. You have to read
the block you're writing to from the disk drive, merge the 512 bytes, and
write it back. For cases where you're writing a 132 byte chunk of data to the
middle of a file on a block boundary, you have to copy the first part to one
block (read, copy, write) and the other half to the other block (read, copy,
write).

> 	(In fact, everybody's mother except the floppy trackdisk device does
> this).  That isn't to say copying is never used.... it is used quite a bit in
> the OS, but it isn't used 100% of the time.

I'd say it's used most of the time. How many programs check the size of
the disk blocks being referenced and adjust their I/O requests to match?

A program that's efficient under the SFS will definitely be a hog on the FFS.
-- 
		Peter da Silva  `-_-'  peter@sugar.uu.net
		 Have you hugged  U  your wolf today?