Path: utzoo!mnetor!uunet!lll-winken!lll-tis!ames!necntc!ima!think!ephraim
From: ephraim@think.COM (ephraim vishniac)
Newsgroups: comp.sys.mac
Subject: Re: Mac disk sizes
Message-ID: <17405@think.UUCP>
Date: 2 Mar 88 21:57:41 GMT
References: <1054@ut-emx.UUCP>
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Reply-To: ephraim@vidar.think.com.UUCP (ephraim vishniac)
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Organization: Thinking Machines Corporation, Cambridge, MA
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In article <1054@ut-emx.UUCP> ayac071@ut-emx.UUCP (William T. Douglass) writes:
>Is there a limit in the OS or hardware that restricts the size or number of
>hard disks that you can attach to a mac II?  Thanks,

Disk size: the number of sectors in the drive is held in a 32-bit
field in the drive queue element.  Each sector is 512 bytes, so
there's an obvious limit of 2^9 * 2^32 = 2^41 bytes.  However, the
dCtlPosition field used in low-level calls to the driver is only 32
bits and indicates a byte position, not a sector position, so there's
a smaller obvious limit of 2^32 bytes (4GB).  I'd be pleasantly
surprised if the file system doesn't accidentally use signed
arithmetic somewhere, which would cause an effective limit of 2^31
bytes (2GB).  I've actually heard of people attaching >1GB disk
systems to a Mac, so the real limit is somewhere between that and 4GB.

You can avoid the drive size limit by partitioning the disk at a low
level.  The Driver Partition Map (DPM) uses 32-bit sector numbers, so
this moves you back toward the 2^41 byte limit.

Number of drives: You can only attach seven SCSI devices, but a single
SCSI device could be several logical drives.  Also, there are other
attachment methods than the Apple-provided SCSI bus.  For example, you
could build a Nubus card with your own SCSI (or other) interface. Each
mounted volume uses space in the system heap, so your heap will
eventually get crowded.  This is a soft limit: you'd have to
experiment to find out how much of a problem it is.

Ephraim Vishniac					  ephraim@think.com
Thinking Machines Corporation / 245 First Street / Cambridge, MA 02142-1214

     On two occasions I have been asked, "Pray, Mr. Babbage, if you put
     into the machine wrong figures, will the right answers come out?"