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From: markv@kuhub.cc.ukans.edu
Newsgroups: comp.sys.amiga.tech
Subject: Re: "Device" vs. "Handler"??
Message-ID: <25339.26d3ba57@kuhub.cc.ukans.edu>
Date: 23 Aug 90 16:13:27 GMT
References: <082090.120745@ckctpa.UUCP>
Organization: University of Kansas Academic Computing Services
Lines: 73

In article <082090.120745@ckctpa.UUCP>, crash@ckctpa.UUCP (Frank J. Edwards) writes:
> The Subject/Summary lines say it all:  what's the difference?  I've noticed
> that most of the PD-type stuff are handlers, with the notable exception of
> the MSH: software.

Actually the difference is quite simple.  A "device" is an Exec
device, like trackdisk.device.  It typically is the lowest level of
driver above the hardware.  In the case of a floppy, the
trackdisk.device handles commands like read a block, write a block,
etc.  It has no knowledge of a file system etc.  You have to do I/O to
a device at the programming level, via Do/Begin/SendIO and messages.

A "handler" is a DOS level driver, like DF0:.  It understands the
concept of a file, and can be opened, closed, checked for EOF, etc.
It can be "block structured" (ie: understand a file system, like DF0:,
RAD:, etc.), or stream oriented (like SER:, PAR:, AUX: etc).  A
handler can be accessed at the use level like "COPY FOO RAM:FOO" or 
"DIR >PRT:".

Most but not all handlers are layered/paired with a device driver,
like:
DFX:	->	trackdisk.device
SER:	->	serial.device
PAR:	->	parallel.device
PRT:	->	printer.device
SPEAK:	->	narrator.device

But not all handlers have a device driver, mainly those that dont use
any hardware other than memory like PIPE:, NULL:, RAM:, etc.  And of
course not all devices have handlers like timer.device and
audio.device.

At a the programmed level handlers and devices are different.  Devices
resemble libraries and take messages and function calls.  I/O is done
via messages with (somewhat) standardized commands like CMD_WRITE,
CMD_READ, etc.  But there is quite a bit of difference between
devices.  Devices are free to vary the interface and behavior of these
commands quite a bit since they are only accessed at the programming
levels.

Handlers on the other hand respond to DOS packets (which are
messages, but different than exec I/O messages) and dont
look much like a library.  They also must respond to the packets in a
consistant and defined way since the packet actions are things like 
Lock()/Open()/Close()/Read()/Write() at file, etc.

Hope this somewhat long explanation helps.  Devices and handlers are
different and perform different functions.  "PD" handlers are
generally more common since they tend to perform software only
functions, or provide new interfaces to existing hardware.  On the
other hand devices tend to be to interface to specific hardware, and
most Amiga hardware (standard or addon) has a device driver provided.

Finally the example of MSH: is BOTH a device and a handler.  The MSH:
part is a handler that provides a AmigaDOS interface to a MS-DOS file
system.  In practice this would be all that is needed.  MSH also
provides a device driver to access the floppy hardware because of bugs
in trackdisk.device, so messydisk.device is a device.

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