What does SX, DX, IDE, MFM mean?
sga1
What do the SX or DX suffixes mean, eg 386DX?
What is an MFM or IDE hard disk?
Steve
Peter da Silva
> What do the SX or DX suffixes mean, eg 386DX?
DX originally referred to a chip run that had a specific hardware bug fix
in it. Later, SX and DX were used to refer to the width of the memory
bus (I guess they decided D stood for Double and so came up with S for
Single, though it's more like half-width). In a later incarnation, SX
was used to refer to a 486 with no FPU. These days "D" means "whatever
the top end of this chip line is" and "S" means "stripped"...
Ignore DX/SX/DLC/ABCDEFGHI specs and ask: does it have an MMU? An FPU?
A cache? How big is the cache? What software will you guarantee it's
compatible with? What's the performance on *my* benchmarks? What's the
off-chip cache? Do you guarantee that cache works? Lotsa stuff more
important than alphabet soup.
> What is an MFM or IDE hard disk?
In the beginning (in the PC world) drives used the same electrical interface,
which was referred to as ST506 after the Seagate drive that pioneered it.
There were two main ways to store data on a ST506 drive, MFM (modified
frequency modulated, like a floppy) and RLL (run length limited... actually
any disk data format is some kind of RLL code, this is just a code that
allowed 50% more than the code used in MFM). An RLL drive was just an
ST506 drive that was good enough quality to use with RLL format (as determined
by the controller hardware).
Then there came ESDI, which was electrically similar to ST506, but defined
more signals so you could have bigger drives, and up to 7 devices attached
to a single controller (though for software compatibility with a kludge that
IBM came up with to simplify things for manufacturing XTs, PC-based ESDI
controllers usually only supported 2 drives). This was further enhanced and
heavily reworked to become SCSI: which allows 7 devices on a single cable
and is the best thing to use in a PC because it allows you to use high-
performance CDROMs and tape drives as well as disks. But back to our story.
IDE stands for Internal Drive Electronics. It's sort of "let's start over
with MFM and rework what went into SCSI, but we'll do it as cheap as
possible and won't change the software interface". It's kinda kludgy and
doesn't always work: basically they split the old MFM controller board
into an AT bus interface part and a disk controller part, and ran a cable
between the two... then put the disk controller on the drive. The problem
with this is they left the disk ID decoding on the controller side, so when
you have two drives they have to do a lot of careful handshaking to work
right and it doesn't always work for two drives unless they're the same make
and model. And occasionally it fails even then.
The advantage? You can manufacture an IDE controller card for like a few
dollars because all it does is adjust the bus signal levels and interrupts.
There's cheap IDE drives up to 525MB. There's a "New IDE" spec that's
supposed to support larger drives and CDROMs.
The disadvantage: With two drives *both* drives are talking to the same
I/O port and with the same interrupts, and have to be careful not to
accidentally mistake the other drive's commands as theirs.
IDE's OK for a rock-bottom system where you don't plan on adding a second
drive, and don't mind low performance tapes and CDROMs (you can get used
250MB SCSI tapes for as low as $100, and used 60MB ones for $60, and you'll
be a lot happier with them than a 250-MB-compressed "floppy tape").
If you're doing anything real, shell out the bucks for an Adaptec or Bustek
SCSI controller with DMA. I use a Bustek/Buslogic 747 and it's great.
--
Peter da Silva `-_-'
Network Management Technology Incorporated 'U`
1601 Industrial Blvd. Sugar Land, TX 77478 USA
+1 713 274 5180 "Hast Du heute schon Deinen Wolf umarmt?"
Scott G. Hall
>
>What do the SX or DX suffixes mean, eg 386DX?
>What is an MFM or IDE hard disk?
When Intel uses the SX suffix, it can be read as "something less than
the full DX version". Unfortunately, its use isn't consistent.
386DX - full 386 chip, 32-bit data interface
386SX - 386 chip, but with 16-bit data interface
(made for motherboard manufacturers with AT-type motherboards
to minimize their production line changes to use the 80386
chip instead of the 80286 chip)
486DX - full 486 chip, complete with numeric coprocessor
486SX - 486 chip without numeric coprocessor
486DX2 - full 486 chip with an internal clock-doubler
(internal clock runs twice as fast as the motherboard clock,
meaning a 66MHz 486DX2 machine really has a 33MHz motherboard
and RAM access)
Harddisk types:
MFM - "Modified Frequency Modulation" technically describes the
recording method on the disk patters; typically used to
designate ST-506 style drives using the ST-506 interface
first used with the Seagate ST-506 disk
{one 34-pin data cables for up to four drives, separate
20-pin control cable for each drive}
RLL - "Run Length Limited"; drives with an ST-506 interface but
recording data in a way that usually gives about 50% more
storage
{same cables as for MFM}
ESDI - (I don't remember the what accronym stands for anymore);
describes the controller interface used; one of the surfaces
was dedicated to storing the sectoring information, and so you
can think of this as a "hard-sectored" or "firm-sectored" disk
{same cables as for MFM}
SCSI - "Small Computer Systems Interface"; uses an arbitrated bus to
connect to harddisks, printers, tape drives, and so on; each
harddisk has an embedded harddisk controller that interfaces
to the SCSI bus
{50-pin cable; header-pin connector internally, Amphenol
connector externally; except for Apple's weird SCSI connector}
SCSI-II - a SCSI interface using a different external connector, and
where the loading resisters on not used at the last device in
a chain
IDE - "Integrated Devices ??"; each harddisk has an embedded harddisk
controller that interfaces to a modified and condensed ISA bus
{40-pin cable}
VESA - a video adapter interface that some really-fast RAID (fault-tolerant)
harddisk sub-systems are using via a VESA interface board
(such as Teradata and Fujitsu)
PCI - a compact "PC Interface" that was developed for laptop systems;
you'll see this mostly with 2.5" disks
--
- sgh We don't play golf to feel bad... We play bad golf to feel good!
- Leslie Nielson "Bad Golf Made Easier"
Scott G. Hall (s...@cbvox.cb.att.com)
AT&T Bell Laboratories - GCBS, Columbus, OH, USA
William J Buffam
>From: S.G.Ad...@ukc.ac.uk (sga1)
>Subject: What does SX, DX, IDE, MFM mean?
>Date: Thu, 15 Sep 94 14:06:00 GMT
>What do the SX or DX suffixes mean, eg 386DX?
>What is an MFM or IDE hard disk?
MFM = modified frequency modulation, which refers to the way the data is
encoded on the disk.
IDE = internal drive electronics, which means the disk drive controller lives
within the device itself, rather than being on a separate board.
SX, DX: don't know what they actually stand for, but 80386SX processors are 32
bits internally but only 16 bits wide access to memory. 80386DX has 32 bit
access to memory. I believe that, in relation to the 80486, the suffixes mean
the same as with 386, with the additional qualification that the 80486SX is
missing the built-in floating point unit.
Bill Buffam
Unisys Corp, Malvern, PA, USA