QUANTUM QUIPS, QUIRKS AND QUIDDITY
By Chris Birch
Copyright (c) 1990 Apple Users' Group, Sydney
Republished from Applecations, a publication of the Apple Users' Group, Sydney, Australia.
A hard disk is probably the most important peripheral an Apple II or Mac owner will ever connect to their computer. Increasingly, owners are purchasing hard disks as opposed to additional RAM or floppy drives and not surprisingly they often select a Quantum hard drive mechanism.
Hard drives come in various configurations. They can be internal drives which means they are housed inside the computer on a peripheral card placed in an expansion slot or often incorporated into a new power supply which replaces the original power supply.
Alternatively a hard disk (HD) could be an external device and as such it is placed inside a casing, sometimes with a fan and connected to the computer with a cable. The casing is often colour coordinated with the computer and usually approximates the footprint of the computer. This means the HD's width and breadth are similar to the computer's dimensions.
A disadvantage with purchasing an internal drive is that they have to be manufactured to work with a specific model or subset of Apple models. If you decide to change computers then chances are the hard disk will not be fully hardware compatible with the new computer.
Alternatively, most of the newer external HDs which claim to work with one type of Apple computer will actually work with any Apple computer. This is due to the adoption, by Apple, of a standard way of interfacing or connecting peripherals to its computers.
The standard is known as the Small Computer Systems Interface or SCSI (pronounced "SCUH-zee") and its main rival is the Enhanced Small Device Interface or ESDI. I won't confuse things here by attempting a comparison. Suffice to say the two protocols are fundamentally very different ways of approaching the question of connecting devices to a computer system and they operate on different levels.
From the computer owner's point of view, a single SCSI device would be slightly out performed by the equivalent ESDI device. The real difference is in large, powerful networked systems where the SCSI solution is clearly superior and the performance difference not so trivial.
There is no reason why a SCSI HD advertised in a Macintosh magazine cannot work with an Apple IIe. In fact owners of an Apple II type computer should check out the SCSI HDs which claim to be Macintosh peripherals. The only problem would be that any software bundled with the drive would be unusable but probably superfluous.
In this way, Apple II owners can take advantage of the Mac's higher profile and the competitiveness of the Mac marketplace. They may also pickup a newer, faster drive than some of the HDs typically advertised in Apple II journals.
Whatever your computer, the Quantum HD is fast becoming the standard Apple HD. In fact, if you take a look inside any
off the shelf Mac which comes with an internal HD, chances are that Apple themselves have used a Quantum mechanism.
Quantum HDs obviously come highly recommended and this article will use a 105Mb ProDrive to illustrate the discussion. The performance specifications of the Quantum range are indeed impressive and they are discussed later. The drive mechanism is also interesting in that several patented features are included.
If you think of the HD as similar to a record player with a read head or needle fixed to one end of an arm able to sweep across the record. Now imagine several records (3 with the 105Mb ProDrive) mounted atop each other with a head/arm assembly for each side of each record.
The records as such are the same size as the common 3.5" floppies but these are rigid and of aluminium alloy construction. The nominal rotational speed is much faster than any record player or floppy disk drive. A 105Mb Quantum spins at 3,662 RPM +/- 0.3% compared with 300 RPM +/- 1% for the original Disk II 5.25" drive and 394-590 RPM with the Apple 3.5 Drive.
The platters have a carbon overcoat to lubricate the surface in case a head should touch the disk. The heads, at 7 microinches (0.18 microns), do fly close to the disk surface but they would never actually come into contact with the data area. The overcoat is principally intended to protect the drive heads when outside the data area.
If the disk should slow down to 3423 RPM or less for more than 2 seconds then the heads are automatically pulled over to the shipping/landing zone located toward the centre of the disk stack assembly.
A drive head simultaneously handles read and write functions and is mounted at one end of an arm which in turn is connected to a rotor. The rotor is technically called a rotary voice coil and this entire rotary positioning assembly is known as an actuator.
The disk surface is broken up into thin bands of data known as cylinders. The number of cylinders is determined by the precision of the actuator and the sensitivity of the drive heads. Our example Quantum has 1,019 of these compared with 80 on the Apple 3.5 Drive and 35 with a Disk II.
The width of a track is the same as a cylinder on the Apple 3.5 Drive but an 800K 3.5" floppy is double sided so we would say that the Apple 3.5 Drive has 160 tracks. In comparison our Quantum drive also has tracks the same width as a cylinder. With 6 surfaces the Quantum then has 6,114 data tracks.
The data is stored on any HD in an encoded format due to hardware requirements. Various schemes include NRZ (non return to zero), MFM (modified frequency modulation) and RLL (run length limited). The RLL scheme is superior and the example Quantum uses a "RLL 2,7" encoding scheme.
Each cylinder is segmented into sectors of a uniform 512 byte size. The number of sectors per cylinder varies. The outermost 831 cylinders on a 105Mb ProDrive have 209
physical sectors per cylinder, including specially reserved defect areas. The inner 188 cylinders have 167 sectors. Each 512 byte sector also has 6 bytes allocated as an ECC (error correction code).
Each sector has a number allocated sequentially amongst adjacent sectors on a track from cylinder 0, head 0 through to cylinder 1018, head 5. When the ProDrive needs to access a sector it knows the sector will always be in a predetermined position. This means Quantum drives are hard sectored drives.
In comparison the various Apple disk operating systems over the years have supported soft sectored floppy drives. It is up to the disk operating system to lay down a sector pattern. During the life time of a disk the sectors will actually move around, although they will always be located on the same track.
Each soft sector disk requires much more than the Quantum's 6 bytes of overhead. They need at least a header/prefix containing data to identify the sector to the disk operating system in addition to any error or checksum code. The disk operating system must also manage the space between the sectors.
A hard sectored drive has an advantage in terms of the density of data stored on each track. Additionally, the computer's disk operating system is freed up from sector management and similar considerations. The drive is able to benefit from this set up by allocating a sectoring scheme optimised to the drive, rather than the computer. As a HD is a mechanical device and hence slower at manipulating data than a computer it makes sense to do any optimising at the HD end.
A problem with many disk drives, hard and floppy, is rotational latency time. We have already seen that the ProDrives are voice coil, SCSI devices with RLL 2,7 encryption. This is good technology but the Quantum HDs excel with their very low latency times.
Latency time refers to the delay caused between sector accesses when the HD must wait for the next sector to spin around under the drive head and/or for the head to be repositioned. Most HDs and all floppy drives minimise this time by employing sector interleaving.
Each sector has a physical address or sector number as discussed above and even the various soft sector based Apple disk operating systems use a sequential numbering system (the exception is Dos 3.2.1 and earlier).
But the disk operating systems fiddle this allocation by assigning a sequential access order which skips a certain number of sectors when the next sector is required. This is the effective or logical sector numbering system. For instance, if the next logical sector is physically located four sectors later then this is referred to as a 4:1 interleave.
A 2:1 interleave system is ascending if the next logical sector is physically located two sectors away against the
direction of disk spin (HFS) or descending if in the direction of disk spin (Dos 3.3).
The ProDrives are so fast that sector interleaving is unnecessary although we would say the HD has a 1:1 interleave. Not so long ago this technology was so expensive it would only be employed on large capacity mainframe drives or DASD (direct access storage device).
The Quantums do employ track and cylinder skewing though. The ProDrive uses track skewing to ensure that when a different head on the same cylinder is needed, the head will be above the next sequential sector.
For example, an outer track on the 105Mb drive would have all the sectors shifted or skewed by 7 between tracks. This results in a sequential head switch time of 3.0 msec which is extremely fast.
Cylinder skewing minimises the latency time between head switches when accessing sequential sectors across adjacent cylinders. The Quantum typically uses a 15 sector cylinder skew.
The result of all this skewing is a very fast 16.4 msec maximum rotational latency time and an average of 8.2 msec.
The above timing standards are not usually quoted in HD advertisements. You would often see reference to the average seek time though. This is the average time to complete a specified number (say 1000) of random seeks anywhere in the drive.
You would need voice coil technology to achieve a sub 30 msec average seek time. Resellers will usually boast of times in the 20's for a high performance drive to hang off an 020 or 386 cpu. Quantum come in at only 19 msec and you can hang these off any of Apple's current cpus.
What is more remarkable is the effective average seek time is even lower due to another of the ProDrive's features. The DisCache (R) is a 64K programmable buffer which anticipates future sector needs. As 50% or more of sector accesses are sequential then the cache virtually eliminates the 8 msec rotational latency time.
An average effective seek time of around 12-13 msec is often quoted for the Quantum ProDrives and these claims would be difficult to dispute.
The ProDrives really shine in multiuser environments where both head seek and latency times are more pronounced. If the data is in the cache then Quantum claim 27 msec on average can be saved.
The example drive is able to transfer up to 4 Mb/sec from the cache to the SCSI cable in any one direction. This is fast enough to support animation or multiple graphic images being displayed on the monitor every second.
In the Apple II world there is a much talked about demonstration of several hundred digitised grey scale images from "The Empire Strikes Back" played at just over 30 frames per second. The Quantum could transfer the data four times faster than this if the Apple IIgs were able to handle it.
Such a demonstration would also be possible on a directly connected Mac SE/30 for instance.
A 4 Mb/sec transfer rate is not quite up to scratch to justify a bank of Quantums for a 3090 mainframe's DASD needs. Yet in the land of Motorola and WDC cpus the ProDrives offer superior transfer rates.
It is obvious from the above discussion the ProDrives achieve their remarkable performance from some very exacting technology. The drive was assembled in a Class 100 purified air environment and remains closed during its life. The drive is sealed with a metal cover, gasket and a ferro-fluid magnetic seal around the spindle bearing.
Inside the drive are two 0.3 micron filters. One is a breather filter for internal/external pressure equalization purposes and the other is a circulation filter to help maintain the stringent cleanliness.
The standard measure of reliability is the mean time between failure (MTBF), expressed as a number of power on hours (POH). The afore mentioned 25-30 msec average seek time drives are in the 20,000-30,000 POH vicinity. Our example drive has a MTBF of 50,000 POH which is exceptional.
Random or soft errors do inevitably occur. The 105Mb drive has a maximum random data error rate of one every 1e+10 bits read. These are relatively easy to recover from.
A hard error is due to factory defects and partially explains why a 105Mb drive, when formatted by your computer's software may have only 100Mb of available storage. Prior to shipment any defects such as scratches or thin spots in the media are detected and these sectors are deallocated.
In addition, one sector for every 6 tracks is set aside for field found/grown hard errors which might occur during the life of the drive. The example drive thus has a minimum of 3,057K set aside as defect zones. The corresponding maximum defect data error rate is one every 1e+12 bits read.
How the errors and defects are managed is beyond the scope of this discussion. Suffice to say the management is efficient and transparent to the computer's disk operating system and the SCSI protocols.
A final feature which sets a ProDrive apart from the rest is the Airlock (R). It is merely an airvane which is capable of intersecting the actuator to force the headstack to be held over a non data area of the disks. This protects the heads from coming into contact with the storage media during transportation.
The Airlock is additional to the automatic parking/placement of the heads in the shipping/landing zone when DC power is removed from the spindle motor. It provides additional insurance against the head stack moving out of the landing zone.
When purchasing a HD you should enquire about the mechanism inside the packaging. It is certainly a more important consideration than any software bundled with the drive or the colour of the drive case.
Surprisingly, the ProDrives are price competitive and occasionally cheaper than their counterparts. Unusually, it is cheaper to purchase a Quantum here in Australia than through US mailorder. Little wonder Quantums are increasingly the hard drive of choice for Apple owners.
QED

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