Atari ST and hard disks
Here we will focus mostly on ST and STE machines, but many of written stays for Atari Falcon and TT machines too.
Little history for better understanding:
When Atari ST was launched, in 1985 hard disks were not much used -
they were too expensive, exotic pieces of hardware. Standards were not
established, and there was no IDE (ATA), later winner in mass storage
system for masses :-) Atari did some support for attaching
(in that time yet future) hard disks in form of ACSI port. It was
typical for that time: usage of own system, not compatible with any
other. But it was not so bad move, considering lack of standards. And
ACSI port is actually some kind of simplified SCSI port. After it, some
hard disks appeared on market. More success than Atari had independent
manufacturers. Typical hard disk kit for Atari ST machines was:
ACSI-SCSI adapter and SCSI disk + power supply + driver, partitioner
software. In meantime IDE hard disks appeared on market. They were
designed to be cheap, simple interfaceable. Atari made support for IDE
disks somewhere in 1991-92. Code for boot from such interface/disk is
added to TOS 2.06, and latest machines as ST-Book and Falcon had
built-in IDE interface.
However, some people did not realised in that time what really
happening: I read in some article in German Atari magazine that it is
no wise to make IDE interface, because SCSI is just little more
expensive, and IDE has no perspective. Now we know that they were
wrong. Biggest problem with IDE interface is that it can not be simple
connected to machine - you need to open case and solder lot of lines.
In case of Mega STs it was little simpler, but still some soldering and
mounting connector was required.
Now we have absurd situation that some Atari users have ACSI-SCSI-IDE
chain of interfaces. Main problem here is that Atari bankrupted
after 1992, and that there is no expansion port/bus on ST machines.
How it works?
Unlike by modern PCs, there is only very little support for hard disks
in TOS. Actually, in TOS is only short code for loading master boot
record from hard disks. All further is done by driver and it's loader.
TOS has support for FAT16 filesystem - MS DOS compatible. Later TOS
versions added some support for bigger FAT16 partitions in form of 'Big
sectors'. All that required (compatible) hard disk drivers.
TOS 1.0 was limited to partition sizes of max 16MB due to bug (without
big sectors). FAT16 partition has max size of 32MB. FAT16 with big
sectors, called BGM may be up to 1GB, depending on TOS version.
ACSI in it's initial form has limit of 1GB for total size of attached
hard disk, because of addressing system. It is expanded later in XHDI.
But we should not blame Atari much for those limits. By PC, situation
was even worse - max half GB total disk size accessible with CHS
addressing by IDE/MFM disks. Not to mention mess in partition tables.
IDE (ATA) went through numerous expansions. Most important is LBA addressing, what accessess hard disks up to 128GB capacity.
BigDOS partitions on PC have limit of 2GB in size, and they are
readable on Atari with help of free program BIGDOS + hard disk driver
(at least AHDI 3.0 compat.)
Partitioning of hard disks:
In very first sector of every hard disk is partition table. Atari uses
so called GEMDOS partition table. It is defined in AHDI. However, we
are not forced to use that system at all - because there is nothing in
TOS related to it. All is done by hard disk driver. Usage of MS DOS
partition system offers as other possibility - it uses same FAT16
partitions as GEMDOS. And we have benefit that such disk will be
readable on PC without any special software.
Where we are today (2007) ?
Still some people uses Atari ST machines. Things changed a lot until
80-es of last century. We have now cheap Flash mass storage medias,
with low power consumption, fast work. SCSI is developed too, but it is
not for mass-market. Nobody manufactures ACSI-SCSI adapters. Ideal
would be ACSI-IDE or ACSI-some Flash card interface. But from some
reasons they are not available, although there are some projects under
development/manufacturing. IDE has advance that CD ROM/DVDs can be
Problem is that ACSI and IDE (ATA) are not compatible. For instance IDE
has 16-bit data bus, while ACSI 8 . Even bigger problem is control:
ACSI first gives command code, then parameters, while IDE does it in
opposite order. So, designing ACSI-IDE IF is not so simple.
There is still a lot of working, good quality ACSI-SCSI adapters used
with Atari machines, and they are best supported by software. IDE
is not so good supported, and most of drivers is obsolete.
Problem is when someone wants to buy adapter and mass storage media: what to buy today?
May go to E-bay and look for some second-hand combination. Prices are
not to good, considering that new Flash card of 4GB costs under 50
Euros, and will be gradually cheaper.
P. Putnik , March 2007.