While there is also a new way of addressing sectors on the hard disk, Logical Block Address (LBA), this is merely a simplification. Contrary to common myth, LBA has nothing to do with breaking the famous 528 MB barrier. In fact, even in the old ATA and IDE standards, the theoretical size limit was well over 100GB. The 528 Mbyte limit is in fact a combination of the combined limits of two mechanisms.
Fast-ATA from Seagate provided the Enhanced BIOS specification but did not make use of the ATAPI standards and did not provide support for CDROM drives.
Enhanced IDE (EIDE) not only broke the 528 Meg barrier, it also provided support for up to four EIDE devices. These devices could be physical hard drives or devices like CDROM drives, high capacity removable media and Tape Backup drives. The four drive capability is provided by having Primary and Secondary IDE channels, ea ch with a Master and a Slave device.
With local bus technology the data throughput can be as high as 33MB/s, but when used on an ISA bus the data rate would only be 2 to 3 MB/s, limited by the bus width and the bus speed.
While ATAPI CDROMs use the hard disk interface, this does not mean that they look like a Hard Disk drive. From the Operating System point of view, they are a completely different kind of animal. ey most closely resemble a SCSI device.
At first the most common solution was to provide a BIOS Extension ROM on the IDE interface card, or to supply a device driver that was loaded from the config.sys file when the Operating System loaded. These two solutions supplied alternative Interrupt 13h routines and provided support for drives as large as 8.4GByte. Removing the Interrupt 13h barrier allows for support of high-capacity hard drives. This was a messy solution and sometimes caused compatibility issues.
The BIOS on the System Board of modern computers has an enhanced Interrupt 13h routine built in. Look in the CMOS setup and you will see parameters like "Normal", "LBA" and "Large", this is an indication of support for large hard drive partitions.
The enhanced BIOS Interrupt 13h routine can recognise the size of the Hard Drives as well as the nature of the non hard disk EIDE devices connected to the interface. This means the computer can be booted from an EIDE CDROM drive or from a high capacity removable media devices like ZIP or JAZ drives.
Windows 95 OSR2 released late in 1996, provided a 32 bit FAT option for larger drives and this carried on to Windows 98. The only problem with using the 32 bit FAT is DOS and older versions of Windows will not recognise a FAT 32 drive.
Why the original size limit existed ?
The original IDE capacity limitation existed due to the number of bits allocated for specifying the Cylinder, Head, and Sector address information at both the Interrupt 13h level and at the IDE interface level. Because both the Interrupt 13h routine and the IDE interface specify differing maximum values, combining these two sets of limits, produces the 528 MByte barrier that meant large hard drives had to be split into partitions no larger than 528 MByte.
This table helps to explain where the 528 MByte limit came from. The maximum capacity limits of the technology was the combination of the lowest values for each parameter.
| Limit imposed by the INT13 routine | Limits imposed by the IDE interface | Limit on the original IDE | |
|---|---|---|---|
| Maximum sectors per track | 63 | 255 | 63 |
| Maximum number of heads | 255 | 16 | 16 |
| Maximum number of cylinders | 1024 | 65536 | 1024 |
| Maximum capacity | 8.4GB | 136.9GB | 528MB |
LBA uses so-called LBA addressing to access the hard disk. The advantage of LBA is that it theoretically is a little faster. The disadvantages are that not all drives support it, not even all large ones, and it sometimes turns out to be slower, depending on the drive.
Large or Extended CHS tells the BIOS to use CHS translation. It uses a different geometry (Cylinders/Heads/Sectors) when talking to the drive than when talking to the BIOS. This type of translation works with all drives.
WARNING: The BIOS may change the (translated) geometry if you change from Normal or Large to LBA. The same thing may happen if you transfer a disk that has been formatted on an old, non-LBA computer to a new one that uses LBA. This WILL often destroy the data on the disk.
The LBA translation is loaded into the drive's task file registers. Bit 6 of the drive's SDH register is set to indicate to the drive's firmware that it should interpret the information in its task file registers as LBA rather than Cylinder, Head and Sector information. This scheme allows for the full use of all of the bits allocated for CHS information at the Interrupt 13h interface, thereby supporting up to 8.4GB.
Using a logical block addressing scheme is attractive primarily because it is 100 percent compatible with BIOS Interrupt 13 routine and allows for reduced overhead, producing higher performance. The logical block addressing scheme provides the compatibility essential for personal computer usage as well as enables the implementation of higher capacity disk drives required in today’s personal computers.
The EIDE interfa specifications has made use of the Secondary addresses that were assigned to a Secondary hard drive controller but never implemented with ST506 or ESDI. The Secondary IDE channel uses I/O addresses of 0170 to 0177 and IRQ15
The EIDE standard has also added two more EIDE I/O channels and these are
Tertiary channel at I/O addresses 01E8 - 01EF and assigned IRQ 11 or 12
Quaternary channel at I/O addresses 0168 - 016F and assigned IRQ 10 or 11
These extra IDE channels may be used to provide for extra CDROM drives, Optical drives, Tape Back-up drives, and removable media drives (Zip, Jaz, LS120 and Syquest). With the EIDE interface becoming so powerfull and flexible these will be a great asset in the near future. All these EIDE channels will mean the SCSI interface is not neccessary for many devices that once used it. Many sound cards provide an EIDE interface intended for connecting a CDROM drive. These interfaces can often be configured as the Secondary, Tertiary or Quaternary channel, via jumpers or soft-setup. When using EIDE channels provided by sound cards remember if the card is an ISA bus card, the data rate will be limited to 2 to 3MHz because of the bus speed and width.
Most 486 computers may not support LBA or similar technology. Look at the CMOS setup for the computer you are working on and see if it mentions Logical Block modes or Large partitions. If it does not then you will have to use a utility like Disk Manager, available from the hard drive vendors, to provide support for large drives. This support is provided by modifying the boot record and partition sector on the disk and the procedure adopted depends on who's version of Disk Manager you are using. Follow the instructions on the Disk Manager disk. Some "large" hard drives are being supplied with Disk Manager already on the disk but only use it if you have to. Do not use Disk Manager if the BIOS supports LBA or Large partitions.
Some very old IDE devices may not coexist with other IDE and EIDE devices and this may cause problems if you wish to transfer data from on old drive to a new drive, in a master/slave configuration. If you are transfering an old hard drive from an older system and wish to keep the data on it, it is advisable to record the cylinder, sector, head and mode details, used in the old system, so you can set them up the same in the new system. You cant rely on the Auto Detect Hard Drive utility in a new system to get it the same.
When connecting hard drives and CDROM drives to EIDE controllers you have several options in configuration. For example if you have one hard drive and one CDROM drive you could connect them both to channel one with the hard drive as the master and the CDROM as a slave.
This configuration has the disadvantage of the slowest device, the CDROM drive, slowing down the access to the fastest device, the hard drive. It is better to connect hard drives to the primary channel and the CDROM, as a master, to the secondary channel.
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