Phil Storrs PC Hardware book

PC/XT 8bit bus PCs, Hard Drives
and Real Time Clocks

This chapter is of historical interest only, but it does answer some questions posed by the origins of the DOS computer.

Support for a Hard Drive was not built into the XT's BIOS ROM.

The XT type computer does not have BIOS Support for hard drives built into it's own BIOS ROM so hard drive support is supplied by a BIOS Extension ROM located on the hard drive interface card. This ROM is mapped into the computers memory starting at memory address C8000, or expressed in Segment and Offset, C800:0000.

The Hard Drives usually fitted to an old XT type computer was an ST506 type the first step in the process of formatting the hard drive involves setting up the Low Level Format.

Most eight bit (XT type) hard drive interface cards have a routine built into the BIOS Extension ROM to perform a Low Level Format. The most common address for the start of this routine is C800:0005 but you should be aware other addresses have been used by some card manufacturers.

This routine is run from Debug with the command G= followed by the starting address. Form this point on the actual procedure can be different for cards from different manufacturers.

Not all eight bit hard drive interface cards have the Low Level Format routine built in but most do - the interface card used in the original IBM XT did not have the routine built in and you require a Diagnostic to perform a Low Level Format on the hard drive in these computers.

The steps involved in Formatting the hard drive in an old XT (8 bit bus) DOS computer.

Once the disk has been prepared you should then setup DOS and UTIL subdirectories.

Don't forget the command lines that run the files you put into the UTIL subdirectory to set the RTC, to park the hard drive and to set the turbo mode.

Important. If you are going to reformat an old hard drive you must first save any of the hardware support files that are used on that computer and a copy of the AUTOEXEC.BAT file that calls them. You may also wish to save a Bootable DOS floppy disk and a all the DOS support files and External DOS commands so you can put the old version of DOS back on this machine.

The Real Time Clock, Parking The Hard Drive and Turbo Programs

The old XT type computer did not have a built in Real Time Clock and this facility was usually supplied by an after market RTC on the I/O card that also supplied the parallel and serial I/O ports.

The RTC function was activated by a Clock Program called from the Autoexec.bat file. This file transferred the Time and Date from the Battery backed-up Real Time Clock on the I/O card to the computers own Time and Date clock on the system board. The problem with this arrangement is that because the RTC function was not designed into the system by IBM no I/O address assignments were ever made for it. This lead to the after-market suppliers of RTC boards using any old I/O addresses they liked. This lead to many versions of the Clock Program being used and each will only activate it's own version of the hardware. The names used for these clock programs are names like CLOCK, TIMER, GETCLOCK, and the same name is often used for quite different versions of the clock program.

The AUTOEXEC.BAT file should run the "clock program" at boot-up and as these old hard drives did not "Park" the hard drive's heads automatically we used to often load an "AUTOPARK" utility from the AUTOEXEC.BAT .this utility parks the hard drive after a period of inactivity.

The IBM PC and PC/XT computers operated with a clock speed of 4.7Mhz but it was not long before the clone manufacturers started to supply TURBO computers which could run with a 4.7MHz clock or a faster clock, usually 8MHz but 10 and 12MHz machines were available towards the "end of the era".

Some computers used a hardware switch and some used a software switch that required a "TURBO" program to be run from the AUTOEXEC.BAT file.

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Copyright © Phil. Storr, last updated 4th December 1998