Finding CHS-Values for HDD of an old 486-DX2/50 Highscreen notebook

By dose | December 23, 2017
Under: Uncategorized

Last week, I received a call for help from someone who inherited an old Highscreen 486-DX2/50 notebook from her father that contained some important documents that she wanted to recover.
Now as the Notebook hadn’t been in operation for approx. 10 years, CMOS battery was empty and the Notebook refused to boot DOS, as the CHS-Values for the Harddisk in BIOS got lost.
Now as there was no manual available anymore, I just unscrewed the 2 screws in the back of the Notebook, lifted the Keyboard and had a look at the harddisk.
So I discovered that it contained a Connor Peripherals CP30174E harddisk with approx. 160MB in size.
Now I thought that this is going to be easy, found a manual on the Internet that showed me the correct CHS values:

903 Cylinders
8 Heads
46 Sectors
0 Precomp
903 Landing zone

So I entered the information in BIOS, attempted to boot and…. Nothing, the Boot loader code in the MBR still showed me that the OS cannot be found.
Therefore I booted up with a floppy disk and was able to access the internal drive. But of course, I wanted to find out the right settings in the BIOS to let the Notebook boot again.
So I started up Norton Disk Editor:
Object -> Drive -> [x] Physical -> Hard drive 0
ALT+A (Object -> Partition table), F6 (View / As partition table)
There I was able to see the following table:

  Starting Location Ending Location Relative Number of
System Boot Side Cylinder Sector Side Cylinder Sector Sectors Sectors
BIGDOS Yes 1 0 1 5 1004 55 55 331595

Conclusion #1: Cylinder count must be >= 1004
Conclusion #2: Sectors may be 55
Conclusion #3: It has at least 5 Heads

Next, checking the Boot sector:
Object -> Drive -> [x] Logical -> C:
ALT + B (Object -> Boot Record)

There is showed:
Sides: 6
Sectors per track: 55

So we know that we have 6 Heads and 55 Sectors and >= 1004 Cylinders
Now judging from the CHS values of the Harddisk vendor, we know that the drive has a capacity of 162.257 MB
Therefore we can approximate the value of the Cylinders to closely match that value and find out that it has to be approx. 1007 Cylinders.
Therefore, I ended up with these values:

1007 Cylinders
6 Heads
55 Sectors
0 Precomp
1007 Landing zone

Entered them in the BIOS and it booted up fine.

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