🚀 go-pugleaf

Gerry Wheeler (wheels@mks.uucp) asked me how to look at the
partition info on the hard disk. Since it seems that my answer
didn't get through the gateways, and since this is certainly of
general interest, I send it to this list.

So here's how I read and modified the hard disk's partition
information:

The partition info is located on logical sector 0. Normally you
cannot access it because this is the sector just before the boot
sector of the first partition (i.e. BEFORE sector 0 of this
partition). The reason for this is that all sector numbers you
specify are translated according to the partition information.
But there is a hook!
Take the BIOS function rwabs (function #4). It's calling sequence
is

 rwabs(rwflag,buffer,count,recnr,dev)
 long buffer;
 int  rwflag,count,recnr,dev;

where:
 dev:        number of device (A = 0, B = 1,...)
 recnr:      number of first sector you want to read
 count:      number of sectors
 buffer:     buffer to read into
 rwflag:     special parameter, individual bits are interpreted
                                  as follows:

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |X|X|X|X|X|X|X|X|X|X|X|X| | | | |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       \__________ __________/ | | | \______ 0 = read, 1 = read
                  V            | | |________ 1 = ignore media change
            "don't care"       | |__________ (hard disk only)
                               |               1 = no retries
                               |____________ (hard disk only)
                                               1 = recno is absolute

This means: if you set rwflag = 0/1, you read/write the sectors
of the specified partition. If you set it to 8/9 you read/write
the sectors of the whole hard disk INDEPENDENT OF THE PARTITION.
(This is an undocumented feature, I discovered it by
disassembling the hard disk driver program.)

To read the hard disk's boot sector you write
   rwabs(8,buffer,1,0,2);

To write it, call rwabs(9,...)

The length of the sector is 512 bytes, so buffer should be at
least as long as this.

Content of the boot sector:
In the little booklet you got together with your hard disk there
is a table what is contained in sector 0 and how the partition
information is written. Just look at the addresses given there.
The rest of the boot sector you can use for your own purposes.
(Konrad Hahn's hard disk boot program does this.)

   Rainer Schoepf
   <SCHOEPF@DMZRZU71.BITNET>