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From: gohyongkwang AT hotmail DOT com (Goh, Yong Kwang)
Newsgroups: comp.os.msdos.djgpp
Subject: Interpreting return value from _bios_disk
Date: 28 Jul 2004 02:36:52 -0700
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To: djgpp AT delorie DOT com
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Reply-To: djgpp AT delorie DOT com

Hi.

I'm writing a simple program to read read a floppy at the sector
level, so I'm using _bios_disk.

Apparently, there are some bugs in the documentation and example for
_bios_disk.
After reading some previous postings by other users, it appears to me
that

the line in the example:

di.buffer = &record_buffer;

ought to be changed to:

di.buffer = record_buffer;

since record_buffer is declared as: 

char record_buffer[512];

Secondly, the way of interpreting the return value is not just
comparing it to zero for success and non-zero otherwise. Since AL also
contains the number of sectors read (I'm only currently experimenting
with _DISK_READ so far), hence some masking or bit shifting ought to
be done. I've copied my short program below to double check if I'm
interpreting the return value correctly.

----
#include <bios.h>
#include <stdio.h>

int main(void){
    char record_buffer[512];
    struct diskinfo_t di;
    unsigned ax_value = 0;

    di.drive = 0x00;
    di.head = 0;
    di.track = 0;
    di.sector = 1;
    di.nsectors = 1;
    di.buffer = record_buffer;

    ax_value = _bios_disk(_DISK_READ, &di);
    printf("AH=%X AL=%X\n", (ax_value & 0x0000FF00) >> 8, ax_value &
0x000000FF);
    
    return 0;
}
----

The confusing thing is that it returns an unsigned (integer) which is
4 bytes huge in DJGPP and AX is only 16 bits and therefore 2 bytes
huge. Hence I think the first two bytes are just for padding purpose
and unused and the last 2 bytes store the actual value of AH and AL
respectively. Therefore some bit masking and shifting may be necessary
to parse the AH and AL value.

Is my code and interpretation correct? Just need a confirmation from
you guys. Initially I was wondering why I kept getting "Disk error"
with the _bios_disk example because the return value was non-zero.

Lastly, I notice that I'm always given the Disk Time Out error AH=0x80
instead of a Drive not ready error AH=0xAA like DOS does whenever a
floppy is not present in the drive and my program tries to access the
non-available floppy disk. DOS reports, "Not ready reading drive A."
Why is this so?

Thanks a lot.

Goh, Yong Kwang
Singapore