From:	"A. Sinan Unur" <sinan DOT unur AT cornell DOT edu>
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: Problem with function biosdisk() ... unexplained !
Date:	Mon, 05 Jan 1998 10:05:26 -0600
Organization:	Cornell University
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Mr. Lin wrote:
> 
> Hi ,
> 
> I was experimenting with the calls biosdisk() and _bios_disk() from 
> DJGPP 2.0 It seems like both of these calls gives different return 
> values and I can't seem to explain it. 

<snip>
>   unsigned char errorCode ;   /* Error code returned from bios disk call */
<snip>

>        /* The error code returned from this call is 1 */
>        errorCode = _bios_disk(0x02,&diskInfo) ;
> 
>   */

if you look at the documentation for _bios_disk (info libc alpha
_bios_disk), you will see that it is declared as:

: unsigned _bios_disk(unsigned cmd, struct diskinfo_t *di)

that is, the return type is unsigned int rather than unsigned char. the
same page explains the meaning of the return value:

: Return value of AX register. The meaning of high-order byte (AH):

so, only the value of the high order byte is meaningful. i presume, when
you put the return value in a byte rather than a 32 bit word, you only
get the contents of the AL register. compare this to the docs on
biosdisk:

: int biosdisk(int cmd, int drive, int head, int track,
        int sector, int nsects, void *buffer);

: Return Value
: ------------

: The value of AH returned by the BIOS.

and the reason for the difference is explained.

> Could it be a bug ??

it helps to look at one's own code first, and check it against the
documentation before jumping into conclusions on such issues. i don't
know the reason for the difference between the two functions, though.

  -- Sinan.

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