Newsgroups: comp.os.msdos.djgpp
From: "Arthur Bredrick" <sammy AT blast DOT net>
Subject: three .externs whose purpose and origin I know not
Lines: 64
Message-ID: <EHG2yv.AnA@nonexistent.com>
Date: Thu, 2 Oct 1997 22:12:22 GMT
To: djgpp AT delorie DOT com
DJ-Gateway: from newsgroup comp.os.msdos.djgpp
Precedence: bulk

I have seen the following declared in Martynas Kunigelis' keyboard interrupt
handler (the assembly file keyboard.s):

  .extern ___djgpp_base_address
  .extern ___djgpp_ds_alias
  .extern ___djgpp_dos_sel

___djgpp_ds_alias is used very early on in the code:

>#
 ># Load DS with our data selector
>#
>movw    %cs:___djgpp_ds_alias, %ds

Later on, when the handler memory is being unlocked (in the handler shutdown
procedure), ___djgpp_base_address is used:

 >#
 ># Unlock the region we locked at initialization
 >#
>leal        locking_region_start, %ecx
>leal        locking_region_end, %edi
 >subl        %ecx, %edi
>addl       ___djgpp_base_address, %ecx
 >shldl      $16, %ecx, %ebx
 >shldl      $16, %edi, %esi
 >movw    $0x0601, %ax    # unlock linear region
>int            $0x31

Towards the end of the code, when the handler is chaining to the old
handler, ___djgpp_dos_sel is used:

>#
># void keyboard_chain(int toggle);
>#
>.align 4
>_keyboard_chain:
>cmpl    $0, 4(%esp)
>je          chain_off
>
>chain_on:
 >#
 ># Set the chain_flag and clear BIOS shift/ctrl/alt status bits:
 >#
 >movl    $1, chain_flag
>
>push    %es
>movw   ___djgpp_dos_sel, %es
>andb    $0xf0, %es:0x417
>pop      %es
>jmp      chain_done
>
>chain_off:
>movl    $0, chain_flag
>chain_done:     ret

I am very new to protected mode programming, DJGPP, and assembly in general.
Where do these externals come from and what are they being used for?  Thanks
for any help.

Art