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Date: Tue, 01 Nov 2005 12:46:01 -0500
From: Diego Cueva <dcueva AT todo1 DOT com>
Subject: Why the call instruction  points 2 bytes before to the real start
 sub-routine ?
To: djgpp AT delorie DOT com
Message-id: <003201c5df0c$20f5abe0$2432a8c0@dcueva>
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Reply-To: djgpp AT delorie DOT com


Hi:
Please help me with the next issue:

In the next assembler program:
====================== ipl.s ================
.code32
.text
.global start
start:
        nop
        # Clear screen
        call clrscr
        # Print Hello world
        movw    $m1,    %si
        call    impstr
        # End
        int     $0x20
# Clear screen code using assembler
clrscr:
        pushw   %ax
        pushw   %bx
        pushw   %cx
        pushw   %dx
        movb    $6,     %ah
        movb    $0,     %al
        movb    $0,     %ch
        movb    $0,     %cl
        movb    $24,    %dh
        movb    $79,    %dl
        movb    $7,     %bh
        int     $0x10
        popw    %dx
        popw    %cx
        popw    %bx
        popw    %ax
        ret
# Putchar using assembler 
( ...more code here...)
=============================================

I am using the compiler in this way (to generate a ipl.com file or pure
code machine file):

..\..\tools\as -o  ipl.o   ipl.s
..\..\tools\ld -m i386go32  -Ttext 0x0100 -s --oformat binary -o ipl.com
ipl.o

When I compile using ".code16" at the top of the code, the ipl.com file
works very fine, that I expected (clear the screen, print "Hello
world\n" and end).

But if I omit, or put explicit ".code32", at the top of the file, the
ipl.com file has the next problem:

1579:0100 90            NOP
1579:0101 E80B00        CALL    010F
1579:0104 0000          ADD     [BX+SI],AL
1579:0106 66            DB      66
1579:0107 BE5C01        MOV     SI,015C
1579:010A E83400        CALL    0141
1579:010D 0000          ADD     [BX+SI],AL
1579:010F CD20          INT     20

NOTE!!!
CALL    010F: Must point to the "clrscr" subroutine, but this subroutine
start at 0111.
CALL    0141: Must point to the "impstr" subroutine, but this subroutine
start at 0143.

Note 2 bytes of displacement in the CALL instruction.

Obviously the ipl.com file compiled with .code32 does not work fine.
010F points directly to INT 0x20 :(     :'(

Thanks a lot for your help and time.

Att.
Diego Cueva.

PD: I can accept a RTFM as a response, but say me the section or page.



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