Why can not GNU linker translate relocatable symbol address to absolute address?

Why can not GNU linker translate relocatable symbol address to absolute address?

[戴 志勇]

Hi:
   I have a problem that the gnu assembler and linker can not relocate symbols address in 16 bit real mode of x86 architecture. Below is the code:



# file: boot.asm

.code16

.text

.global _start

_start:

# initialize

movw %cs, %ax

movw %ax, %ds

movw %ax, %es

movw %ax, %ss

movw $base, %sp



# clear screen

movw $0x0600,  %ax

movw $0x0700,  %bx

movw $0,       %cx

movw $0x184f,  %dx

int $0x10



# set focus

movw $0x200,   %ax

movw $0x0,     %bx

movw $0x0,     %dx

int $0x10



# display on screen: Starting boot

movw $0x1301,  %ax

movw $0x000f,  %bx

movw $0x0,     %dx

movw $msg_len, %cx

pushw %ax

movw %ds,      %ax

movw %ax,      %es

popw %ax

movw msg,      %bp # Assign msg address to %bp, But cannot

                   # translate relocation address to

                   # absolute address. I must use:

                   # movw $msg_addr, %bp

int $0x10



# reset disk

xorb %ah, %ah

xorb %dl, %dl

int $0x13

hlt



msg: .ascii "Starting Boot"

msg_len = . - msg

base = 0x7c00

msg_addr = msg + 0x7c00





.org 510

.word 0xaa55



# file: end


Assembling the file boot.asm in Cygwin shell:
as -o boot.o bootloader5.asm --warn --32 && ld -r -Ttext 0x7c00 -e _start -s -o  boot.tmp boot.o -m i386pe &&  objdump -m i8086  -D boot.tmp


Below is output:

boot.tmp:     file format pe-i386


Disassembly of section .text:

00007c00 <_main>:
    7c00:       8c c8                   mov    %cs,%ax
    7c02:       8e d8                   mov    %ax,%ds
    7c04:       8e c0                   mov    %ax,%es
    7c06:       8e d0                   mov    %ax,%ss
    7c08:       bc 00 7c                mov    $0x7c00,%sp
    7c0b:       b8 00 06                mov    $0x600,%ax
    7c0e:       bb 00 07                mov    $0x700,%bx
    7c11:       b9 00 00                mov    $0x0,%cx
    7c14:       ba 4f 18                mov    $0x184f,%dx
    7c17:       cd 10                   int    $0x10
    7c19:       b8 00 02                mov    $0x200,%ax
    7c1c:       bb 00 00                mov    $0x0,%bx
    7c1f:       ba 00 00                mov    $0x0,%dx
    7c22:       cd 10                   int    $0x10
    7c24:       b8 01 13                mov    $0x1301,%ax
    7c27:       bb 0f 00                mov    $0xf,%bx
    7c2a:       ba 00 00                mov    $0x0,%dx
    7c2d:       b9 0d 00                mov    $0xd,%cx
    7c30:       50                      push   %ax
    7c31:       8c d8                   mov    %ds,%ax
    7c33:       8e c0                   mov    %ax,%es
    7c35:       58                      pop    %ax
    7c36:       8b 2e 43 00             mov    0x43,%bp   # Here is problem: msg address is not absolute address
    7c3a:       cd 10                   int    $0x10
    7c3c:       30 e4                   xor    %ah,%ah
    7c3e:       30 d2                   xor    %dl,%dl
    7c40:       cd 13                   int    $0x13
    7c42:       f4                      hlt
    7c43:       53                      push   %bx
    7c44:       74 61                   je     7ca7 <_main+0xa7>
    7c46:       72 74                   jb     7cbc <_main+0xbc>
    7c48:       69 6e 67 20 42          imul   $0x4220,0x67(%bp),%bp
    7c4d:       6f                      outsw  %ds:(%si),(%dx)
    7c4e:       6f                      outsw  %ds:(%si),(%dx)
    7c4f:       74 00                   je     7c51 <_main+0x51>
        ...
7dfd:       00 55 aa                add    %dl,-0x56(%di)


As you can see, address 0x7c36 is disassembly of: movw msg, %bp. But the absolute address of msg is not 0x7c43. Is there a mistake in my command options ? Thanks!

Best wishes to you!
Zhiyong Dai