[TAG] a question about linker script

[Ben Okopnik]

----- Forwarded message from lion ? <helo54ljm at yahoo.com.tw> -----

Date: Thu, 17 Jun 2004 16:40:11 +0800 (CST)
From: lion ? <helo54ljm at yahoo.com.tw>
To: TAG <tag at lists.linuxgazette.net>
Subject: a question about linker script
To: ben at callahans.org

Dear Mr.Okopnik:
My name is Liang Jun-Ming,a student from Taiwan.
I'm suffering from a programming problem,I would very much
appreciate receving an answer from you !
I wrote a testing program named hello.s , and a linker script named
ldscript as follows:
/******hello.s*******/
    .global _start
    .text
       _start:
            movl  $len,%edx
            movl  $msg,%ecx
            movl  $1,%ebx
            movl  $4,%eax
            int   $0x80      #print string
            movl  $0,%ebx
            movl  $1,%eax
            int   $0x80      #exit
 
     .data
        msg:
            .string "Hello !\n"
            len = . - msg
/*******ldscript********/
      SECTIONS
      {
         .text 0x1000 : AT(0x3000000) { *(.text) }
         .data .  : AT(LOADADDR(.text) + SIZEOF(.text) )
            {  *(.data)  }
      }
after " as hello.s -o hello.o" and "ld -Tldscript hello.o -o hello"
I get an executable named hello.
My question is:
-----What does "AT(0x3000000)" mean ?
     Does it mean that the loader will load this executable at
"physical address " 0x3000000 ?
     I know the "virtual address" of the first byte of this executable
would be 0x1000, but in Linux , can we put data at any physical
address as we wish ? If it does, why the program still works even if the RAM
is 32M in my PC ?(0x3000000 > 32M). What tool in Linux can we have to
let us "watch" the contents of memory by specifying "physical addresses"?
If it doesn't, what is the function when we write "AT(xxxx)" ?
Hope I can receive an answer when you are not busy. Thank you very much!

           


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* Ben Okopnik * okopnik.freeshell.org * Editor-in-Chief, Linux Gazette *
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