Re: "Put an instruction into a specified location"

[Michael Meissner <meissner at cygnus dot com>]

On Wed, May 16, 2001 at 09:35:50AM -0700, Xinan Tang wrote:
> Hi
> 
>   Yesterday I asked the question of "how to put an instruction
> into a specified location?" The answer I received is to use
> .org directive.
> 
>   However, that is not what I want. What I really want is to
> generate such equivalent infortion at runtime, i.e., assume no
> such .org directive in the source code.
> 
>   For example, suppose there are such `strange' instructions:
> 
>   1: @0x20, MOV R1, 0x1000
>   2: @0x40, ADD R1, R2, R3
> 
>   The address after the symbol @ is the address that the corresponding
> instruction is supposed to go. Now, how to generate such relocation
> infortion when the instruction is parsed? I guess Fixups might be
> helpful?
> 
>   Furthermore, there is `gap' between these two instructions
> 
> 	(0x40-0x20=0x20)
> 
>   How can I dump such `non-continuous' but `neighbour' two instructions?
> 
> Thanks

What you need to do is put all of the code that needs to be hard wired into
hard addresses into its own section.  Then write your linker script so that the
section is bound to that address.  Quoting from the ld manual:

File: ld.info,  Node: Simple Example,  Next: Simple Commands,  Prev: Script
Format,  Up: Scripts

Simple Linker Script Example
============================

   Many linker scripts are fairly simple.

   The simplest possible linker script has just one command:
`SECTIONS'.  You use the `SECTIONS' command to describe the memory
layout of the output file.

   The `SECTIONS' command is a powerful command.  Here we will describe
a simple use of it.  Let's assume your program consists only of code,
initialized data, and uninitialized data.  These will be in the
`.text', `.data', and `.bss' sections, respectively.  Let's assume
further that these are the only sections which appear in your input
files.

   For this example, let's say that the code should be loaded at address
0x10000, and that the data should start at address 0x8000000.  Here is a
linker script which will do that:
     SECTIONS
     {
       . = 0x10000;
       .text : { *(.text) }
       . = 0x8000000;
       .data : { *(.data) }
       .bss : { *(.bss) }
     }

   You write the `SECTIONS' command as the keyword `SECTIONS', followed
by a series of symbol assignments and output section descriptions
enclosed in curly braces.

   The first line inside the `SECTIONS' command of the above example
sets the value of the special symbol `.', which is the location
counter.  If you do not specify the address of an output section in some
other way (other ways are described later), the address is set from the
current value of the location counter.  The location counter is then
incremented by the size of the output section.  At the start of the
`SECTIONS' command, the location counter has the value `0'.

   The second line defines an output section, `.text'.  The colon is
required syntax which may be ignored for now.  Within the curly braces
after the output section name, you list the names of the input sections
which should be placed into this output section.  The `*' is a wildcard
which matches any file name.  The expression `*(.text)' means all
`.text' input sections in all input files.

   Since the location counter is `0x10000' when the output section
`.text' is defined, the linker will set the address of the `.text'
section in the output file to be `0x10000'.

   The remaining lines define the `.data' and `.bss' sections in the
output file.  The linker will place the `.data' output section at
address `0x8000000'.  After the linker places the `.data' output
section, the value of the location counter will be `0x8000000' plus the
size of the `.data' output section.  The effect is that the linker will
place the `.bss' output section immediately after the `.data' output
section in memory

   The linker will ensure that each output section has the required
alignment, by increasing the location counter if necessary.  In this
example, the specified addresses for the `.text' and `.data' sections
will probably satisfy any alignment constraints, but the linker may
have to create a small gap between the `.data' and `.bss' sections.

   That's it!  That's a simple and complete linker script.

-- 
Michael Meissner, Red Hat, Inc.  (GCC group)
PMB 198, 174 Littleton Road #3, Westford, Massachusetts 01886, USA
Work:	  meissner@redhat.com		phone: +1 978-486-9304
Non-work: meissner@spectacle-pond.org	fax:   +1 978-692-4482