HOW-TO: Customizing FPC target

  1. Introduction
  2. The things we need first
  3. Programme internals
  4. Building RTL
  5. Compiling your programme
  6. Resume

1. Introduction

A question about developing system or OS-free software has been arised several times at the Free Pascal dedicated forums. I won't tell a lot about modifing huge mass of source code but I shall try to add a special target by simply providing a custom RTL.

The main purpose of this document is to get ready to compile a Multiboot OS kernel which will be launched by GRUB. There's an answer on how to produce a plain binary as well.

I'm not an FPC developer, I'm just a user. So for the further info consider the programmers of the FPC.

Contact: Juras B. (Yury Benesh aka ybx) mailto:ybxsoft(put @)tut.by
http://ybx.narod.ru - don't be afraid of the "ru", the index is in three languages.

This manual is too short, so ask questions and I may expand it.

2. The things we need first

3. Programme internals

The first piece, i.e. the entry point is a startup code written in the assembler language. We need to specify a low-level initialization and finalization here. For Win32 target the file wprt0.as contains two functions: _mainCRTStartup and _WinMainCRTStartup. The code in the FPC source passes the needed entry point function to the linker. So that _mainCRTStartup is specified for a console (default) target.

The prt0.as file for Linux target contains function _start. The common things between these startup files is that they both pass control to a corresponding inside (in-library) function or directly call the PASCALMAIN routine. The routine is placed in the output main programme module. The generated assembly file for a GO32V2 and WIN32 main programme is almost the same. It contains PASCALMAIN label, which is the actual start point, HEAPSIZE, HEAP labels, a table of unit's initialization functions and several other definitions, the major of them are used by the system unit.To generate a main module listing, write a programme with an empty "begin;end." and execute "fpc -a -Anasmobj myprog.pas".

Now when we know some peculiarities of the already defined targets we can write our own startup files (named accroding to the compiler presumptions) and the system unit. For instance, if we use the Win32 target we must use wprt0.as as the startup code for a console application and syswin32.pas as a system unit. When compiling for the Win32 target the compiler uses aswand ldw while for the Linux target it uses ld and as. I'll use the Linux target because I want to use ELF-targeted binutils. The compiled binutils package for Win32 which produces ELF executables is available at my home site.

4. Building RTL

So we selected the Linux target and as a result we'll use Linux units directory. To make both real Linux and our customized target units available, write a simple script which will rename the units directories, i.e. to swap the names.

The next step is to create a basic RTL, mainly the system unit and friends, for me they were system and strings units.

To create a unit we should take system.pp from the "source/rtl/template" directory, write a custom makefile and startup file.
  1. Create a directory in the /source/rtl/ tree. Let it be custom.
  2. Copy system.pp template as syslinux.pp.
  3. Create prt0.asm or prt0.as. I used the first one as I prefer using NASM.
    The file must contain _start function and a memory for stack reserved in the bss section.
    The _start function must initialize ESP register and FPU, then start PASCALMAIN procedure.
  4. Create Makefile.fpc and write corresponding rules to build system, strings, mmx, cpu and some other needed units.
  5. Run fpcmake Makefile.fpc, then run make and copy all .ppu, .o, .a files to your units directory (/pp/units/mytarget/).

5. Compiling your programme

Now you're ready yo compile your programmes for the custom target. I used the following
command line:
fpc -Tlinux -k-Ttext -k0x220000 -XX sample.pas
I specified the starting address for the text section because I need the kernel to be loaded at a certain address.

6. Resume

As you can see I explained everything as briefly as possible. So if you have several questions feel free to ask me via email. The samples for this document you'll find at my home site.

6 September, 2003