Ada Bare Bones

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Difficulty level

Medium
Kernel Designs
Models
Other Concepts

In this tutorial we will compile a simple Ada kernel and boot it.

WAIT! Have you read Getting Started, Beginner Mistakes, and some of the related OS theory?

Preface

This tutorial is based on my multiboot kernel which I developed some time ago and placed on my site [1] and will also be the basis for my own kernel TAMP.

One of the first things people ask on the Ada IRC channel on Freenode is "Can Ada be used for OS development?" to which the answer is a resounding yes. But there are 2 problems:

  1. The people asking this question are new to Ada, and
  2. GNAT is not the easiest compiler to build.

Therefore these users don't understand what it takes to get the compiler into a useable state.

As you may have seen from other bare bones tutorials on this site, they state that you must have a compiler built which can handle ELF files, the usual way is by building GCC which targets i386-elf or some other similar architecture. The problem here is that GNAT will not build for these targets out of the box without messing with it's makefile. You have to do this as the makefile builds the RTS and then the gnat tools (gnatmake, gnatbind, et al) which must all be built to have a working compiler - even though we will be replacing the RTS with our own cut down version.

For this tutorial, we will use the system GNAT compiler to build for a PC i386. GNAT is part of GCC. Later I will show how to build an arm-elf compiler and tools. My OS is Debian testing 64-bit with GNAT 4.6.

GNAT and the Ada runtime system (RTS)

For this kernel we will be configuring a zero footprint RTS profile. This basically means, we have a compiler, tools and not much else.

Directory structure

We need a place to structure this kernel, 

mkdir -p bare_bones/src/pc
cd bare_bones
mkdir -p rts/boards/i386/adalib
mkdir -p rts/boards/i386/adainclude
mkdir -p rts/src
mkdir -p obj

RTS files to copy

You will need to copy the following files from your compiler's RTS directory into rts/src and then create symbolic links from them to rts/boards/<arch>/adainclude where your arch is i386 or arm, etc.

N.B: You need to modify the location where these files are copied from, I've just used the location from my machine, which is most likely different to yours. 

for f in "ada.ads" "a-unccon.ads" "a-uncdea.ads" "gnat.ads" "g-souinf.ads" "interfac.ads" "s-atacco.adb" "s-atacco.ads" "s-maccod.ads" "s-stoele.adb" "s-stoele.ads"
do
cp /usr/lib/gcc/x86_64-linux-gnu/4.6/adainclude/$f rts/src/
ln -s `pwd`/rts/src/$f `pwd`/rts/boards/i386/adainclude/$f
done

Files

gnat.adc

This file in the root directory of the build tells GNAT there are some configuration pragmas to apply to the build. These pragmas can also be placed at the start of your custom sytem.ads (see below), but we'll place them here for now.

What these do is to tell GNAT how much of the RTS we can use in our kernel, which is not a lot really. 

pragma Discard_Names;
pragma Restrictions (No_Enumeration_Maps);
pragma Normalize_Scalars;
pragma Restrictions (No_Exception_Handlers);
pragma Restrictions (No_Finalization);
pragma Restrictions (No_Tasking);
pragma Restrictions (No_Protected_Types);
pragma Restrictions (No_Delay);
pragma Restrictions (No_Recursion);
pragma Restrictions (No_Allocators);
pragma Restrictions (No_Dispatch);
pragma Restrictions (No_Implicit_Dynamic_Code);
pragma Restrictions (No_Secondary_Stack);

Discard_Names

In Ada, the compiler generates strings for various data types, e.g. enumerations, these strings can then be used in I/O. 

type Fruit is (Orange, Banana, Apple);
--  Ada defines the following strings, "Orange", "Banana" and "Apple" in an array.

--  These strings can be accessed using the 'Image attribute, as in
Put (Fruit'Image (Orange));
--  Prints "Orange" to the console.

Normalize_Scalars

Forces all scalars to be initialised, se the latest GNAT RM:Normalize_Scalars for more information.

No_Exception_Propagation

No_Finalization

Max_Tasks

No_Protected_Types

No_Delay

No_Recursion

No_Allocators

No_Dispatch

No_Implicit_Dynamic_Code

No_Secondary_Stack

system.ads

Every Ada program must have access to the System package, this essentially tells the compiler what kind of hardware we are building for, therefore there will be 1 system.ads file per architecture your kernel supports.

Copy a system.ads from GCC that matches the target you are working with, in our case this is gcc-<version>/gcc/ada/system-linux-x86.ads, name it system.ads and place it into rts/boards/i386/ we need to edit this a bit.

We don't need to change anything from the top as all the integer sizes should be correct. Go to the private part of the spec and change the following values:

  1. Command_Line_Args => False
  2. Configurable_Run_Time => True
  3. Exit_Status_Supported => False
  4. Stack_Check_Probes => False
  5. Suppress_Standard_Library => True
  6. ZCX_By_Default => False
  7. GCC_ZCX_Support => False

For more information on these options, see gcc-<version>/gcc/ada/targparm.ads.

Last chance handler

When you start to write and compile Ada using this custom environment, the compiler will automatically place calls from the runtime into your final binary (this is what the compiler normally does, but we've restricted it a lot). One of these calls is to Last_Chance_Handler so create 2 new files and place into rts/boards/<arch>/adainclude, as follows.

last_chance_handler.ads

with System;

procedure Last_Chance_Handler
  (Source_Location : System.Address; Line : Integer);
pragma Export (C, Last_Chance_Handler, "__gnat_last_chance_handler");

last_chance_handler.adb

procedure Last_Chance_Handler
  (Source_Location : System.Address; Line : Integer) is
   pragma Unreferenced (Source_Location, Line);
begin
   --  TODO: Add in code to dump the info to serial/screen which
   --  is obviously board specific.
   loop
      null;
   end loop;
end Last_Chance_Handler;

As you can see, the meat of the handler is actualy a null loop at the moment, this is something you need to complete for your OS kernel and also, per platform.

Compiling the runtime

Create a file called gnat.gpr in the root directory and copy the following into it: 

library project gnat is
   type Arch_Name is ("i386", "arm");
   type Board_Name is ("pc", "rpi");

   Arch  : Arch_Name  := "i386";
   Board : Board_Name := external ("Board");

   case Board is
      when "pc" =>
         Arch := "i386";
      when "rpi" =>
         Arch  := "arm";
   end case;

   for Source_Dirs use ("rts/boards/" & Arch & "/adainclude");
   for Object_Dir use "obj"; --"rts/boards/" & Arch & "/adalib";

   package Builder is
      Basic_Switches := ("-gnat2005", "-g", "-x", "-a", "-gnatg");

      case Board is
         when "pc" =>
            for Default_Switches ("Ada") use Basic_Switches &
               ("-m32", "-march=i386");
         when "rpi" =>
            for Default_Switches ("Ada") use Basic_Switches &
               ("-march=armv6zk", "-mfpu=vfp", "-mfloat-abi=hard", "-marm",
                "-mcpu=arm1176jzf-s", "-mtune=arm1176jzf-s");
      end case;
   end Builder;

   package Compiler is
      for Default_Switches ("Ada") use ("-O2", "-ffunction-sections", "-fdata-sections");
   end Compiler;

   for Library_Kind use "static";
   for Library_Name use "gnat-4.6";
   for Library_Dir use "rts/boards/" & Arch & "/adalib";
end gnat;

Now compile with the following command: 

gnatmake -XBoard=pc -Pgnat.gpr

Inside rts/boards/i386/adainclude/ you should have the RTS sources symbolically linked along with the custom last_chance_hander and system files. Inside rts/boards/i386/adalib/ you should have the libgnat-4.6.a and also *.ali matching the source which are required by GNAT.

startup.s

This is PC specific so place this in the src/pc directory.

GAS

.global startup                         # making entry point visible to linker

# setting up the Multiboot header - see GRUB docs for details
.set ALIGN,    1<<0                     # align loaded modules on page boundaries
.set MEMINFO,  1<<1                     # provide memory map
.set FLAGS,    ALIGN | MEMINFO          # this is the Multiboot 'flag' field
.set MAGIC,    0x1BADB002               # 'magic number' lets bootloader find the header
.set CHECKSUM, -(MAGIC + FLAGS)         # checksum required

.align 4
.long MAGIC
.long FLAGS
.long CHECKSUM

# reserve initial kernel stack space
.set STACKSIZE, 0x4000                  # that is, 16k.
# On my binutils the following line didn't work as the .lcomm instruction takes 2 parameters.
#.lcomm stack, STACKSIZE, 32             # reserve 16k stack on a doubleword boundary
.lcomm stack, STACKSIZE                 # reserve 16k stack
.comm  mbd, 4                           # we will use this in kmain
.comm  magic, 4                         # we will use this in kmain

startup:
    movl  $(stack + STACKSIZE), %esp    # set up the stack
    movl  %eax, magic                   # Multiboot magic number
    movl  %ebx, mbd                     # Multiboot data structure

    call  main                          # call main created by gnatbind

    cli
hang:
    hlt                                 # halt machine should kernel return
    jmp   hang

multiboot.ads

--  TODO

Console

The following 2 files give you access to the VGA console at 80x25 characters. As they are PC specific, they go into the src/pc directory.

console.ads

with System;

package Console is
   pragma Preelaborate (Console);

   type Background_Colour is
     (Black,
      Blue,
      Green,
      Cyan,
      Red,
      Magenta,
      Brown,
      Light_Grey);

   for Background_Colour use
     (Black      => 16#0#,
      Blue       => 16#1#,
      Green      => 16#2#,
      Cyan       => 16#3#,
      Red        => 16#4#,
      Magenta    => 16#5#,
      Brown      => 16#6#,
      Light_Grey => 16#7#);

   for Background_Colour'Size use 4;

   type Foreground_Colour is
     (Black,
      Blue,
      Green,
      Cyan,
      Red,
      Magenta,
      Brown,
      Light_Grey,
      Dark_Grey,
      Light_Blue,
      Light_Green,
      Light_Cyan,
      Light_Red,
      Light_Magenta,
      Yellow,
      White);

   for Foreground_Colour use
     (Black         => 16#0#,
      Blue          => 16#1#,
      Green         => 16#2#,
      Cyan          => 16#3#,
      Red           => 16#4#,
      Magenta       => 16#5#,
      Brown         => 16#6#,
      Light_Grey    => 16#7#,
      Dark_Grey     => 16#8#,
      Light_Blue    => 16#9#,
      Light_Green   => 16#A#,
      Light_Cyan    => 16#B#,
      Light_Red     => 16#C#,
      Light_Magenta => 16#D#,
      Yellow        => 16#E#,
      White         => 16#F#);

   for Foreground_Colour'Size use 4;

   type Cell_Colour is
      record
         Foreground : Foreground_Colour;
         Background : Background_Colour;
      end record;

   for Cell_Colour use
      record
         Foreground at 0 range 0 .. 3;
         Background at 0 range 4 .. 7;
      end record;

   for Cell_Colour'Size use 8;

   type Cell is
      record
         Char   : Character;
         Colour : Cell_Colour;
      end record;

   for Cell'Size use 16;

   Screen_Width  : constant Natural := 80;
   Screen_Height : constant Natural := 25;

   subtype Screen_Width_Range  is Natural range 1 .. Screen_Width;
   subtype Screen_Height_Range is Natural range 1 .. Screen_Height;

   type Row    is array (Screen_Width_Range)  of Cell;
   type Screen is array (Screen_Height_Range) of Row;

   Video_Memory : Screen;

   for Video_Memory'Address use System'To_Address (16#000B_8000#);

   pragma Import (Ada, Video_Memory);

   procedure Put
     (Char       : in Character;
      X          : in Screen_Width_Range;
      Y          : in Screen_Height_Range;
      Foreground : in Foreground_Colour := White;
      Background : in Background_Colour := Black);

   procedure Put
     (Str        : in String;
      X          : in Screen_Width_Range;
      Y          : in Screen_Height_Range;
      Foreground : in Foreground_Colour := White;
      Background : in Background_Colour := Black);

   procedure Clear (Background : in Background_Colour := Black);
end Console;

console.adb

package body Console is
   procedure Put
     (Char       : in Character;
      X          : in Screen_Width_Range;
      Y          : in Screen_Height_Range;
      Foreground : in Foreground_Colour := White;
      Background : in Background_Colour := Black) is
   begin
      Video_Memory (Y)(X).Char              := Char;
      Video_Memory (Y)(X).Colour.Foreground := Foreground;
      Video_Memory (Y)(X).Colour.Background := Background;
   end Put;

   procedure Put
      (Str        : in String;
       X          : in Screen_Width_Range;
       Y          : in Screen_Height_Range;
       Foreground : in Foreground_Colour := White;
       Background : in Background_Colour := Black) is
   begin
      for Index in Str'First .. Str'Last loop
         Put (Str (Index),
              X + Screen_Width_Range (Index) - 1,
              Y,
              Foreground,
              Background);
      end loop;
   end Put;

   procedure Clear (Background : in Background_Colour := Black) is
   begin
      for X in Screen_Width_Range'First .. Screen_Width_Range'Last loop
         for Y in Screen_Height_Range'First .. Screen_Height_Range'Last loop
            Put (' ', X, Y, Background => Background);
         end loop;
      end loop;
   end Clear;
end Console;

bare_bones.adb

This is platform independent and therefore goes into the src directory. 

with Console; use Console;

procedure Bare_Bones is
begin
   Clear;

   Put ("Hello, bare bones in Ada.",
        Screen_Width_Range'First,
        Screen_Height_Range'First);
end Bare_Bones;
pragma No_Return (Bare_Bones);

linker.ld

This is a PC specific script so goes into the src/pc directory. 

OUTPUT_FORMAT(elf32-i386)
STARTUP(startup.o)
ENTRY (startup)

SECTIONS
{
    . = 0x00100000;

    .text :{
        code = .; _code = .; __code = .;
        *(.text)
        *(.rodata)
    }

    .rodata ALIGN (0x1000) : {
        *(.rodata)
    }

    .data ALIGN (0x1000) : {
        data = .; _data = .; __data = .;
        *(.data)
    }

    .bss : {
        sbss = .;
        bss = .; _bss = .; __bss = .;
        *(COMMON)
        *(.bss)
        ebss = .;
    }
    end = .; _end = .; __end = .;
}

makefile

Place this file in the root directory. 

ARCH		=	i386
RTS_DIR		=	`pwd`/rts/boards/$(ARCH)

ifeq ($(ARCH),i386)
GNATMAKE	=	gnatmake
AS		=	as
ASFLAGS		=	--32 -march=i386

OBJS		=	obj/startup.o obj/multiboot.o obj/console.o
BOARD		=	pc

.PHONY: obj/multiboot.o obj/console.o

endif

all: bare_bones

bare_bones: $(OBJS) src/bare_bones.adb
	$(GNATMAKE) --RTS=$(RTS_DIR) -XBoard=$(BOARD) -Pbare_bones.gpr

obj/startup.o: src/$(BOARD)/startup.s
	$(AS) $(ASFLAGS) src/$(BOARD)/startup.s -o obj/startup.o

.PHONY: clean

clean:
	-rm obj/* *~ bare_bones

bare_bones.gpr

Place this file in the root directory. 

project Bare_Bones is
   type Arch_Name is ("i386", "arm");
   type Board_Name is ("pc", "rpi");

   Arch  : Arch_Name  := "i386";
   Board : Board_Name := external ("Board");

   -- TODO: Add in a case statement that adds an arch dir to source.

   case Board is
      when "pc" =>
         for Source_Dirs use ("src", "src/pc");
      when "rpi" =>
         for Source_Dirs use ("src", "src/rpi");
   end case;

   for Object_Dir use "obj";
   for Exec_Dir use ".";
   for Main use ("bare_bones.adb");

   package Builder is
      Basic_Switches := ("-gnat2005", "-g", "-x", "-a", "-gnatg",
                         "-gnatec=../gnat.adc", "-gnaty-I", "-gnaty+d");

      case Board is
         when "pc" =>
            for Default_Switches ("Ada") use Basic_Switches &
               ("-m32", "-march=i386");
         when "rpi" =>
            for Default_Switches ("Ada") use Basic_Switches &
               ("-march=armv6zk", "-mfpu=vfp", "-mfloat-abi=hard", "-marm",
                "-mcpu=arm1176jzf-s", "-mtune=arm1176jzf-s");
      end case;
   end Builder;

   package Compiler is
      case Board is
         when "pc" =>
            for Default_Switches ("Ada") use
               ("-O0", "-g", "-ggdb", "-ffunction-sections", "-fdata-sections");
         when "rpi" =>
            for Default_Switches ("Ada") use
               ("-O0", "-g", "-ggdb", "-ffunction-sections", "-fdata-sections");

      end case;
   end Compiler;

-- To reduce size of final binary.
   package Linker is
      for Default_Switches ("Ada") use
         ("-Wl,--gc-sections", "-static", "-nostartfiles", "-nodefaultlibs",
          "-T../src/" & Board & "/linker.ld", "-v");
   end Linker;
end Bare_Bones;

Testing

Make sure you have built the RTS above before this next stage otherwise you won'g have a kernel. 

make

qemu -kernel bare_bones

On the QEMU window, it should clear the screen, the the cursor won't move so it will be in the middle of the screen, in the top-left corner will be the message "Hello, bare bones in Ada."

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