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Ada is a strongly typed programming language that enjoys widespread use within the embedded systems and safety-critical software industry. It was originally developed under contract to the United States Department of Defense, as part of a project to develop a single unified language meeting the safety and reliability requirements of the department's embedded systems projects.

The language was first standardized in 1983, and has since undergone multiple revisions. The latest version is Ada 2012. The language itself is defined by the ISO standard ISO/IEC 8652, and the Military standard MIL-STD-1815A. In 1991, the US Department of Defense mandated the use of Ada for all software, though exceptions to this rule were often granted. This mandate was effectively removed in 1997, as the DoD began to embrace 'commercial off the shelf' (COTS) technology. Similar requirements existed in other NATO countries for command and control systems, and Ada was the mandated or preferred language for defense-related applications in countries such as Sweden, Germany, and Canada.



Ada has a number of useful features that are of particular interest for low-level programming and operating system development.

Custom types

In addition to being a strongly typed language, Ada allows for the definition of new scalar, enumerated and record types. Custom primitive types can also be constrained to a predefined range of values. The example below demonstrates the definition of a new integer type based upon Ada's native Natural type, restricted to a predefined range. The use of the subtype directive informs the compiler that other variables of the Natural type are compatible with the newly defined subtype.

   VGA_COL_COUNT : constant := 80;
   VGA_ROW_COUNT : constant := 24;
   subtype Col is Natural range 0 .. VGA_COL_COUNT - 1;
   subtype Row is Natural range 0 .. VGA_ROW_COUNT - 1;

The below example illustrates the creation of incompatible custom integer types. While their base type and range constraints are identical, Ada treats both as separate, incompatible types. An assignment of a variable of one type to the value of another is illegal, and will trigger a compile-time error.

   type Integer_1 is range 1 .. 10;
   type Integer_2 is range 1 .. 10;
   A : Integer_1 := 8;
   B : Integer_2 := A; -- illegal!

The following example demonstrates the creation of a custom enumerated type. It also demonstrates a subtype of an enumerated type with a constrained range of values.

   type Day_Of_Week is (Monday, Tuesday,
     Wednesday, Thursday, Friday, Saturday, Sunday);
   subtype Work_Day is Day_Of_Week range Monday .. Friday;

A variable with the type of Work_Day is restricted to its constrained range. Any attempt to assign a value outside of this range to a variable of this type will raise a Constraint_Error exception at runtime.

Representation clauses

Ada allows for explicitly defining the in-memory representation of scalar and compound types. The following example demonstrates the definition of a record type (equivalent to structures in C), as well as its associated representation in memory.

   --  The format of the System Table Descriptor pointer used by the processor
   --  to load descriptor tables like the GDT and IDT.
   type System_Table_Descriptor is
         Size   : Unsigned_16;
         Offset : System.Address;
      end record
   with Size => 48;
   for System_Table_Descriptor use
         Size   at 0 range 0  .. 15;
         Offset at 0 range 16 .. 47;
      end record;

The Size aspect specifier instructs the compiler that the System_Table_Descriptor type must be 48 bits in size. The record representation clause instructs the compiler as to the required layout of this record type in memory. This example specifies that the Size member should occupy bits 0 to 15, and the Offset member should occupy bits 16 to 47. This feature is analogous to C's bit-fields.

The following example demonstrates defining the in-memory representation of an enumerated type.

   --  The privilege level for a particular descriptor.
   --  These correspond to the 'protection ring' that this descriptor is
   --  accessible from.
   type Descriptor_Privilege_Level is (
   with Size => 2;
   for Descriptor_Privilege_Level use (
      Ring_0 => 0,
      Ring_1 => 1,
      Ring_2 => 2,
      Ring_3 => 3

The Size aspect specifier instructs the compiler that the Descriptor_Privilege_Level type must be 2 bits in size. The representation clause instructs the compiler as to required representation of each possible value of the enumerated type in memory. In this example the value of Ring_0 will be represented by a value of 0x0 in memory, the value of Ring_1 will be represented by 0x1, and so on.


The following example, and accompanying comparison with C, demonstrates the configuration of a hypothetical UART device by interfacing with an 8-bit memory-mapped configuration register. This example has been adapted from a presentation by AdaCore viewable at:

with System.Storage_Elements; use System.Storage_Elements;
--  Main
procedure Main is
   --  Baud rate type.
   type Baud_Rate_T is (b_9600, b_14400, b_115200);
   for Baud_Rate_T use (
     b_9600   => 0,
     b_14400  => 1,
     b_115200 => 7
   --  Parity Select Type
   type Parity_T is (None, Even, Odd);
   for Parity_T use (
     None => 0,
     Even => 1,
     Odd  => 2
   --  Control Register for a hypothetical UART device.
   type UART_Control_Register_T is
         Baud_Rate : Baud_Rate_T;
         Parity    : Parity_T;
         Unused    : Boolean := False;
         ISR       : Boolean;
      end record
   with Volatile_Full_Access,
     Size => 8;
   for UART_Control_Register_T use
         Baud_Rate at 0 range 0 .. 2;
         Parity    at 0 range 3 .. 4;
         Unused    at 0 range 5 .. 6;
         ISR       at 0 range 7 .. 7;
      end record;
   --  The address of the UART control register.
   UART_CONTROL_REG_ADDR : constant System.Address
     := To_Address(16#8000_0040#);
   --  The UART control register itself.
   UART_Control_Register : UART_Control_Register_T
   with Import,
     Convention => Ada,
     Address    => UART_CONTROL_REG_ADDR;
   --  Configure the UART.
   UART_Control_Register.Baud_Rate := b_115200;
   UART_Control_Register.Parity    := Even;
end Main;

Contrast this with the same functionality implemented in C. Despite being shorter in length, the register cannot be altered without using bitwise operators to manipulate the individual fields. This approach is generally considered more error-prone than using a record in Ada overlaid at the register's memory-mapped address. It is possible to define a struct type in C with bit-fields for the individual elements, however the C standard does not guarantee the layout and order of the individual fields (refer to section paragraph 11 of the C1X standard).

#include <stdint.h>
#define UART_CNTL_REG_ADDR       0x80000040
#define UART_CNTL_BAUD_MASK      0x07
#define UART_CNTL_BAUD_SHIFT     0
#define UART_CNTL_PARITY_MASK    0x18
#define UART_CNTL_IE_MASK        0x80
#define UART_CNTL_BAUD_9600      0
#define UART_CNTL_BAUD_14400     1
#define UART_CNTL_BAUD_115200    7
#define UART_CNTL_PARITY_ODD     2
#define UART_CNTL_ISR_ENABLE     2
int main(int argc, char **argv)
	/** The UART control register pointer. */
	volatile uint8_t *uart_control_reg = (uint8_t*)UART_CNTL_REG_ADDR;
	// Configure the UART.
	*uart_control_reg &= ~UART_CNTL_BAUD_MASK;
	*uart_control_reg |= UART_CNTL_BAUD_115200;
	*uart_control_reg &= ~UART_CNTL_PARITY_MASK;

See Also


Operating System projects using Ada

External Links

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