Ada: Difference between revisions

 

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 [https://www.iso.org/standard/61507.html 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.

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 <tt>Natural</tt> type, restricted to a predefined range. The use of the <tt>subtype</tt> directive informs the compiler that other variables of the <tt>Natural</tt> 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 <tt>Work_Day</tt> 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 <tt>Constraint_Error</tt> exception at runtime.

=== Representation clauses ===

 

 

----------------------------------------------------------------------------

-- The format of the System Table Descriptor pointer used by the processor

Offset at 0 range 16 .. 47;

end record;

 

The <tt>Size</tt> ''aspect specifier'' instructs the compiler that the <tt>System_Table_Descriptor</tt> 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 <tt>Size</tt> member should occupy bits 0 to 15, and the <tt>Offset</tt> 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.

Ring_3 => 3

);

 

The <tt>Size</tt> ''aspect specifier'' instructs the compiler that the <tt>Descriptor_Privilege_Level</tt> 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 <tt>Ring_0</tt> will be represented by a value of <tt>0x0</tt> in memory, the value of <tt>Ring_1</tt> will be represented by <tt>0x1</tt>, 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: https://www.youtube.com/watch?v=qvmDqbuQe-M

 

with System.Storage_Elements; use System.Storage_Elements;

 

-- Baud rate type.

----------------------------------------------------------------------------

for Baud_Rate_T use (

b_9600 => 0,

-- Parity Select Type

----------------------------------------------------------------------------

for Parity_T use (

None => 0,

end Main;

 

 

 

#include <stdint.h>

 

{

/** The UART control register pointer. */

 

// Configure the UART.

}

 

 

==See Also==

*[[Ada Bare bones]]

*[[Ada Runtime Library]]

 

===Operating System projects using Ada===

*[https://muen.codelabs.ch/ Muen Separation Kernel]

*[https://marte.unican.es/ MaRTE OS]