Object-oriented Software in Ada 95

System programming 307
21.1.1 Putting it all together
To print the international telephone code for France the following code can be used:
with System, System.Storage_Elements;
use System, System.Storage_Elements;
procedure Main is
type Country is (USA, France, UK, Australia);
for Colour'Size use Integer'Size;
for Country use (USA=>1, France=>33, UK=>44, Australia=>61);
function Idc is new Unchecked_Conversion( Country, Integer );
begin
Put( "International dialling code for France is ");
Put( Idc(France) );
New_Line;
end Main;
which when run, would produce the following results:
International dialling code for France is 33
It would also be convenient to also include Canada in the Country enumeration for telephone codes.
However, as Canada has the same country code as the USA, this cannot be done directly. The reason for this is
that two enumerations may not have the same physical representation. The way round this is to define a renaming
for Canada as follows:
function Canada return Country renames USA;
which defines Canada as a function that returns the enumeration USA as its result.
21.2 Binding an object to a specific address
In some limited situations it is necessary to read or write from absolute locations in memory. In the historic
operating system MS DOS the time of day is stored in locations (in hexadecimal) 46E and 46C. The exact
specification of what is stored is as follows:
Location (hexadecimal)
046E - 046F
046C - 046D
Contents
The time of day in hours.
The ticks past the current hour.
Each tick is 5/91 seconds.
An object may be bound to an absolute location with the for use clause. For example, to bind the integer
variable Time_High to the absolute location 16#46E# the following declaration can be used:
Time_High_Address : constant Address := To_Address( 16#046C# );
type Time is range 0 .. 65365; --Unsigned
for Time'Size use 16;
-- in 2 bytes
Time_High: Time;
for Time_High'Address use Time_Low_Address;
Note:
Time is a type describing a 16 bit unsigned integer.
The address 16#046E# must be of type Address that is defined in the package System. The child
package System.Storage_elements contains the function to_address which converts an
integer into an address.
© M A Smith - May not be reproduced without permission