Object-oriented Software in Ada 95

308 System programming
A program to print the current time of day in hours, minutes and seconds in a programming running under the
DOS operating system is as follows:
with System, System.Storage_Elements,
Ada.Text_Io, Ada.Integer_Text_Io;
use System, System.Storage_Elements,
Ada.Text_Io, Ada.Integer_Text_Io;
procedure Main is
Time_High_Address : constant Address := To_Address( 16#046C# );
Time_Low_Address : constant Address := To_Address( 16#046E# );
type Seconds_T is range 0 .. 1_000_000_000; --up to 65k * 5
type Time is range 0 .. 65365; --Unsigned
for Time'Size use 16;
-- in 2 bytes
Time_Low : Time;
for Time_Low'Address use Time_High_Address;
Time_High: Time;
for Time_High'Address use Time_Low_Address;
Seconds : Seconds_T;
begin
Put("Time is ");
Put( Time'Image(Time_High) ); Put(" :"); --Hour
Seconds := (Seconds_T(Time_Low) * 5) / 91;
Put(Seconds_T'Image(Seconds/60)); Put(" :"); --Mins
Put(Seconds_T'Image(Seconds rem 60)); --Seconds
New_Line;
end Main;
which when run on a DOS system would produce output of the form:
Time is 17 : 54 : 57
Note:
For this to work, the generated code must be able to access these low locations in DOS.
21.2.1 Access to individual bits
On an MS DOS system memory address 16#0417# contains the status of various keyboard settings. Individual
bits in this byte indicate the settings (set or not set) for the scroll lock, number lock, caps and insert keys. The
layout of this byte is illustrated in Figure 21.1.
Most significant
Bit position
Least significant
7 6 5 4 3 2 1 0
Insert
Caps
lock
Number
lock
Scroll
lock
Figure 21.1 Keyboard status on an MSDOS system.
© M A Smith - May not be reproduced without permission