Low Level Programming

Real-Time Programing Features of Ada:

The facilities that Ada offers as a system-level programming language are very suitable for real-time programming. With Ada you can

• specify word formats,

• control the internal representation of data,

• control where data are placed,

• specify register locations and formats, and

• specify the underlying representation of user-defined types.

• Ada gives you access to machine features:

  • machine code,

  • interrupts,

  • interfaces to other languages,

  • unscheduled access to shared variables, and

  • unchecked type conversion.

1. Define types and subtypes required:

type STATUS_TYPE is (OFF, LOW, NORMAL, HIGH);

subtype TIME_TYPE is INTEGER range 0 .. 2**13-1;

subtype TEMPERATURE_TYPE is INTEGER range 0 .. 2**10-1;

subtype PRESSURE_TYPE is INTEGER range 0 .. 2**6-1;

2. Define record for word formats:

type SENSOR_WORD_FORMAT is

record

STATUS : STATUS_TYPE;

TIME : TIME_TYPE;

TEMPERATURE : TEMPERATURE_TYPE;

PRESSURE : PRESSURE_TYPE;

end record;

3. Use representation specification to map the values and bits: Machine dependent

for STATUS_TYPE use ( OFF => 2#000#,

                                         LOW => 2#001#,

                                         NORMAL => 2#010#,

                                         HIGH => 2#100# );

for SENSOR_WORD_FORMAT use

record

STATUS                 at 0 range 1 .. 3;

TIME                     at 0 range 4 .. 16;

TEMPERATURE   at 1 range 1 .. 10;

PRESSURE            at 1 range 11 .. 16;

end record;

4. Use representation specification to map the values and bits: Machine independent

The other way of handling this word format is to use a pragma. Pragma's

in Ada give directions to the compiler, but do not generate code.

 pragma PACK (SENSOR_WORD_FORMAT);

tells the compiler to pack the words in the computer as tightly as possible with

the fields specified in the machine-independent version of the sensor record.

5. Convert temperature into degree Celsius:

The designer of the equipment might have stated that each step in the temperature input represents 0.25 degree Celsius and that a zero input represents -50 degrees. Thus, to convert the temperature information, we need to multiply the input value by 0.25 and to subtract 50 degrees. This can be done by converting the input information to a floating point type and then performing the calculations in an assignment statement. For example,

INPUT : SENSOR_WORD_FORMAT;

TEMPERATURE_IN_CELSIUS : = 0. 25 * FLOAT (INPUT. TEMPERATURE) - 50. 0;

Problem Statement:

The interface in below figure is an eight-bit interface. ASCII data come into this computer at a specified address A040 (in base 16).

type BINARY_STRING is array (POSITIVE range <>) of BOOLEAN;

INPUT_DATA: BINARY_STRING (1 .. 8);

pragma PACK (BINARY_STRING);

for INPUT_DATA use at 16#A040#;

Package specification:

with UNCHECKED_CONVERSION;

package DATA_CONVERSION is

procedure GET_DATA (C: out CHARACTER);

end DATA_CONVERSION;

Package body:

with UNCHECKED_CONVERSION;

package body DATA_CONVERSION is

procedure GET_DATA (C : out CHARACTER) is

type BINARY_STRING is array (POSITIVE range <>) of BOOLEAN;

INPUT_DATA: BINARY_STRING (1 .. 8);

TEMPORARY : BINARY-STRING (1 .. 8);

pragma PACK (BINARY_STRING);

for INPUT_DATA use at 16#A040#;

function UNCHECKED_CONVERSION is new UNCHECKED_CONVERSION (BINARY_STRING, CHARACTER);

begin

while INPUT_DATA (8) loop -- busy loop on bit 8

null;

end loop;

INPUT_DATA : = not INPUT_DATA; -- changes all bits and bit 8 is set

TEMPORARY : = INPUT_DATA and (FALSE, others => TRUE);

C : = UNCHECKED_CONVERSION (TEMPORARY);

end GET_DATA;

end DATA-CONVERSION;