Re: ada and robots

[mheaney@ni.net (Matthew Heaney)]

In article <33AAC183.34C0@xtra.co.nz>, DesignTools@xtra.co.nz wrote:

I'm not a Modula programmer; perhaps you can walk me through some of your
code examples.

>This code uses the BITSET feature of Modula-2, to greate a PACKED
>BOOLEAN
>ARRAY of 24 Data bits, into 3 BYTES.

This can be done in Ada.

type Bit_Array is 
   array (Positive range 0 .. 23) of Boolean;

for Bit_Array'Size use 24;
for Bit_Array'Component_Size use 1;

or in this case you could just

pragma Pack (Bit_Array);

Ada guarantees that for an array for which the component size is 1 (and
type Boolean is guaranteed to be size 1), that array components are aligned
on contiguous bits.

>   CASE BitTimeL OF
>   | OneMIN..OneMAX :
>     INCL(RxCODE[RxCtr >> 3],RxCtr);
>   | ZeroMIN..ZeroMAX :   (* default is a Zero *)
>   ELSE                   (* bad value, so reset *)
>    TF0 := TRUE;          (* catch Reset, next pass  *)
>    RxCtr := 0;
>   END;
>   INC(RxCtr);

Can you explain what's going on here?

INCL means add an item to a set, right?   If so, you just set the
corresponding bit of the bit array to True.

Of course, we could declare a full-blown set abstract data type,
implemented as a bit array, but when you're twiddling bits anyway, one
doesn't usually bother.

>This code illustrates two methods of loading, one 'collects' BOOLEANs
>and
>is highly readable, the other uses BINARY format.
> IE := {EA,ET0,ES};
> IE := 2#1001_0010;     (* EA.EC.ET2.ES.ET1.EX1.ET0.EX0 *)

type Bit_Array_Range is (EA, EC, ET2, ES, ET1, EX1, ETO, EXO);
type Bit_Array is array (Bit_Array_Range) of Boolean;
pragma Pack (Bit_Array);

To do the first, you can use aggregate assignment:

IE : Bit_Array := (EA, ETO, ES => True, others => False);

To do the second, you have to use unchecked conversion:

function To_Bit_Array is
   new Unchecked_Conversion (
      Source => Interfaces.Unsigned_8, 
      Target => Bit_Array);

IE : Bit_Array := To_Bit_Array (2#1001_0010#);

Oh well, an extra line of code, to do the instantiation.

>This code is from a DUAL 6 BIT Picket fence D to A converter.
>Total code, for Dual DAC is 95 bytes.
>It allows appx 33KHz clock rates, and can run as a background DAC.
>More, or less, BITS / Channels are easy to add / remove.
>Such DACs are highly linear, use just 1 PIN per DAC, and are ideal for
>integration to control voltage outputs.

>  DACa := DAC_A.5;      (* Uses BYTE BITADDRESSABLE syntax *)

If you mean get at an individual bit, that can be done if the type is
already declared as a bit array, or else using unchecked_conversion to a
bit array.

   DAC_A (5)

or

  To_Bit_Array(DAC_A)(5)

You may need an extra pair of parens here; I don't have my Ada 95 compiler
handy.


>This code flips the BIT order in a BYTE, for COMMS protocol usage.
>TYPE hT = ARRAY [0..15] OF BYTE;
>                   (*  0   1   2  3    4  5   6  7  8 9 A B   C  D  E  F
>*)
>CONST MirrorTable =
>hT(0H,08H,04H,0CH,02H,0AH,6,0EH,1,9,5,0DH,3,0BH,7,0FH);
>
>PROCEDURE MirrorF(Par : SHORTCARD REGISTER_2) : SHORTCARD; (* 16
>bytes,14uS *)
>BEGIN
>   RETURN(ROTATE(MirrorTable[Par AND 0FH],4) OR MirrorTable[Par >> 4]);
>END MirrorF;

type hT is array (0 .. 15) of Unsigned_8;
 -- we may have to declare the array index subtype as a modular type
 -- ie type hT is array (Unsigned_8) of Unsigned_8;
 -- but I've never done that, and I don't have my compiler handy...

Mirror_Table : constant hT := 
 (16#0#, 16#8#, ...);

function Mirror_F (Par : Unsigned_8) return Unsigned_8 is
begin
   return Rotate_Right (Mirror_Table (Par and 16#0F#), 4) or
                 Mirror_Table (Shift_Right (Par, 4));
end;

or something like that. 

If my knowledge of Modula were better (I only have Wirth's book, which I
haven't even read), there might be a slicker Ada way to do this, instead of
as a straight translation.

The point is, however, that Ada is no less expressive than Modula with
respect to its low-level facilities.  Nor do I suppose its code generation
is any less efficient, if the types in package Interfaces are used.

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Matthew Heaney
Software Development Consultant
<mailto:matthew_heaney@acm.org>
(818) 985-1271