Array slicing question

Array slicing question

[Mike Silva]

I (I being an Ada novice) am still fiddling around with writing 4 bits of LCD display data into a 32-bit GPIO register.  I have this:

   type Bits_1  is mod 2**1 with Size => 1;
   type Bits_32x1 is array (0 .. 31) of Bits_1 with Pack, Size => 32;
   type Bits_8x1 is array (0 .. 7) of Bits_1 with Pack, Size => 8;

and I want to do things like this:

   a32 : Bits_32x1;
   a8  : Bits_8x1;
...
   a32(4..7) := a8(4..7);

This is not allowed, apparently, because a32 and a8 are different types (even though the elements of the arrays are the same type, so I'm a little confused, but OK).

So can I do this?  I'm guessing if Bits_32x1 and Bits_8x1 were both subtypes of the same type, I could do it - is this correct?  But I don't know how to declare such a type, especially given the Pack and Size attributes.

I note that (ref confusion above) I CAN do this:

      for i in 4..7 loop
         gpio32(i) := b8(i);

Re: Array slicing question

[Simon Wright]

Mike Silva <mjsilva697@gmail.com> writes:

> I (I being an Ada novice) am still fiddling around with writing 4 bits
> of LCD display data into a 32-bit GPIO register.  I have this:
>
>    type Bits_1  is mod 2**1 with Size => 1;
>    type Bits_32x1 is array (0 .. 31) of Bits_1 with Pack, Size => 32;
>    type Bits_8x1 is array (0 .. 7) of Bits_1 with Pack, Size => 8;
>
> and I want to do things like this:
>
>    a32 : Bits_32x1;
>    a8  : Bits_8x1;
> ...
>    a32(4..7) := a8(4..7);
>
> This is not allowed, apparently, because a32 and a8 are different
> types (even though the elements of the arrays are the same type, so
> I'm a little confused, but OK).
>
> So can I do this?  I'm guessing if Bits_32x1 and Bits_8x1 were both
> subtypes of the same type, I could do it - is this correct?  But I
> don't know how to declare such a type, especially given the Pack and
> Size attributes.
>
> I note that (ref confusion above) I CAN do this:
>
>       for i in 4..7 loop
>          gpio32(i) := b8(i);

because the components _are_ the same type.

Would this help?

   type Bits is array (Natural range <>) of Boolean with Pack;

   subtype Bits_32 is Bits (0 .. 31);
   subtype Bits_8 is Bits (0 .. 7);

   A32 : Bits_32 with Size => 32;
   A8 : Bits_8 with Size => 8;

   ...

   A32 (4 .. 7) := A8 (4 .. 7);

Re: Array slicing question

[G.B.]

On 07.07.14 09:31, Mike Silva wrote:
> I (I being an Ada novice) am still fiddling around with writing 4 bits of LCD display data into a 32-bit GPIO register.  I have this:
>
>     type Bits_1  is mod 2**1 with Size => 1;
>     type Bits_32x1 is array (0 .. 31) of Bits_1 with Pack, Size => 32;
>     type Bits_8x1 is array (0 .. 7) of Bits_1 with Pack, Size => 8;
>
> and I want to do things like this:
>
>     a32 : Bits_32x1;
>     a8  : Bits_8x1;
> ...
>     a32(4..7) := a8(4..7);

Since this expression stays brief only as long as the slices
have no holes in them – and if atomicity of the update is
required, not touching the other bits in a32 – creating
a temporary doesn't seem so bad, perhaps hidden behind
a subprogram serving as abstraction.
   The assignment could not use a simple loop then, either,
so a bit-wise assignment from the components of a8 to components
of a32 seems still more direct than shifts and masks.

My 2¢ (for what might be too general a scheme)