Re: Creating several types from a base type and conversion

[Shark8 <onewingedshark@gmail.com>]

On Saturday, January 25, 2020 at 3:37:17 AM UTC-7, Ken Roberts wrote:
> It might be that I can only really concentrate on learning Ada on my weekends (thursday night -> saturday) subject to $HoneyDoList.
> 
> Needless to say, I'm not always the brightest nowadays between age and free time without some extra help, so sorry for seeming a little dense right now.
That's fine; we all know life happens.

> 
> Here's the computer representation as expressed in documentation and how I'm used to working on the hardware:
> 
> 30-bit words (both memory storage and hardware registers)
> 
> Bit Number : 29 28 .. 01 00
> MSBit on left, LSBit on right
Isn't MSB on left, rather arbitrary?
(eg "Your left or mine?")

What really matters is that you're consistent.

> Instruction format:
> 
> Bit number: | 29 .. 24 | 23 .. 21 | 20 .. 18 | 17 .. 15 | 14 .. 0 |
> Field     : | Op code  |    J     |    K     |    B     |    Y    |
> 
> Op code : Instruction
> j       : Skip next instruction modification
> k       : Source or destination of data
> b       : Index register to use to modify Y
> y       : Constant or memory address

I would recommend that you use a proper enumeration for your op-codes, and a full record for your instruction. The enumeration can help by making use of the required case-coverage (absent an OTHERS option) for case-statements and discriminated-records; the record is an excellent way to "keep things together" and organize things. — If you *do* use a record for the instruction-type, consider a record discriminated on the op-code enumeration.

> 
> Based on Niklas Holsti's recommendation, it looks like the following would be what I want to do:
> 
> <code>
> 
> with Interfaces;
> package core is
> 
> subtype Word is Interfaces.Unsigned_32 with range 0 .. 2**30 - 1;
> 
> subtype D_Word is Interfaces.Unsigned_64 with record
>     Ru : Word;
>     Rl : Word;
> end record
> 
> for D_Word use record
>     Ru : at 0 use 59 .. 30;
>     Rl : at 0 use 29 .. 0;
> end record
> 
> -- NOTE: I_Word is basically read-only
> --       The only use is to make it easier to break down Word into
> --       the fields to interpret the instruction and will not change
> --       during the instruction execution cycle
> 
> I_Word is Word with record
>     -- Not sure how to define this part yet
>     -- See below for breakdown of bits-to-fields
> end record
> 
> for I_Word use record
>     f : at 0 range 29 .. 24;
>     j : at 0 range 23 .. 21;
>     k : at 0 range 20 .. 18;
>     b : at 0 range 17 .. 15;
>     y : at 0 range 14 .. 00;
> end record
> 
> end package core;
> 
> </code>
I_Word's syntax is off you can only do "Type X is new Y with record ..." for tagged types. — in order to make it work as you have it laid out you're going to need the appropriate sized types to occupy those locations.

> -- Circular left shift register
> procedure shift_left_logical (R     : <> Word'Class, 
>                               Count : in Integer) is
You can only use S'Class on tagged types; while you /could/ have your word-types as tagged-types, it's probably a really bad idea considering you indicate you want to model the bits. (Tagged-types have a tag which would throw off your record's bit-layouts.)

> 
> Thanks for all of your help - I wish I could catch on a little quicker, but age and $DayJob (plus $HoneyDoList) make learning a new language a little challenging lately.
Again, it's fine.