🚀 go-pugleaf

Hi,

Real world examples abound where access to low level bit operations
are required.  My particular application requires analysis of data
recorded from an embedded system.  These analysis applications are 
usually run on only one type of computer, but it would be nice if 
the source code were portable.

Type transfer functions (CASTS) to and from BITSETs (and custom sets)
are a low level operation that is required for many situations.  Bit 
twiddling is required for many situations.  

Examples which come to mind are (1) converting a MIL-STD-1750 floating 
point single precision number into IEEE format (2) a number is stored 
in a bitfield within a 16 bit word, and (3) a fixed point number where 
the least significant bit has a value other than one.

These all require a knowledge of how the data is represented in the 
computer.  Given the various methods with which this data can be
represented causes these to be non-portable

In article <DM2KB6.983@nyongwa.montreal.qc.ca> Michel De Rosa,
aliath@nyongwa.montreal.qc.ca writes:
>Mark Morgan Lloyd (mark_tbu@cix.compulink.co.uk) wrote:
>: > neither does it do any sort of interpretation of the bits,
>: > i.e. it's a straight copy of the bitmap, if you will..
>: 
>: Watch it! I've argued this one in the past with TopSpeed, who IIRC under 
>: certain circumstances _do_ apply transformations.
>
>Really? They do that?... hmmmm do you have any cases, where they do this,
>so i can avoid 'debugging' time.. ;{)
>
>I really don't see what aspect of this 'feature', they felt needed changing,
>PIM is really quite clear on this... I would have argued the point too..

Type transfer functions (CASTs) do not simply copy the bits, but rather
they have some very simple sematics.  

[1: Type transfer between same sizes]

   Straight copy of the bits.

[2: Type transfer from long to short]

   This should simply truncate off the most significant bits, leaving 
   the least significant N bits.  This may actually modify bits form 
   the original data.  This must not cause a run-time error.

[3: Type transfer from short to long]

   Signed numbers need help. An cast from an INTEGER to a LONGINT 
   (or possibly    SHORTINT -> INTEGER, and SHORTINT -> LONGINT) 
   would extend the sign bit (assuming twos complement) into the 
   upper bits of the LONGINT.  This does not modify any of the 
   original bits though.  

   All other transfers should be treated as unsigned, in particular
   CHARs. 

In addition, type transer fucntions are restricted to only simple types 
(ie. not records or arrays), but do allow bitsets, and custom sets if 
they fit within a word or longword.  Some compilers allow very large 
sizes of sets (ie. 256 elements for SET OF CHAR), but this is very
non-standard.  

NOTE: all comments are made without knowledge of the ISO-xxxx standard.
If any of my comments contradict this standard please inform me.

Peter
-----