Re: Bitordering? was Re: Bitmanipulation in Ada

[Jim Rogers <jimmaureenrogers@worldnet.att.net>]

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"Nick Roberts" <nick.roberts@acm.org> wrote in 
news:opsc2yf8zvp4pfvb@bram-2:

> On Fri, 20 Aug 2004 22:57:40 +0200, Alfred Hilscher  
> <SPAM@alfred-hilscher.de> wrote:
> 
>> When I declare an boolean array with eight elements 1..8 (e.g. to
>> address a hardware port with eight status bits), how are they
>> ordered is X(1) the msb or the lsb of the byte?
> 
> It is undefined.

It is undefined in an array. It can be defined in a record.

The Ada LRM, section 13.5.3 deals with bit ordering issues.
To quoute:

A bit ordering is a method of interpreting the meaning of the storage 
place attributes. High_Order_First (known in the vernacular as �big 
endian�) means that the first bit of a storage element (bit 0) is the 
most significant bit (interpreting the sequence of bits that represent a 
component as an unsigned integer value). Low_Order_First (known in the 
vernacular as �little endian�) means the opposite: the first bit is the 
least significant. 

For every specific record subtype S, the following attribute is defined: 

S�Bit_Order Denotes the bit ordering for the type of S. The value of this 
attribute is of type System.Bit_Order. Bit_Order may be specified for 
specific record types via an attribute_definition_clause; the expression 
of such a clause shall be static. 

If Word_Size = Storage_Unit, the default bit ordering is implementation 
defined. If Word_Size > Storage_Unit, the default bit ordering is the 
same as the ordering of storage elements in a word, when interpreted as 
an integer. 

The storage place attributes of a component of a type are interpreted 
according to the bit ordering of the type. 

Implementation Advice 

The recommended level of support for the nondefault bit ordering is: 

If Word_Size = Storage_Unit, then the implementation should support the 
nondefault bit ordering in addition to the default bit ordering.

Jim Rogers