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Section 8.2.2.3:
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types SAME = HEXTET ; HEXTET = seq of BIT inv oct == len oct = 16
This class is the primitive immutable class which has sixteen bits, but otherwise may have any meaning. Operations for creation, conversion and individual bit testing and manipulation are provided in addition to equality testing (which is solely a test of identical bit-patterns).
The following feature is required to be implemented for this class in accordance with the specification given in $IS_EQ of which $BIT_PATTERN is a sub-type :-
The following features are required to be implemented for this class in accordance with the specifications given by inheritance in $STR of which $BIT_PATTERN is a sub-type :-
The following feature is required to be implemented for this class in accordance with the specification given in $HASH of which $BIT_PATTERN is a sub-type :-
The following features are required to be implemented for this class in accordance with the specifications given in $BINARY of which $BIT_PATTERN is a sub-type :-
The following features are required to be implemented for this class in accordance with the specifications given in the abstract class $BIT_PATTERNS of which $BIT_PATTERN is a sub-type:-
The following features are required to be implemented for this class in accordance with the specifications given in the abstract class $BIT_PATTERN :-
This reader routine provides the same value as asize - for a hextet!
Hextet_Bits() : res : CARD
Since this is a reader routine, the pre-condition is vacuously true.
post (res = asize(self))
This reader routine returns the number of bits contained in a hextet (16!).
This reader routine returns the maximum unsigned value which can be represented in a hextet bit-pattern.
Hextet_Max() : res : CARD
Since this is a reader routine, the pre-condition is vacuously true.
post (res = 2 ** asize(self) - 1)
This reader routine returns the maximum numeric value representable as an exact unsigned number in the hextet bit-pattern.
This creation feature returns a new hextet all the bits of which are clear (ie having the value clearbit).
create | : SAME |
create() res : SAME
Because there are no arguments to this feature, the pre-condition is vacuously true.
post let resbits : seq of BIT be st resbits = res in resbits = [clearbit, clearbit, clearbit, clearbit, clearbit, clearbit, clearbit, clearbit, clearbit, clearbit, clearbit, clearbit, clearbit, clearbit, clearbit, clearbit]
This routine creates a new hextet value all the components of which are clear.
This second creation routine returns a new hextet in which the bit-pattern is the representation of val.
create2(val : CARD) res : SAME
pre val <= Hextet_Max
post card(res) = val
This routine returns a new hextet which takes the value val as a bit-pattern.
This third creation routine returns a new hextet in which the bit-pattern is the representation of the character val in sixteen bits.
create3(val : CHAR) res : SAME
pre let extra = tl tl CHAR.binstr(val) in forall idx | idx in inds extra & extra(idx) = null
post char(res) = val
This routine returns a new hextet which takes the value val as a bit-pattern.
This iter, which is not relevant to all classes sub-typing from $BIT_PATTERN, yields in sequence the individual octets which make up self - from the least significant group of eight upwards.
Note that the formal name of the iter has been changed to replace the exclamation mark iter symbol to a name acceptable to vdm tools.
octet_iter(self : SAME) res : OCTET
pre len history mod 8 = 0
This post-condition makes use of the history concept from vdm++ (see the vdm dialect notes).
post let loc_res : seq of BIT be st (res = loc_res and len loc_res = 8) in ((history~ = [] and (history = loc_res)) or (history = history ^ loc_res)) and loc_res = self((len history + 1), ..., (len history + 8))
For quit actions see the specification of the quit statement.
errs QUIT : len history~ = len self -> quit
This iter yields in turn the sequence of octets making up self, st\rting with the least significant group of eight bits.
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or enquiries should be made toKeith Hopper. Page last modified: Monday, 28 May 2001. |
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