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doc/pascal/conf.doc

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+.sp 1.5i
+.nr H1 3
+.NH
+Conformant Arrays
+.nh
+.LP
+.sp
+A fifth kind of parameter, besides the value, variable, procedure, and function
+parameter, is the conformant array parameter (\fBISO 6.6.3.7\fR). This
+parameter, undoubtedly the major addition to Pascal from the compiler writer's
+point of view, has been implemented. With this kind of parameter, the required
+bounds of the index-type of an actual parameter are not fixed, but are
+restricted to a specified range of values. Two types of conformant array
+parameters can be distinguished: variable conformant array parameters and
+value conformant array parameters.
+.sp
+.NH 2
+Variable conformant array parameters
+.LP
+.sp
+The treatment of variable conformant array parameters is comparable with the
+normal variable parameter.
+Both have in common that the parameter mechanism used is \fIcall by
+reference\fR.
+.br
+An example is:
+.br
+.in +5m
+to sort variable length arrays of integers, the following Pascal procedure could be used:
+
+.nf
+\fBprocedure\fR bubblesort(\fBvar\fR A : \fBarray\fR[low..high : integer] \fBof\fR integer);
+\fBvar\fR i, j : integer;
+\fBbegin
+        for\fR j := high - 1 \fBdownto\fR low \fBdo
+                for\fR i := low \fBto\fR j \fBdo
+                        if\fR A[i+1] < A[i] \fBthen\fI interchange A[i] and A[i+1]
+\fBend\fR;
+.fi
+.in -5m
+
+For every actual parameter, the base address of the array is pushed on the
+stack and for every index-type-specification, exactly one array descriptor
+is pushed.
+.sp
+.NH 2
+Value conformant array parameters
+.LP
+.sp
+The treatment of value conformant array parameters is more complex than its
+variable counterpart.
+.br
+An example is:
+.br
+.in +5m
+an unpacked array of characters could be printed as a string with the following program part:
+
+.nf
+\fBprocedure\fR WriteAsString( A : \fBarray\fR[low..high : integer] \fBof\fR char);
+\fBvar\fR i : integer;
+\fBbegin
+        for\fR i := low \fBto\fR high \fBdo\fR write(A[i]);
+\fBend\fR;
+.fi
+.in -5m
+
+The calling procedure pushes the base address of the actual parameter and
+the array descriptors belonging to it on the stack. Subsequently the procedure
+using the conformant array parameter is called. Because it is a \fIcall by
+value\fR, the called procedure has to create a copy of the actual parameter.
+This implies that the calling procedure knows how much space on the stack
+must be reserved for the parameters. If the actual-parameter is a conformant
+array, the called procedure keeps track of the size of the activation record.
+Hence the restrictions on the use of value conformant array parameters, as
+specified in \fBISO 6.6.3.7.2\fR, are dropped.
+
+A description of the EM code generated by the compiler is:
+
+.nf
+.ft I
+load the stack adjustment sofar
+load base address of array parameter
+compute the size in bytes of the array
+add this size to the stack adjustment
+copy the array
+remember the new address of the array
+.ft R
+.fi