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