Enumerations and Subranges

Unpacked Enumeration or Unsigned Subranges

Table 5-9 “Allocation and Alignment of Unpacked Enumeration or Unsigned Subrange Variables (HP Pascal Packing Algorithm) ” shows how the HP Pascal packing algorithm allocates and aligns unpacked enumeration or unsigned subrange variables.

An unpacked, signed subrange is always allocated four bytes.

Example

The value zero is always included in the subrange when the minimum number of bits is calculated.

TYPE
   enum_type  = (red,blue,yellow);  {3 elements}
   subr_type1 = 1..300;             {Including zero, 2 bytes}
   subr_type2 = 1..66000;           {Including zero, 4 bytes}
   subr_type3 = 100000..100010;     {Including zero, 4 bytes}
   subr_type4 = -1..200;            {4 bytes}
 
VAR
   enum_var  : enum_type;   {Allocated 1 byte, byte-aligned}
   subr_var1 : subr_type1;  {Allocated 2 bytes, 2-byte-aligned}
   subr_var2 : subr_type2;  {Allocated 4 bytes, 4-byte-aligned}
   subr_var4 : subr_type4;  {Allocated 4 bytes, 4-byte-aligned}
 
   unpacked_array : ARRAY [1..3] OF enum_type;  {Each element is
                                                 allocated one byte
                                                 and is byte-aligned}
 
   unpacked_record : RECORD
                        f1 : subr_type1;  {Allocated 2 bytes,
                                           2-byte-aligned}
                        f2 : subr_type2;  {Allocated 4 bytes,
                                           4-byte-aligned}
                     END;

Packed Array Elements of Enumeration or Subrange Types

A packed enumeration or subrange variable requires the minimum number of bits needed to represent its values in a record. It is bit-aligned.

If the enumeration or subrange variable belongs to a packed array, the HP Pascal packing algorithm allocates it the smallest power of two bits that is greater than or equal to the number of bits it requires, and aligns it on that boundary.

Table 5-10 “Allocation and Alignment of Packed Array Elements of Enumeration or Subrange Type (HP Pascal Packing Algorithm)” shows the relationship between the number of bits that a packed array element of an enumeration- or subrange-type array requires, the number of bits that the HP Pascal packing algorithm allocates to it, and its alignment.

Example

TYPE
   direction = (north,south,east,west);
   day = (sun,mon,tues,wed,thurs,fri,sat);
 
VAR
   pa1 = PACKED ARRAY [1..5] OF direction;
   pa2 = PACKED ARRAY [1..5] OF day;

Each element of the array pa1 requires two bits. Two is a power of two, so each element is allocated two bits. The entire array occupies 10 bits. It is allocated two bytes:

Figure 5-18 Enumerations and Subranges Example 1

Each element of the array pa2 requires three bits. The smallest power of two that is greater than or equal to three is four, so each element is allocated four bits. The entire array occupies 20 bits. It is allocated three bytes:

Figure 5-19 Enumerations and Subranges Example 2

Packed Record Elements of Enumeration or Subrange Types

If the variable belongs to a packed record, the HP Pascal packing algorithm allocates it as many bits as it requires, and bit-aligns it.

Example

TYPE
   day = (sun,mon,tues,wed,thurs,fri,sat);
 
VAR
   r : PACKED RECORD
          f1,f2,f3,f4,f5,f6,f7,f8,f9,f10,f11 : day;
       END;

Each field of the record r requires three bits. The entire record occupies 33 bits. It is allocated five bytes:

Figure 5-20 Enumerations and Subranges Example 3

	NOTE: Subranges can cross 4-byte boundaries, but code is less efficient when they do. Packed records (such as those above) are byte-aligned. Code is more efficient when their alignment is specified with the ALIGNMENT compiler option.