HLASM Language Reference

DS Instruction
The size of a storage area that can be reserved by a DS instruction is limited only
by the size of virtual storage or by the maximum value of the location counter,
whichever is smaller.
How to Use the DS Instruction
Use the DS instruction to:
Reserve storage
Force alignment of the location counter so that the data that follows is on a
particular storage boundary
Name fields in a storage area.
To Reserve Storage: If you want to take advantage of automatic boundary
alignment (if the ALIGN option is specified) and implicit length calculation, you
should not supply a length modifier in your operand specifications. Instead, specify
a type subfield that corresponds to the type of area you need for your instructions.
Using a length modifier can give you the advantage of explicitly specifying the
length attribute value assigned to the label naming the area reserved. However,
your areas are not aligned automatically according to their type. If you omit the
nominal value in the operand, you should use a length modifier for the binary (B),
character (C), graphic (G), hexadecimal (X), and decimal (P and Z) type areas;
otherwise, their labels are given a length attribute value of 1 (2 for G and CU type).
When you need to reserve large areas, you can use a duplication factor. However,
in this case, you can only refer to the first area by its label. You can also use the
character (C) and hexadecimal (X) field types to specify large areas using the
length modifier. Duplication has no effect on implicit length.
Although the nominal value is optional for a DS instruction, you can put it to good
use by letting the assembler compute the length for areas of the B, C, G, X, and
decimal (P or Z) type areas. You achieve this by specifying the general format of
the nominal value that is placed in the area at execution time.
If a nominal value and no length modifier are specified for a Unicode character
string, the length of the storage reserved is derived by multiplying by two the
number of characters specified in the nominal value (after pairing).
To Force Alignment: Use the DS instruction to align the instruction or data that
follows, on a specific boundary. You can align the location counter to a
doubleword, a fullword, or a halfword boundary by using the correct constant type
(for example, D, F, or H) and a duplication factor of zero. No space is reserved for
such an instruction, yet the data that follows is aligned on the correct boundary.
For example, the following statements set the location counter to the next
doubleword boundary and reserve storage space for a 128-byte field (whose first
byte is on a doubleword boundary).
DS
D
AREA DS CL128
Alignment is forced whether or not the ALIGN assembler option is set.
To Name Fields within an Area: Using a duplication factor of zero in a DS
instruction also provides a label for an area of storage without actually reserving the
area. Use DS or DC instructions to reserve storage for, and assign labels to, fields
176 HLASM V1R5 Language Reference