# Choosing efficient computational data items

When you use a data item mainly for arithmetic or as a subscript, code `USAGE BINARY` on the data description entry for the item. The operations for manipulating binary data are faster than those for manipulating decimal data.

However, if a fixed-point arithmetic statement has intermediate results with a large precision (number of significant digits), the compiler uses decimal arithmetic anyway, after converting the operands to packed-decimal, decimal floating point, or vector decimal form. For fixed-point arithmetic statements, the compiler normally uses binary arithmetic for simple computations with binary operands if the precision is eight or fewer digits. Above 18 digits, the compiler always uses decimal arithmetic. With a precision of nine to 18 digits, the compiler uses either form.

To produce the most efficient code for a `BINARY` data item, ensure that it has:

• A sign (an `S` in its `PICTURE` clause)
• Eight or fewer digits

For a data item that is larger than eight digits or is used with `DISPLAY` or `NATIONAL` data items, use `PACKED-DECIMAL`. The code generated for `PACKED-DECIMAL` data items can be as fast as that for `BINARY` data items in some cases, especially if the statement is complicated or specifies rounding.

To produce the most efficient code for a `PACKED-DECIMAL` data item, ensure that it has:

• A sign (an `S` in its `PICTURE` clause)
• An odd number of digits (`9`s in the `PICTURE` clause), so that it occupies an exact number of bytes without a half byte left over
• 15 or fewer digits in the `PICTURE` specification on `ARCH(7)` machines. If a `PACKED-DECIMAL` data item has more than 31 digits, library routines are used. For a `PACKED-DECIMAL` data item with 16-31 digits on `ARCH(8)` or higher level machines, the compiler uses instructions that are more efficient than library calls, but not as fast as if the data item has 15 or fewer digits.