You can specify the length field in an SS-format instruction. This lets you indicate explicitly the number of bytes of data at a virtual storage location that is to be used by the instruction. However, you can omit the length specification, because the assembler computes the number of bytes of data to be used from the expression that represents the address of the data.

See topic SS format for more information about SS-format instructions.

Implicit Length

When a length subfield is omitted from an SS-format machine instruction, an implicit length is assembled into the object code of the instruction. The implicit length is either of the following:

• For an implicit address, it is the length attribute of the first or only term in the expression representing the implicit address.
• For an explicit address, it is the length attribute of the first or only term in the expression representing the displacement.

Explicit Length

When a length subfield is specified in an SS-format machine instruction, the explicit length always overrides the implicit length.

An implicit or explicit length is the effective length. The length value assembled is always one less than the effective length. If you want an assembled length value of 0, an explicit length of 0 or 1 can be specified.

In the SS-format instructions requiring one length value, the allowable range for explicit lengths is 0 through 256. In the SS-format instructions requiring two length values, the allowable range for explicit lengths is 0 through 16.