How should the 'System Use' field be interpreted in a 'Directory Record'?

Question

In the ECMA 119 specifications (freely available here), I am trying to understand how to fetch the content of the System Use field:

How is one supposed to compute the length of the System Use field, i.e. how is the value of the LEN_SU found in the left column ?

Answer 1

The value of LEN_SU is given implicitly. From BP1 you know the total number of bytes in the directory record (LEN_DR). LEN_SU is then given (implicitly) as the bytes remaining in the directory record after 33+LEN_FI+possible_padding(1), where you get length LEN_FI from BP33.

Hence

LEN_SU = LEN_DR - (33+LEN_FI+possible_padding(1))

From the spec.:

Padding Field [BP (34 + LEN_FI)]

This field shall be present in the Directory Record only if the number in the Length of the File Identifier field is an even number.

System Use [BP (LEN_DR - LEN_SU + 1) to LEN_DR)

This field shall be optional. If present, this field shall be reserved for system use. Its content is not specified by this Standard. If necessary, so that the Directory Record comprises an even number of bytes, a (00) byte shall be added to terminate this field.