'System Use' 字段在 'Directory Record' 中应该如何解释？
How should the 'System Use' field be interpreted in a 'Directory Record'?
在 ECMA 119 规范（免费提供 here）中，我试图了解如何获取 System Use 字段的内容：
应该如何计算 System Use 字段的长度，即如何在左列中找到 LEN_SU 的值？
LEN_SU 的值是隐式给出的。从 BP1 您知道目录记录中的总字节数 (LEN_DR)。 LEN_SU 然后（隐含地）给出 33+LEN_FI+possible_padding(1) 之后目录记录中剩余的字节，从 BP33 获得长度 LEN_FI。
因此
LEN_SU = LEN_DR - (33+LEN_FI+possible_padding(1))
来自规范：
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.
在 ECMA 119 规范（免费提供 here）中，我试图了解如何获取 System Use 字段的内容：
应该如何计算 System Use 字段的长度，即如何在左列中找到 LEN_SU 的值？
LEN_SU 的值是隐式给出的。从 BP1 您知道目录记录中的总字节数 (LEN_DR)。 LEN_SU 然后（隐含地）给出 33+LEN_FI+possible_padding(1) 之后目录记录中剩余的字节，从 BP33 获得长度 LEN_FI。
因此
LEN_SU = LEN_DR - (33+LEN_FI+possible_padding(1))
来自规范：
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.