If you use the programming options below together with the registers and accumulators listed, you must make sure you restore the contents of the registers or accumulators yourself, otherwise errors may occur.

Using the following higher-language constructions may cause the contents of the DB register and address register AR1 to be modified:

Fully-qualified DB access (for example, DB20.DBW10) as an actual parameter for a function (FC)

Function block (FB) and multiple-instance calls

Structure components of a formal parameter as an address within an FC or FB

Structure components of a formal parameter as an actual parameter for an FC or FB

With FB, FC, and multiple-instance calls, you should not use the RLO or accumulator 1 or 2 as additional (implicit) parameters.

The DI register and address register AR2 are used on the system side for the FB and multiple-instance calls and should not therefore be modified within FBs.

Address register AR1 is used by some of the loadable standard blocks.

The command "L P#parameter_name" loads the address offset of the specified parameter within an FB, relative to address register AR2. In order to determine the absolute offset in the instance data block in multiple-instance type FBs, you must also add the area-internal pointer (address only) of the AR2 register to this value

这是STEP在线帮助中的说明
对于第二条，我当初曾深受其苦，在FB中调用了系统自带的作为多重背景调用的FB后，当前DB号也被改变了，导致程序错误