How to make SFC83 source block input variable? unlink DB

Good day

I am currently having problems on my work memory space of my 315-2 dp. I used SFC 83 and 84 to solve this issue but still im lacking some kilobytes more. I want to ask if i can make the Source and Destination block on the SFC83 'variable'?

I cannot make this scenario work:

OPN DB [#Unlinked_DBs]

CALL SFC 83
REQ :=M120.1
SRCBLK :=DBX0.0 BYTE 2298 <<<< part of the unlinked data block
RET_VAL:=#Retval
BUSY :=M120.7
DSTBLK :=P#DB2.DBX 0.0 BYTE 2298

-----------------------------

but this one actually works:

CALL SFC 83
REQ :=M120.1
SRCBLK :=DB211.DBX0.0 BYTE 2298 <<<< unlinked data block
RET_VAL:=#Retval
BUSY :=M120.7
DSTBLK :=P#DB2.DBX 0.0 BYTE 2298

if i can just make the source block variable then it will save some work memory. Please help me.

Thanks in advance.

Daniel