Your Assembler-Rrogramm will receive the Parm via Register-One.
The first HalfWord specifies the length of the Parm, passed to the programm. So the first, the programm has to do, after beeing entered, is check length and store the parm.
L R3,0(r1) ...load Parm
SR R2,R2 ...clear reg-2
LH R2,0(R3) ...load length
LTR R2,R2 ...chck length
BCTR R2,0 ...length - 1
EX R2,EX_MVC ...move Parm in Field