Please Explain me the code

I do have experience in Triconex TMR PLC, ABB AC-450 controllers and many others apart from Siemens systems and i am working on them for the first time, i know some basics of siemens product but i am not proficient with them.
I have to code in STL as it is the requirement,thanks for the info about OB1, could you expain me how FB and FC works i mean the concept for the same and also i would like to know when to use "interface" with IN,OUT, IN-OUT,TEMP,STAT.

i am sending you one small code could you expain me step by step working

L #START_ADDR
T #SRCADDR
L 0
ADD: L W#16#1002
T LW 0
L #NO_BYTES
T LW 2
L #SRC_DB
T LW 4
L #SRCADDR
SLD 3
OD DW#16#84000000
T LD 6
L W#16#1002
T LW 10
L #NO_BYTES
T LW 12
L #DEST_DB
T LW 14
L #DEST_START_ADDR
SLD 3
OD DW#16#84000000
T LD 16
CALL "BLKMOV"
SRCBLK :=#SANY
RET_VAL:=#RET_VAL
DSTBLK :=#DANY

CALL FC 2
NM_BREAKER_1:="DEVICE".NM_breaker

CALL "BLKMOV"
SRCBLK :=#DANY
RET_VAL:=#RET_VAL
DSTBLK :=#SANY

L #SRCADDR
L #NO_BYTES
+I
T #SRCADDR
L #DEV_CNT
L 1
+I
T #DEV_CNT
L #NODEV
>=I
JC A001
JU ADD
A001: BLD 130


Thanks in Advance

please try to explain me clearly i mean what each step is suppose to do

Have you tried placing your cursor over each instruction and pressing F1 ?

yeah i did, but it was not that helpful, it will be better if someone can explain me how the things are going on each register level pls