Guys could you explain to me this program START: MOV P1,#00H MOV A,#0FFH LOOP: MOV P1,A DEC A and what happening?

ORG 00H
P1 EQU 90H
SJMP START
START:
MOV P1,#00H
MOV A,#0FFH
LOOP:
MOV P1,A
DEC A
CALL DELAY
SJMP LOOP
DELAY:
MOV R5,#10
DLY1:
MOV R4,#180
DLY2:
MOV R3,#255
DLY3:
DJNZ R3,DLY3
DJNZ R4,DLY2
DJNZ R5,DLY1
RET
END