Dynamic data on Screen -Display data Based On the First Field

Question

hello Helping Minds..
I have Two Fields In MODULEPOOL SCREEN.
1--- BUKRS
2--- BELNR ..... Both From BKPF table.
My requirement is When i select a CompanyCode ( BUKRS ) In d first field,all the DocumentNO ( BELNR ) related to that CompCOde Shd be displayed in the 2nd field,
For Ex-
If i choose BUKRS = 1000,
and Then clicked the Belnr field, All d DocumentNO for BUKRS 1000 will be populated in d Dropdown list..
        BUKRS = 0001,
then BELNR = 0100000000 ,
                      0100000001 in d dopdown. 
source code will be Appreciated...
Thnks in advance To D helping Minds

Answer 1

Process on value-request.
filed Bukrs module f4_bukrs.
field belnr module f4_belnr.
*module f4bukrs input.*_
if belnr is not initial.
select belnr bukrs from bkpf whre belnr = belnr.   
else.
select belnr bukrs from bkpf whre belnr = belnr.
endif.               ""   Hey both these Condition lead to same thing...den Whts d use??
Firstly Belnr will be always Intial. cos it will be populated after we select the BUKRS.  , Here am confused?
if t_bkpf is not initial.
  CALL FUNCTION 'F4IF_FIELD_VALUE_REQUEST'        "  No idea Abt dis FUncn module, pls explain
endif.
end module.
module f4_belnr input.
if bukrs is not initial.
select belnr bukrs from bkpf where bukrs = bukrs.
else.
select belnr bukrs from bkpf whre belnr = belnr.
endif.
if t_bkpf is not initial.          ""   What is t_bkpf and y its used
call fm to CALL FUNCTION 'F4IF_FIELD_VALUE_REQUEST'
endif.
end module.