include 'win32a.inc'
  format PE GUI 4.0

section '.text' code readable executable

    mov                 ebx,crc32_table ; base address
    xor                 ecx,ecx ; index used to update table entries
begin:
    mov                 eax,ecx
    and                 edx,0 ; inner loop counter
inner_loop:
    cmp                 dl,8
    jz                  update_table
    test                al,1 ; check for odd number
    jz                  shr_and_loop_back
    shr                 eax,1
    xor                 eax,0EDB88320h ; reversed (lil endian)
    inc                 dl
    jmp                 inner_loop
shr_and_loop_back:
    shr                 eax,1
    inc                 dl
    jmp                 inner_loop
update_table:
    mov                 [ebx+ecx*4],eax
    inc                 ecx
    or                  ch,ch
    jz                  begin
finish: ; as of here, we have full table ... the rest of the code is clear i think
    xor                 edx,edx
    and                 eax,edx
    not                 edx
    mov                 ecx,[input_length]
    mov                 esi,input_data
calc_crc32_checksum:
    lodsb
    xor                 al,dl
    movzx               edi,al
    mov                 eax,[ebx+edi*4]
    shr                 edx,8
    xor                 edx,eax
    loop                calc_crc32_checksum
    not                 edx
    push                10h
    push                crc32_checksum
    push                edx
    call                [itoa]
    add                 esp,0Ch
    push                0
    push                eax
    push                0
    push                0
    call                [MessageBox]
    sub                 eax,eax
    ret
    int3

section '.data' data readable writeable

  crc32_table rd 400h
  input_length dd 4
  input_data db 'fasm',0
  crc32_checksum rb 9


section '.idata' import data readable

  library user32,'user32.dll',\
          ntdll,'ntdll.dll'

  import user32,\
         MessageBox,'MessageBoxA'

  import ntdll,\
         itoa,'_itoa'

section '.reloc' fixups data readable discardable