Multiboot 2 uses a different structure than the original multiboot (namely, it uses tag structures).
Here's an example header from one of my older projects:
# multiboot 2 header (see http://download-mirror.savannah.gnu.org/releases/grub/phcoder/multiboot.pdf)
.balign 8
mbhdr:
.long 0xe85250d6 # magic
.long 0 # architecture (i386, 32-bit)
.long .LhdrEnd-mbhdr # header length
.long -(.LhdrEnd-mbhdr+0xe85250d6) # checksum
# tags
# module align
.word 6 # type
.word 0 # flags
.long 8 # size in bytes (spec says 12?)
.balign 8
# loader entry
.word 3
.word 0
.long 12
.long entry
.balign 8
# console flags
.word 4
.word 0
.long 12
.long 0x03 # EGA text support, require console
.balign 8
# info request
.word 1
.word 0
.long 4*6+8
.long 5 # BIOS boot device
.long 1 # command line
.long 3 # modules
.long 9 # ELF symbols
.long 6 # memory map
.long 10 # APM table
.balign 8
# address info
.word 2 # type
.word 0 # flags
.long 24 # size
.long mbhdr # header load addr
.long 0x100000 # load addr
.long 0 # load end addr (entire file)
.long 0 # BSS end addr (no BSS)
.balign 8
# terminating tag
.word 0
.word 0
.long 8
.LhdrEnd: