I try to make a segment that has the base at B8000 and a size of 4000 bytes.Here is my descriptor but it dosen't work.Please tell me what is the mistake

sys_video: dw 0x0FA0
dw 0x8000
db 0x0B
db 0x92
db 0x40

: I try to make a segment that has the base at B8000 and a size of 4000 bytes.Here is my descriptor but it dosen't work.Please tell me what is the mistake
:
: sys_video: dw 0x0FA0
: dw 0x8000
: db 0x0B
: db 0x92
: db 0x40
:

//You leftout the highest descriptor byte
//Compare
: sys_video: dw 0x0FA0
: dw 0x8000
: db 0x0B
: db 0x92
: db 0x40
: db 0x00