mov ax,0FFFFh                 ;see note 1
mov word ptr [ds:tempWord],ax ;store hexadecimal value FFFF into tempWord

mov bl, 80h
mov byte ptr [ds:tempByte],bl ;the register size needs to match the data type

;Note 1:
;UASM doesn't like leading hex digits A-F so a 0 is placed in front.
;It doesn't change the storage type of the operand. (i.e. this is still a 16 bit value even though there are 5 digits)