What do you understand by DB assembler directive?  

DB (Define Byte): The DB directive explains by a byte-type variable (that is a variable that occupies one byte of memory space). Within a given directive statement, there may be particular initial value or multiple values of the defined variable. When there is one initial value, so one byte of memory space is reserved. When there are multiple values, so one byte of memory space is reserved for every value. The general format of this:

Name of Variable       DB      Initial value or values.