Q. What is Base Register Addressing ?

An addressing technique in which content of an instruction specifies base register is added to address field or displacement field of the instruction. (Refer to Figure below).The displacement field is taken to be a positive number. For illustration if a displacement field is of 8 bits then a memory region of 256 words beginning at address pointed to by base register can be addressed by this mode. This is same as indexed addressing scheme besides that role of Address field and Register is reversed. In the indexing Address field of instruction is fixed and index register value is changed while in Base Register addressing Base Register is common and Address field of the instruction in different instructions is changed. In this particular case:

EA = A+ (B) 

D = (EA)

(B) Refers to the contents of a base register B.

Contents of the base register can be changed in the privileged mode only. No user is permitted to alter the contents of the base register. Base-addressing scheme offers protection of users from one another.

This addressing technique is generallyused to relocate the programs in memory particularly in multiprogramming systems in which only value of base register needs updating to reflect beginning of a new memory segment. 

Like index register a base register can be a general-purpose register or a special register reserved for base addressing.