Question

(d) Give one reason why on the AVR the result of the multiply instruction is always placed in RO and R1, rather than having the destination specified by the instruction. [2 marks] (e) Some processors use one of the registers in the register file to hold the program counter (PC). The AVR doesn't do this but instead keeps the PC in a separate register. What are the advantages of each of these two methods for storing the program counter? I4 marks]

Answer 1

(d) Since multiplication of two numbers could result in product whose size is beyond the storage capacity of single register and hence it require 2 registers . Since one destination register is not sufficient to hold the result, hence AVR keeps result in R0 and R1 instead of specifying single register in instruction itself.

(e) The advantage of keeping same register for PC and other registers is that it gives user flexibility to read and write the content of program counter like other registers.

The advantage of keeping separate PC register is that validity of address can be checked during jump to particular address so that program should not jump to space not valid for user. Hence it provides better security .

Please comment for any clarification.