In computer processors, the data path often consists of the following functional

Question

In computer processors, the data path often consists of the following functional blocks. The instruction register stores the current instruction to be executed. The program counter (PC) stores the address of the next instruction to be fetched. The memory address register (MAR) is a register that either stores the memory address from which data will be fetched to the CPU or the address to which data will be sent and stored. The memory data register (MDR) is a register of a computer's control unit that contains the data to be stored in the computer storage (e. g. RAM), or the data after a fetch from the computer storage.

What are the advantages and disadvantage of fixed size instructions and varying size instructions? How does the datapath hardware deal with both?

Explanation / Answer

Fixed-length instructions
· Advantage: fixed-length instruction is fast and results in better performancewhen instruction levelpipelining is
   used.
                    Simpler for control unit to fetch and decode
· Diadvantage: wastes space

Variable-length instructions
· Advantage: Variable-length instruction saves storage space as it uses primary and secondary storage more
   efficiently.
· Disadvantage: more complex to decode

Datapath Consists of hardware elements that deal with and transform data signals
With Fixed Size , it follows the simple alignment pattern for instruction placement in memory . Due to this control instructions( Jump ,branch) speed up when there target instructions do not cross boundaries

If performance is most important, use fixed length instructions:

(1)Simple to decode, so use hardware
(2)Wastes code space because of simple operations
(3)Works well with pipelining

If code size is most important,use variable length instructions:

(1)Difficult control design to compute next address.
(2)complex operations, so use microprogramming.
(3)Slow due to several memory accesses.

With Varying size instructions , parallel processing has been exploited for execution complex instructions, where multiple operations are performed in parallel.
As the size is not fixed , Number of instructions are reduced by having multiple operations within a single instruction

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