1. Processor Organization and Architecture
Module III
Processor Organization and Architecture

2. Nano-programming
In this method, microinstructions does not generate control signals directly but uses a second control memory called Nano Control Memory (NCM)
There are two levels of control memory:
higher level Micro Control Memory &
lower level NCM that stores nano-instructions

4. Nano-programming
Suppose there are n = 2048 micro-ops each of 41 bits wide,
The memory required for single control ROM is 2048 x 41 =83,968 bits

5. Nano-programming Suppose there are 100 unique control words in ROM.
These 100 words can be uniquely saved in nano control ROM saving space as shown:

6. Nano-programming Advantage: Disadvantage:
Reduces total size of required control memory
Greater design flexibility
Disadvantage:
Less speed due to two level memory

7. Classification based on instruction sets
Complex Instruction Set Computers
Reduced Instruction Set Computers

8. Complex Instruction Set Computers
CISC processor mostly uses a unified cache for both data and instructions and shares the same path
They mostly use microprogrammed CU and hence control memory is required and it slows down the execution

9. CISC Architecture

10. CISC Characteristics
It tries to simplify the compilation and improve the overall computer performance.
CISC architecture attempt to provide a single machine instruction for every statement in a high-level language.
It incorporates variable-length instruction formats.
Packing variable instruction formats in memory requires special decoding circuits.

11. CISC Characteristics Instructions manipulate operands in memory
Uses a variety of addressing modes—from 5 to 20
Uses a large number of instructions—from 100 to 250
Includes specialized instructions which are used infrequently
Examples : Motorola family, Intel x86 CPUs-Pentium

12. Reduced Instruction Set Computers
It uses separate instruction and data caches and their access paths are different
It uses hardwired control unit and hence faster execution.

13. RISC Architecture

14. RISC Characteristics
Uses a small set of instructions with register-to-register operations and simple load and store operations for memory.
Uses only a few addressing modes
Follows a simple instruction format
instruction length is fixed and aligned on word boundaries.
Instructions are easy to decode.
Hardwired control makes faster execution

15. RISC Characteristics Execute one instruction per clock cycle
Includes pipelining
Load or store may require two clock cycles.
Uses a large number of registers
overlapped register windows speed-up procedure call and return.
Thus register-to-memory operations is minimized
Examples : SPARC, Alpha,...

16. RISC vs CISC RISC CISC Contains simple instructions of one clock cycle
Very few instructions refer to memory
Hardwired Control Unit
Fixed Format Instructions
Contains complex instructions taking more than one clock cycle
Most of the instructions may refer memory
Micro-programmed Control Unit
Variable Format Instructions

17. RISC vs CISC RISC CISC Few Instructions Few Addressing Modes
Many Registers
Complexity is in compiler
Faster clock rates
Reduces the cycles per instruction at the cost of the number of instructions per program.
Many Instructions
Many Addressing Modes
Few Registers
Complexity is in microprogram
Slower clock rates
Minimize the number of instructions per program sacrificing the number of cycles per instruction.