1. CISC & RISC Architecture
Suvendu Kumar Dash
M.Tech in ECE
VTP1492

2.  History Of CISC & RISC
 Need Of CISC
 CISC
 CISC Characteristics
 CISC Architecture
 The Search for RISC
 RISC Characteristics
 Bus Architecture
 Pipeline Architecture
 Compiler Structure
 Commercial Application
 Reference
Overview

3. History Of CISC & RISC
 1950s IBM instituted a research program.
 1964 Release of System/360.
 Mid-1970s improved measurement tools demonstrated on
CISC.
 1975 801 project initiated at IBM’s Watson Research Center.
 1979 32-bit RISC microprocessor (801) developed led by
Joel Birnbaum.
 1984 MIPS (Microprocessor without Interlocked Pipeline Stages)
developed at Stanford, as well as projects done at Berkeley.
 1988 RISC processors had taken over high-end of the
workstation market.

4. Need Of CISC
 In the past, it was believed that hardware design was easier than
compiler design
 Most programs were written in assembly language
 Hardware concerns of the past:
 Limited and slower memory
 Few registers

5. The Solution
 As limited registers so …
Instructions have do more work, thereby minimizing the number of
instructions called in a program.
 Allow for variations of each instruction
 Usually variations in memory access.

6.  CISC, which stands for Complex Instruction Set Computer.
 Each instruction executes multiple low level operations.
 Ex. A single instruction can load from memory, perform an
arithmetic operation, and store the result in memory.
 Smaller program size.
CISC

7. CISC Characteristics
 A large number of instructions.
 Some instructions for special tasks used infrequently.
 A large variety of addressing modes (5 to 20).
 Variable length instruction formats.
Disadvantages :
However, it soon became apparent that a complex instruction set has
a number of disadvantages:
 These include a complex instruction decoding scheme, an
increased size of the control unit, and increased logic delays.

8. CISC Architecture
 The essential goal of a CISC architecture is to attempt to provide a
single machine instruction for each high level language instruction
 Ex:
 IBM/370 computers
 Intel Pentium processors

9. The Search for RISC
 Compilers became more prevalent.
 The majority of CISC instructions were rarely used.
 Some complex instructions were slower than a group of simple
instructions performing an equivalent task:
 Too many instructions for designers to optimize each one.
 Smaller instructions allowed for constants to be stored in the unused bits
of the instruction
 This would mean less memory calls to registers or memory.

10. RISC
 RISC Stands for Reduced Instruction Set Computer.
 It is a microprocessor that is designed to perform a smaller
number of types of computer instruction so that it can operate
at a higher speed.

11. RISC Characteristics
 Relatively few instructions
 128 or less
 Relatively few addressing modes.
 Memory access is limited to LOAD and STORE instructions.
 All operations done within the registers of the CPU.
 This architectural feature simplifies the instruction set and encourages
the optimization of register manipulation.
 An essential RISC philosophy is to keep the most frequently accessed
operands in registers and minimize register-memory operations.

12. RISC Characteristics Cont..
 Fixed Length, easily decoded instruction format
 Typically 4 bytes in length
 Single cycle instruction execution
 Done by overlapping the fetch, decode and execute phases of
two or three instructions known as Pipelining!!
 Large number of registers in the processor unit.
 Use of overlapped Register Windows.

13. BUS Architecture
 Bus Interconnection of Processor units to memory and IO
subsystem

14. BUS Architecture Cont..
Memory Bus:
 Memory bus (also called system bus since it interconnects the
subsystems)
 Interconnects the processor with the memory systems and also
connects the I/O bus.
 Three sets of signals –address bus, data bus and control bus

15. BUS Architecture Cont..
System Bus :
 A system’s bus characteristics --- according to the needs of the
processor, speed, and word length for instructions and data.
 Processor internal bus(es) characteristics differ from the system
external bus(es).

16. BUS Architecture Cont..
Buses to interconnect the processor Functional units to memory and
IO subsystem

17. BUS Architecture Cont..
Address Bus
 Processor issues the address of the instruction byte or
word to the memory system through the address bus
 Processor execution unit, when required, issues the
address of the data (byte or word) to the memory
system through the address bus.

18. Data Bus
BUS Architecture Cont..
 When the Processor issues the address of the instruction,
it gets back the instruction through the data bus When
it issues the address of the data, it loads the data through
the data bus.
 When it issues the address of the data, it stores the data
in the memory through the data bus.

19. BUS Architecture Cont..
Control Bus
 Issues signals to control the timing of various actions
during interconnection.
 Bus signals synchronize the subsystems

20. Pipeline Architecture
 A technique used in advanced microprocessors where the
microprocessor begins executing a second instruction before the first
has been completed.
 A Pipeline is a series of stages, where some work is done at each stage.
The work is not finished until it has passed through all stages.
 With pipelining, the computer architecture allows the next instructions
to be fetched while the processor is performing arithmetic operations,
holding them in a buffer close to the processor until each instruction
operation can performed.

21. Pipeline Architecture
 The pipeline is divided
into segments and
each segment can
execute it operation
concurrently with the
other segments.
 Once a segment
completes an
operations, it passes
the result to the next
segment in the
pipeline and fetches
the next operations
from the preceding
segment.
Instruction 1 Instruction 2
X X
Instruction 4 Instruction 3
X X
Four sample instructions, executed linearly

22. Pipeline Architecture
 CISC instructions do not fit pipelined architectures very well.
 For pipelining to work effectively, each instruction needs to have
similarities to other instructions, at least in terms of relative instruction
complexity.

23.  Instruction Pipelining
 Similar to the use of an assembly line in manufacturing plant.
 New inputs are accepted at one end before previously accepted
inputs appear as outputs at the other end.
 Pipeline requires instruction to be divided into more stages.
 So, that at every clock cycle, new instruction can be inserted for
processing
Pipeline Architecture

24. Pipeline Architecture

25. Pipeline Architecture Cont..
Various instruction phases:
 Fetch Instruction(FI): fetch the next instruction
 Decode Instruction(DI): determine the opcode and operand
 Calculate Operands(CO):calculate the effective address of
source operands.
 Fetch Operands(FO):fetch each operand from memory.
 Execute Instructions(EI): perform the indicated operation and
store the result.
 Write result or Operand(WO): store the result into memory.

26. Pipeline Architecture Cont..
RISC Pipeline.
 Different from normal one.
 Based on type of instruction.
 According to instruction type, decide the number of phases in
pipeline.
 Number of stages in pipeline are not fixed.

27. Pipeline Architecture Cont..
RISC Pipeline.
 Most instructions are register to register
 Two phases of execution
 I: Instruction fetch
 E: Execute
 ALU operation with register input and output
 For load and store
 Three phase execution
 I: Instruction fetch
 E: Execute
 Calculate memory address
 D: Memory
 Register to memory or memory to register operation

28. Pipeline Architecture Cont..
Effects of Pipelining(1)

29. Pipeline Architecture Cont..
Effects of Pipelining(2)

30. Pipeline Architecture Cont..
Increase the Speedup Factor:
 I and E stages of two different instructions are performed
simultaneously.
 Which yields up to twice the execution rate of serial scheme.
 Two problem prevents to achieve this the maximum speedup:
 Single port memory is used so only one memory access is possible
per stage.
 Branch instruction interrupts the sequential flow.

31. Pipeline Architecture Cont..
Four stage pipeline:
 E stage usually involves an ALU operation, it may be longer. So
we can divide into two stages:
 E1: Register file read.
 E2: ALU operation and register write.

32. Pipeline Architecture Cont..
Effects of Pipelining(3)

33. Pipeline Architecture Cont..
Optimization of RISC Pipelining:
 Delayed branch:
 Does not take effect until after execution of following instruction.
 This following instruction is the delay slot.
 Increased performance can be achieved by reordering the
instructions!!!
This can be applicable for unconditional branches.

34. Pipeline Architecture Cont..
Normal and Delayed Branch:
Address Normal
Branch
Delayed
Branch
Optimized
Delayed
Branch
100 LOAD X, rA LOAD X, rA LOAD X, rA
101 ADD 1, rA ADD 1, rA JUMP 105
102 JUMP 105 JUMP 106 ADD 1, rA
103 ADD rA, rB NOOP ADD rA, rB
104 SUB rC, rB ADD rA, rB SUB rC, rB
105 STORE rA, Z SUB rC, rB STORE rA, Z
106 STORE rA, Z

35. Compiler Structure
 A compiler is a Computer Program (or set of programs) that
transforms Source Code written in a Programming Language (the
source language) into another computer language (the target language,
often having a binary form known as Object Code).
 The most common reason for wanting to transform source code is to
create an Executable program.

36. Compiler Structure

37. Compiler Structure Cont..
 In a compiler,
 linear analysis
 is called Lexical Analysis or Scanning and is performed by
the Lexical Analyzer or Lexer,
 hierarchical analysis
 is called Syntax Analysis or Parsing and is performed by
the Syntax Analyzer or Parser.
 During the analysis, the compiler manages a Symbol Table by
 recording the identifiers of the source program
 collecting information (called Attributes) about them: storage
allocation, type, scope, and (for functions) signature.

38. Compiler Structure Cont..
 When the identifier x is found by the lexical analyzer
 generates the token id
 enters the lexeme x in the symbol-table (if it is not already there)
 associates to the generated token a pointer to the symbol-table
entry x. This pointer is called the Lexical Value of the token.
 During the analysis or synthesis, the compiler may Detect Errors and
report on them.
 However, after detecting an error, the compilation should proceed
allowing further errors to be detected.
 The syntax and semantic phases usually handle a large fraction of the
errors detectable by the compiler.

39. Commercial Applications
RISC:
 First commercially available RISC processor was MIPS
 R4000
 Supports thirty-two 64-bit registers
 128Kb of high speed cache
 SPARC
 Based on Berkeley RISC model
 PowerPC.
 Motorola.
 Nintendo Game Boy Advance (ARM7)
 Nintendo DS (ARM7, ARM9)

40. Commercial Applications Cont..
CISC:
 CISC instruction set architectures are
 System/360 through z/Architecture,
 PDP-11,
 VAX,
 Motorola 68k, and Intel(R) 80x86.

41. Reference
 Computer Organization And Architecture,8th Edition , William Stallings
 http://nptel.ac.in/courses/Webcourse-contents/IIT-
%20Guwahati/comp_org_arc/web/
 http://www.borrett.id.au/computing/art-1991-06-02.htm

42. Thank You