outline

Stored Program

• The most fundamental characteristic of the Von Neumann architecture
• A sequence of machine language instructions stored as binary values in memory
• Task of the Control Unit to
  -   fetch from memory the next instruction to be executed
  -   decode it
  -   execute it

Machine Language Instructions

• Instructions that can be decoded and executed by the control unit of a computer
• Operation code (op code) is a unique unsigned-integer code assigned to each machine language operation
• Address field(s) - the memory addresses of the values on which this operation will work

Format

• Typical Machine Language Format
• 

Instruction Format

• Assume the op code for ADD is a decimal 9
• Cells X and Y correspond to addresses 99 and 100 (decimal)

• op code	address 1	address 2
  8 (bits)	16 (bits)	16 (bits)

• Putting it all together

• op code	address 1	address 2
  00001001	0000000001100011	0000000001100100

Instruction Set

• The set of all operations that can be executed by a processor
• RISC - Reduced Instruction Set Computer - newer processors make instruction sets as small and as simple as possible

Instruction Groups

• 4 basic groups
  -   data transfer
  -   arithmetic
  -   compare
  -   branch

Data transfer

• These operations that move information between or within the different components of the computer
• All data transfer instructions follow the nondestructive fetch/destructive store principle
• Contents of the source cell are never destroyed, only copied.
• Contents of the destination cell are overwritten and its previous contents are lost

Example Data transfer

• Operation	Meaning
  LOAD X	Load register R with the contents of memory cell X
  STORE X	Store the contents of register R into memory cell X
  MOVE X, Y	Copy the contents of memory cell X into memory cell Y

Arithmetic

• These are operations that caue the arithmetic/logic unit to perform computation.
• Typically indluce arithmetic operations like + , - , * , / as well as logic operations such as AND, OR and NOT

Arithmetic Example

• Operation	Meaning
  ADD X, Y, Z	Add the contents of memory cell X to the contents of memory cell Y and put the result in memory cell Z (Three-address ihnstruction)
  ADD X, Y	Add the contents of memory cell X to the contents of memory cell Y. Put the result into memory cell Y (two-address instruction)
  ADD X	Add the contents of memory cell X to the contents of register R. Put the result into register R (one-address instruction)

Compare

• These operations compare two values and set an indicator on the basis of the results of the compare
• Most Von Neumann machines have a special set of bits inside the processor called condition codes and these bits are set by the compare operations

Compare Example

• Operation	Meaning
  COMPARE X, Y	Compare the contents of memory cell X to the contents of memory cell Y and set the condition codes accordingly

• Branch

  • The normal mode of operation of a Von Neumann machine is sequential
  • The branch instructions alter the normal sequential flow of control - based typically on the current settings of the condition codes
  • Branch is almost always proceded by either a compare instruction or some other instruction that sets the condition codes

  Branch Example

  	Condition	How the condition codes are set
  CON (X) > CON (Y)	GT = 1	EQ = 0	LT = 0
  CON (X) = CON (Y)	GT = 0	EQ = 1	LT = 0
  CON (X) < CON (Y)	GT = 0	EQ = 0	LT = 1

  Operation	Meaning
  JUMP X	Take the next instruction unconditionally from memory cell X
  JUMPGT X	If the GT indicator is a 1, take the next instruction from memory cell X. Otherwise, take the next instruction from the next sequential location
  HALT	Stop program execution. Don't go on to the next instruction

Fetch/Execute

• The task of the control unit is to fetch and execute instructions similar to what was shown in the previous slides
• The control unit relies on two special registers to accomplish this:
  -   Program Counter (PC)
  -   Instruction Register (IR)
  -   and on the Instruction Decoder Circuit

Sequence

• The PC holds the address of the next instruction to be executed
• The IR holds a copy of the instruction fetched from memory (holding both the op code and the addresses)
• To determine what the instuction is, the op code portion must be decoded using the instruction decoder

Assembly Language

• Developed by individuals programming on the first-generation computers - designed for people as well as computers
• Today, assembly language viewed as low-level programming languages - which means they are closely related to the machine language

High-level languages

• Because they are more user-oriented, they are not machine-specific, the use both natural language and mathemetical notation in design
  -   BASIC
  -   Pascal
  -   C++
  -   Java

The Continuum of Programming Languages

Source Program

• User written program
• Translated into a machine language program - object program - carried out by the assembler (compiler)

Advantages of assembler

• Use of symbolic operation codes rather than numeric ones
• Use of symbolic memory addresses rather than numeric ones
• Psuedo-operations that provide useful user-oriented services such as data generation

Translation/Load/Execute

Format

• label:

  op code mnemonic

  address field

  --comment

Label

• Attached to any instruction or piece of data in the program
• Becomes a permanent identification for this instruction or data, regardless of where it appears in the program or where it may be moved in memory
• Name followed by a colon placed at the beginning of an instruction
  -   BEGIN:     LOAD X

Op code mnemonic

• Allows the programmer to refer to op codes using the symbolic name rather than by a number
  -   LOAD, ADD, STORE

Address field

• Lets the programmer use symbolic addresses in addition to numeric addresses
  -   JUMP 17

Comment

• Helpful explanation added to the instuction by a programmer and intended for someone reading the program
• Ignored during translation and execution

Instruction Set