1. accepts data (input)
2. processes data arithmetically and logically
3. displays information (output) from the processing and/or
4. stores the results for future use."

The electronic circuits of a computer can recognize and directly execute a limited set of simple instructions. This course examines how this is actually done.

Architecture vs Organization (Implementation &Realization)

• Architecture : appearance of system to user / "virtual" model formed by user/ "attributes that have direct impact on the logical execution of a program" (Stallings p. 4)
• Organization - operational units and their interconnections that realize architectural specifications/ details transparent to user.
  • Implementation : internal structure - how it's really done!
  • Realization : how it's built out of the technology

How Does a Computer Execute Instructions?

1. The electronic circuits of a computer can recognize and directly execute a limited set of simple instructions (e.g. add, test, move). These primitive instructions form the machine language. The instructions are simple in order to reduce cost and complexity.
2. To make the computer easier to program, design a new set of instructions (L1) that is "executed" by the original set of instructions (L0). This is done by translation or interpretation.
3. This can be extended. Design an L2 language that is executed by the L1 language etc.

Machine Levels (top down)

1.	User/Application Level

2.	High-Level Languages

Examples: Fortran, Pascal, C++, Python, Scheme

3.	Assembler Language/Machine Code 

The architecture of a computer is described/defined

by its set of machine code instructions; hence the

phrase Instruction Set Architecture

4.	Control Level (software/hardware interface)

Microprogramming vs. Hardwired

5.	Digital/Hardware Level

Examples (bottom up)

• Digital Logic : AND, OR, NOT Gates. Gates are combined into combinatorial and sequential circuits like Adders, Multiplexors, Decoders, Flip-Flops etc. In turn these circuits are combined into functional units like ALU's, Registers and Memory.

  

• Control Level: Microprogramming : Hardware units like ALU's, registers, memory etc are linked together into a data-path. The flow of bits around the data-path is controlled by "gates" which allow the bits to flow (on) or not flow (off) through the data-path. The binary instructions (1 = on; 0 = off) that control the flow are micro-instructions.

  

• Assembler Language/Machine Code Level: Computers are built out of bi-stable hardware devices - devices that maintain one of two states - high or low which we label 1 or 0. So at the lowest level a computer can only decode and work with patterns of bits. Ultimately everything is binary (which when converted to octal or hexadecimal code is easier for humans to work with).

             0x15 0x00    (Java Byte Code)
             0x15 0x01
             0x60
             0x36 0x02

• An operating system manages a computer's resources and provides a user interface. Thus the operating system of a computer is intimately connect with the instruction set architecture. Certain machine code instructions are reserved for and can only be executed by the operating system (or other privileged users). However, there are ISA level instructions (called interrupts or traps). that call the operating system to provide certain services like I/O.

• An Intel 80x86 Example (prints the character 'A')
  

          mov ah, 02    ; Set up MS-DOS Display Character Routine
          mov dl, 'A'   ;   to display 'A'
          int 21h       ; Invoke Interrupt 21h
   

• Assembly language is a set of mnemonics for machine code which makes programming more user-friendly.

              iload a    (Assembler for Java Byte Code)
              iload b
              iadd
              istore c

              mov ax, a   (Intel 80x86 assembler)
              add ax, b
              mov c, ax
              nop
              int 03

• High Level Language (C++) - User Friendly! and Platform Independent!

  
           c = a + b;

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