CS101 Lecture 26: Low Level Programming. John Magee 30 July 2013 Some material copyright Jones and Bartlett. Overview/Questions

Transcription

1 CS101 Lecture 26: Low Level Programming John Magee 30 July 2013 Some material copyright Jones and Bartlett 1 Overview/Questions What did we do last time? How can we control the computer s circuits? How does the von Neumann Computer work? What is machine language? What is computer programming? 2 1

2 What did we do last time? Reviewed the full adder (the circuit which does binary number addition) Introduced some other circuits which are part of the CPU: the accumulator, the inverter Described the main memory as a location from which to read data into the adder, and to which data can be stored from the accumulator. Introduced the von Neumann architecture. 3 An Abstract View of CPU Circuits Unanswered Questions: How to specify whether to add or subtract. How to specify memory (RAM) addresses. 4 2

3 Stored Program Computer (Von Neumann Architecture) Figure 5.1 The von Neumann architecture 5 Control Processing Unit Central Processing Unit (CPU) Refers to the combination of the Arithmetic/Logic Unit and the Control Unit. 6 3

4 Arithmetic/Logic Unit The ALU performs basic arithmetic operations and logic operations. Examples: Arithmetic: addition, subtraction, multiplication Logical operations: AND, OR, NOT, XOR Most modern ALUs have a small amount of special storage units called registers The accumulator is the main register 7 Arithmetic/Logic Unit Recall how addition and subtraction happen (at the level of the CPU): Addition: take a value from memory send it to the adder add to the value in the accumulator Subtraction: take a value from memory, invert the bits, send to the adder, set carry-in to 1 add to the value in the accumulator 8 4

5 Arithmetic/Logic Unit The difference between addition and subtraction is slight; it can be controlled by sending a signal to the inverter and to the carry-in bit. This signal is like flipping a switch to tell the circuits to behave a certain way. But instead of flipping the switch, we could just use a binary input to an AND gate 9 Arithmetic/Logic Unit Given that: instructions are carried out by sending data through circuits circuits can be controlled by flipping switches. A binary code can be used to control which operation the CPU will do at a given time. 10 5

6 Control Unit The control unit coordinates the flow of operations and data in the computer. coordinates the ALU and memory Instruction register (IR) Contains the binary code indicating which instruction is being executed. Program counter (PC) Contains the address (in memory) of the next instruction to be executed. 11 Flow of Information Bus A set of wires that connect all major components of the computer. Figure 5.2 Data flow through a von Neumann architecture 12 6

7 The Computer Clock The computer has oscillator generates an endless rapid stream of off/on pulses (0,1,0,1 ). Each change from 0 to 1 is called a clock cycle. cycles per second is one measure of CPU speed measured in Hertz (MHz, GHz) cycles per second The clock cycle is used to coordinate the tasks of the various circuits. 13 The Fetch-Execute Cycle Figure 5.3 The Fetch-Execute Cycle 14 7

8 Fetch Execute Cycle The previous diagram shows how it takes 4 steps to execute each instruction. Is cycles per second a valid measure of CPU performance? 15 Stored Program Computer (Von Neumann Architecture) Data and instructions to manipulate the data are logically the same (binary code) and can be stored in the same place (RAM). The machine language of a particular computer is the set of binary coded instructions built into its hardware. 16 8

9 Machine Language Characteristics of machine language: Every processor type has its own set of specific machine instructions. The relationship between the processor type and the instructions it can carry out is completely integrated. Each machine-language instruction does only one very low-level task. 17 Machine Language Example A Virtual Computer is a hypothetical machine designed to contain the important features of a real computer that we want to illustrate. Our examples will use the Pep/7 virtual computer designed by Stanley Warford that has 32 machinelanguage instructions. Note: different than the Super Simple CPU

10 Instruction Format The machine language of a particular computer is the set of binary coded instructions built into its hardware. This machine language has a 3-byte instruction format. Figure 7.2 The Pep/7 instruction format 19 Instruction Format Operation Code Specifies which instruction is to be carried out. Register Specifier Specifies which register is to be used. Addressing-mode Specifier How to interpret the operand part of the instruction. 00 for immediate (value given in operand) 01 for direct (memory address given in operand) 20 10

11 Some Sample Instructions Figure 7.3 Subset of Pep/7 instructions 21 Sample Instructions What does this instruction mean? Load immediate value 7 into the accumulator 22 11

12 Sample Instructions What does this instruction mean? Load value from address 31 into the accumulator 23 Sample Instructions What does this instruction mean? Store value from accumulator into address

13 Sample Instructions What does this instruction mean? Add value from address 522 into the accumulator 25 Sample Instructions What does this instruction mean? Read a character of input into address

14 Sample Instructions What does this instruction mean? Send a character from address 10 to output 27 Computer Programming The CPU does some low-level tasks, and each one is specified as a machine-language instruction. How do we get the CPU to do stuff for us? Computer programming is the process of analyzing a problem, designing a solution, and expressing that solution as a series of computer instructions. Computer programs must be written in the language the computer understands

15 Programming Strategy Divide and Conquer! Break up a large problem into smaller units and solve each smaller problem. Applies the concept of abstraction. The divide-and-conquer approach can be applied over and over again until each subtask is manageable. 29 Machine Language Example Hello, world! program The classic first computer program is called Hello World It prints the words Hello, world to the screen. How to write it in Machine Language for the Pep/7? Specify explicit tasks in unambiguous language. Divide and conquer: Keep refining these tasks until we arrive at a concrete set of instructions which map to machine language instructions

16 Writing a Computer Program Write "Hello, World!" Is this concrete yet? Write "Hello, World!" Write "H" Write "e" Write "l" Write "l" Write "o" Is this concrete yet? 31 Writing a Computer Program Write "H" Write 0x48 Is this concrete yet? Write "e" Write 0x65... Write "o" Write 0x6F 32 16

17 Writing a Computer Program 33 Machine Language Machine language is the set of binary coded instructions built into the hardware of a particular computer, and used directly by ALU/Control Unit. why are the instructions binary encoded? how does this encoding relate to the hardware? 34 17

18 Review: Machine Language Operation Code Specifies which instruction is to be carried out. Register Specifier Specifies which register is to be used. Addressing-mode Specifier How to interpret the operand part of the instruction. 00 for immediate (value given in operand) 01 for direct (memory address given in operand) 35 What the Computer Can Do Modern computer processors can do the following types of basic tasks (CPU instructions): Add, subtract, multiply, divide, increment, decrement Logical AND, OR, XOR, NOT, and NEG operations Load data in from RAM, store data out to RAM Load data from/ send data to input/output Compare register contents for equal to Zero, Less than Zero, Negative Jump to another instruction (by address) Note: by data, we mean one word (e.g. 32 bits) 36 18

19 The JUMP Instruction Recall that the von Neumann computer stores data and instructions in the same RAM. The Program Counter holds the address of the next instruction to be executed. Usually, to find the next instruction, the PrC increments the stored address. The JUMP instruction changes the address stored in the Program Counter programmatically. Conditionally: jump if accumulator equals 0 Conditionally: jump if accumulator is less than 0 Unconditionally: just jump 37 The JUMP Instruction The JUMP instruction enables decision making and repetition inside the CPU. One could argue that JUMP is what makes a computer really programmable. Examples (with some abstraction): If a value is less than 0, then jump to instruction 7 If the quotient is 0, then end the program

20 Lab 09 Learning Objective To deepen your understanding of the central processor unit, the fetch-execute cycle, and the stored program concept. In addition, you will use pseudo code to express an algorithm, and translate the algorithm into machine-language instructions which the CPU can execute

21 Super Simple CPU Input: 1 word (16 bits) Output 1 word (16 bits) Registers: Program Counter Accumulator Temp Instruction Register RAM: 16 words (16 bits each) 41 SSCPU Machine Language Note: the Super Simple CPU has a different instruction format and opcode list than the Pep/7 CPU. Why? What are the implications of this? SSCPU Instruction Format: Each instruction is 16 bits First 4 bits specify the operation code (opcode) Last 12 bits specify either a RAM address, or an immediate value (a.k.a. a literal number) 42 21

22 SSCPU Machine Language Some example instructions: 1111: stop the program 0001: add operand to accumulator 0010: subtract operand from accumulator 0011: load memory cell into accumulator 0101: store accumulator into memory cell 0110: input value into accumulator 0111: output value from accumulator 43 Lab Exercise You will use a simple, simulated CPU to: observe the CPU s fetch-execute cycle in action become acquainted with machine language programming and instructions experiment with programming in a machine language You will write a pseudo code algorithm, and a machine language program to do addition of 2 numeric inputs

23 Pseudo code Pseudo code is a way of expressing algorithms that uses a mixture of English phrases and indention to make the steps in the solution explicit. There are no grammar rules in pseudocode. Pseudocode is not case sensitive. Analogy: pseudo code is like an outline for writing a paper -- it contains all of the main ideas, but is not tied to the structure of grammar, etc. 45 Algorithms Algorithm A set of unambiguous instructions for solving a problem or subproblem in a finite amount of time, and using a finite amount of data. For a given set of inputs, the algorithm will always produce the same outputs

24 Following an Algorithm Grandpa Ron s Chocolate Kahlua Mudslides Ingredient List: ½ cup vodka ½ cup Kahlua ½ cup Bailey s Irish Cream ¼ cup Hershey s chocolate syrup 1 blender full of ice Process: Fill blender with ice. Pours other ingredients onto ice. Blend until smooth consistency. Pour into glasses and enjoy. 47 Following an Algorithm Algorithm for preparing Chocolate Mudslides: If not all ingredients are present Go to store and buy missing ingredients Fill blender with ice Pour other ingredients into the blender While (not smooth) Turn on blender Wait 10 seconds Turn off blender Test consistency with wooden spoon Pour into glasses and serve 48 24

25 Partial Summary Even the modern computer is still a moronic number-crunching, data moving device. Algorithms describe the explicit steps the computer must follow to solve a problem. Pseudocode is a tool to help us express an algorithm, but it is not understood by the computer. Why would anyone want to write a program in machine language? 49 Assembly Language Assembly language A language that uses mnemonic codes to represent machine-language instructions Mnemonic code examples: LOADA means load value into accumulator ADDA means add value into accumulator STOREA means store value from accumulator 50 25

26 The Assembly Process Assembler A program that reads each of the instructions in mnemonic form and translates it into the machine-language equivalent. 51 Pep/7 Assembly Language Mode specifier: i = immediate, d = direct (memory) 52 26

27 Pep/7 Assembly Language An assembly language can have pseudo operations which do not map directly to the underlying machine language operations. 53 Example: Pseudo Code Program Reading and adding three numbers Set sum to 0 Read num1 Add num1 to sum Read num2 Add num2 to sum Read num3 Add num3 to sum Write sum 54 27

28 Example: Assembly Program 55 Partial Summary Assembly language: much easier to write/read than machine language! Why not just use assembly language instead of machine language? Who wrote the assembler program, and what language was it written in? Why not just write programs in pseudo code, and have a program translate that into machine language instructions? 56 28

29 Take-Away Points Von Neumann Architecture Arithmetic/Logic Unit Control Unit Machine Language Divide and Conquer strategy 57 Student To Dos Readings: Reed ch 7, pp Reed ch 6 Homework on course schedule. Wednesday: Meet in CS teaching lab