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What are those bits in memory ???

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• Review: information encoding
  • We have seen that there are many different kinds of information stored in the computer memory:

    integer (= whole number) values: 00000000000000000000000000000101 represents 510 01000000101000000000000000000000 represents 108422758410 (= 230 + 223 + 221) Floating point values: 01000000101000000000000000000000 represents 510 See: click here Character: 00110101 represents the character 5

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• What does these bits represent ???
  • $64,000 question:

    Suppose you see the following bit pattern stored in memory: 01000000101000000000000000000000 Question: What value does this bit pattern encode ??? Does 01000000101000000000000000000000 represent 108422758410 ??? Or does 01000000101000000000000000000000 represent 510 ??? Or something else ???

    Answer:

    You cannot tell what value a bit pattern will represent by looking only at the bit pattern To decode the information that is represented by a bit pattern, you need this information: The encoding method used by the bit pattern (The encoding method is uniquely tied to the data type --- so knowing the data type will tell you the encoding method used) Example: 01000000101000000000000000000000 as an integer value represents: 108422758410 01000000101000000000000000000000 as a floating point value represents: 510 01000000101000000000000000000000 as character (ASCII) code represents: 01000000 10100000 00000000 00000000 '@' unprint NUL NUL And so on !!! (If there are more encoding methods)

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• Illustration to help you overcome this weirdness
  • Weirdness:

    The same bit pattern can represent different value !!! Example: 01000000101000000000000000000000 as an integer value represents: 108422758410 01000000101000000000000000000000 as a floating point value represents: 510

    Let me use the English language to help you overcome this weirdness....

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  • Question:

    Make a correct English sentence that starts with: You is ......

    You (as a human) is so conditioned to use "you" in the context of a personal pronounce (the "me"-generation !), that you don't even think about the fact that:

    The word "you" can also be used in another context

    Example:

    You is an English word

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  • So:

    The same word "you" can have different meaning in different context !!! (This example is completely different from homonyms --- like "bow" (bow down and bow of a ship) !!! Homonyms are different words with the same spelling)

    Just like:

    01000000101000000000000000000000 as an integer value represents: 108422758410 01000000101000000000000000000000 as a floating point value represents: 510

    I'll tell you a joke about Millie and her teacher with the word "I" in class...

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• What information is stored in memory ???
  • Recall:

    Each memory cell stores 8 (eight) bits (There are no boundary or markers in memory that tells you when to stop !!!)

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  • $64,000 question:

    Suppose you see the following memory content at address 2000: Question: What possible values are stored in memory location 2000 ??? The answer will surprise you !!!

    Answer: the possible values stored at memory address 2000 are

    The float value 510: The int value 108422758410: The short value 1657610: The byte value 6410: The character value '@': And so on !!!! (if you have more codes !!!)

    Explanation:

    If the type of the value stored at memory address is float, then: You must take 4 bytes (because a float representation is 32 bits) and use IEEE 754 code to interpret the value If the type of the value stored at memory address is int, then: You must take 4 bytes (because a int representation is 32 bits) and use 2's complement code to interpret the value If the type of the value stored at memory address is short, then: You must take 2 bytes (because a short representation is 16 bits) and use 2's complement code to interpret the value If the type of the value stored at memory address is byte, then: You must take 1 byte (because a byte representation is 8 bits) and use 2's complement code to interpret the value If the type of the value stored at memory address is char, then: You must take 1 byte (because a char representation is 8 bits) and use ASCII code to interpret the value And so on !!! (for different data types)

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  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs255001/demo/java/ShowValueDepOnType2.c

    How to run the program:

    To compile:   gcc ShowValueDepOnType2.c To run:          a.out

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• Summary: to know what value is stored in some variable in memory, you need:

  The memory address where the variable is stored (allocated) in memory Also: the data type of the variable !!!