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2.3: Memory Maps

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  • Every byte of memory in a computer system has an address associated with it. This is a requirement. Without an address, the processor has no way of identifying a specific location in memory. Generally, memory addressing starts at 0 and works its way up, although some addresses may be special or “reserved” in some systems. That is, a specific address might not refer to normal memory, but instead might refer to a certain input/output port for external communication. Very often it is useful to draw a memory map. This is nothing more than a huge array of memory slots. Some people draw them with the lowest (starting) address at the top and other people draw them with the lowest address at the bottom.

    Here’s an example with just six bytes of memory:

    Simple memory map.
    Figure \(\PageIndex{1}\): Simple memory map.

    Each address or slot represents a place we can store one byte. If we had to remember specific addresses we would be doing a lot of work. Instead, the C compiler will keep track of this for us. For example, if we declare a char named X, it might be at address 2. If we need to print that value, we don’t have to say “fetch the value at address 2”. Instead we say; “fetch the value of X” and the compiler generates code to make this work out to the proper address (2). This abstraction eases our mental burden considerably. As many variables require more than one byte, we may need to combine addresses to store a single value. For example, if we chose a short int, that needs two bytes. Suppose this variable starts at address 4. It will also require the use of address 5. When we access this variable the compiler automatically generates the code to utilize both addresses because it “knows” we’re using a short int. Our little six byte memory map could hold 6 char, 3 short int, 1 long int with 1 short int, 1 long int with 2 char, or some other similar combination. It cannot hold a double as that requires 8 bytes. Similarly, it could not hold an array of 4 or more short int (see Chapter Three for details on numeric data types).

    Arrays are of special interest as they must be contiguous in memory. For example, suppose a system has 1000 bytes of memory and a 200 element char array was declared. If this array starts at address 500 then all of the slots from 500 through 699 are allocated for the array. It cannot be created in “scattered” fashion with a few bytes here and a few bytes there. This requirement is due to the manner in which arrays are indexed (accessed), as we shall see later.