- Recall in the
previous webpage
, we learned about the
.data directive:
.data // Tell the assmbler to put the lines that follows .data in the data segment // I.e.: treat these lines as variable definitionsThe data section of the computer program contains the permanent program constants and variables:
- What happens when you define
a (class) variable in
Java, such as
the variable age:
public class JavaProg { public static int age; ... // other stuff omitted because it's not relevant to the discussion }In order to store an integer value into the program variable age, we need to:
-
Reserve
4 bytes of
memory
(We need 4 bytes because an int typed variable in Java can store values between -2147483648 (-231) and 2147483647 (231-1) -- should be knowledge from CS170 !!!!
In order to hold this range of values, we need 32 bits which is equal to 4 bytes.)
In addition to reserving the memory cells to store the value, we need to do one more thing:
- We must
remember the
location (= address)
of these bytes
that are used to
store the value of the
variable called
age !!!
(Because if you do not know where these memory bytes are located, you won't be able to find time again !!!)
So when you define a variable in Java, such as:
public class JavaProg { public static int age; ... // other stuff omitted because it's not relevant to the discussion }The Java compiler will do 2 things:
- Reserve
the required amount of (contiguous) memory cells
to
store the value of
the variable
somewhere in memory
(We don't care where the Java compiler has reserved the memory cells; anywhere in the memory will work !)
- Associate the
address of the
first memory cell with the
identifier age
(Because the reserved memory cells are contiguous, if we know the address of the first memory cell, we can find the rest (the next 3 !!) of the reserved memory cells !!!)
-
Reserve
4 bytes of
memory