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Memory Layout

Picture of Memory Organization

Load and store operations copy the bit pattern from the source into the destination. The source (register or memory) does not change. Of course, the pattern at the destination is replaced by the pattern at the source.

Memory is built to store bit patterns. Both instructions and data are bit patterns, and either of these can be stored anywhere in memory (at least, so far as the hardware is concerned.) However, it is convenient for programmers and systems software to organize memory so that instructions and data are separated. At right is the way MIPS operating systems often lay out memory.

Although the address space is 32 bits, the top addresses from 0x80000000 to 0xFFFFFFFF are not available to user programs. They are used for the operating system and for ROM. When a MIPS chip is used in an embedded controller the control program exists in ROM in this upper half of the address space.

The parts of address space accessible to a user program are divided as follows:

Text Segment: This holds the machine language of the user program (the text).

Data Segment: This holds the data that the program operates on. Part of the data is static. This is data that is allocated by the assembler and whose size does not change as a program executes. Values in it do change; "static" means the size in bytes does not change during execution. On top of the static data is the dynamic data. This is data that is allocated and deallocated as the program executes. In C programming, dynamic allocation and deallocation is done with malloc() and free().

Stack Segment: At the top of user address space is the stack. With high level languages, local variables and parameters are pushed and popped on the stack as procedures are activated and deactivated.


(Thought Question) As the program runs, the data segment grows upward (as dynamic variables are allocated) and the stack grows downward (as procedures get called). Is this sensible? (Hint: how much user memory is left when the two segments meet?)