Page 156 - DCAP103_Principle of operating system

Unit 5: Memory Management

Most systems allow a user process to reside in any part of the physical memory. Thus, although Notes
the address space of the computer starts at 00000, the first address of the user process does
not need to be 00000. This arrangement affects the addresses that the user program can use. In
most cases, a user program will go through several steps-some of which may be optional-before
being executed. Addresses may be represented in different ways during these steps. Addresses
in the source program are generally symbolic (such as count). A compiler will typically bind
these symbolic addresses to relocatable addresses (such as “14 bytes from the beginning of this
module”). The linkage editor or loader will in turn bind these relocatable addresses to absolute
addresses (such as 74014). Each binding is a mapping from one address space to another.
Classically, the binding of instructions and data to memory addresses can be done at any step
along the way:
1. Compile Time: If you know at compile time where the process will reside in memory,
then,

2. Absolute Code can be generated. For example, if you know a priori that a user process
resides starting at location R, then the generated compiler code will start at that location
and extend up from there. If, at some later time, the starting location changes, then it will
be necessary to recompile this code. The MS-DOS. COM-format programs are absolute
code bound at compile time.
3. Load Time: If it is not known at compile time where the process will reside in memory, then
the compiler must generate relocatable code. In this case, final binding is delayed until load

Figure 5.1: Multistep Processing of a user Program

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