Self-relocation

In computer programming, a self-relocating program is a program that relocates its own address-dependent instructions and data when run, and is therefore capable of being loaded into memory at any address. In many cases, self-relocating code is also a form of self-modifying code. Self-relocation is similar to the relocation process employed by the linker-loader when a program is copied from external storage into main memory; the difference is that it is the loaded program itself rather than the loader in the operating system or shell that performs the relocation. One form of self-relocation occurs when a program copies the code of its instructions from one sequence of locations to another sequence of locations within the main memory of a single computer, and then transfers processor control from the instructions found at the source locations of memory to the instructions found at the destination locations of memory. As such, the data operated upon by the algorithm of the program is the sequence of bytes which define the program. Static self-relocation typically happens at load-time (after the operating system has loaded the software and passed control to it, but still before its initialization has finished), sometimes also when changing the program's configuration at a later stage during runtime. As an example, self-relocation is often employed in the early stages of bootstrapping operating systems on architectures like IBM PC compatibles, where lower-level chain boot loaders (like the master boot record (MBR), volume boot record (VBR) and initial boot stages of operating systems such as DOS) move themselves out of place in order to load the next stage into memory. Under CP/M, the debugger Dynamic Debugging Tool (DDT) dynamically relocated itself to the top of available memory through page boundary relocation in order to maximize the Transient Program Area (TPA) for programs to run in. In 1988, the alternative command line processor ZCPR 3.4 for the Z-System introduced so called type-4 programs which were self-relocatable through an embedded stub as well.