Compilers, Assemblers, Linkers, Loaders: A Short Course

Compilers, Assemblers, Linkers, Loaders: A Short Course

This document briefly describes what happens when you compiler and run a program. More details can be found in Compilers, Principles, Techniques, and Tools by Aho, Sethi, and Ullman (CSE 401 book) and Appendix A of Computer Organization and Design by Patterson and Hennesey (CSE 378 book).

Compiling a Program

When you type cc at the command line a lot of stuff happens. There are four entities involved in the compilation process: preprocessor, compiler, assembler, linker (see Figure 1).

The internals of cc

Figure 1: The internals of cc.

First, the C preprocessor cpp expands all those macros definitions and include statements (and anything else that starts with a #) and passes the result to the actual compiler. The preprocessor is not so interesting because it just replaces some short cuts you used in your code with more code. The output of cpp is just C code; if you didn't have any preprocessor statements in your file, you wouldn't need to run cpp. The preprocessor does not require any knowledge about the target architecture. If you had the correct include files, you could preprocess your C files on a LINUX machine and take the output to the instructional machines and pass that to cc. To see the output of a preprocessed file, use cc -E.

The compiler effectively translates preprocessed C code into assembly code, performing various optimizations along the way as well as register allocation. Since a compiler generates assembly code specific to a particular architecture, you cannot use the assembly output of cc from an Intel Pentium machine on one of the instructional machines (Digital Alpha machines). Compilers are very interesting which is one of the reasons why the department offers an entire course on compilers (CSE 401). To see the assembly code produced by the compiler, use cc -S.

The assembly code generated by the compilation step is then passed to the assembler which translates it into machine code; the resulting file is called an object file. On the instructional machines, both cc and gcc use the native assembler as that is provided by UNIX. You could write an assembly language program and pass it directly to as and even to cc (this is what we do in project 2 with sys.s). An object file is a binary representation of your program. The assembler gives a memory location to each variable and instruction; we will see later that these memory locations are actually represented symbolically or via offsets. It also make a lists of all the unresolved references that presumably will be defined in other object file or libraries, e.g. printf. A typical object file contains the program text (instructions) and data (constants and strings), information about instructions and data that depend on absolute addresses, a symbol table of unresolved references, and possibly some debugging information. The UNIX command nm allows you to look at the symbols (both defined and unresolved) in an object file.

Since an object file will be linked with other object files and libraries to produce a program, the assembler cannot assign absolute memory locations to all the instructions and data in a file. Rather, it writes some notes in the object file about how it assumed things were layed out. It is the job of the linker to use these notes to assign absolute memory locations to everything and resolve any unresolved references. Again, both cc and gcc on the instructional machines use the native linker, ld. Some compilers chose to have their own linkers, so that optimizations can be performed at link time; one such optimization is that of aligning procedures on page boundaries. The linker produces a binary executable that can be run from the command interface.

Notice that you could invoke each of the above steps by hand. Since it is an annoyance to call each part separately as well as pass the correct flags and files, cc does this for you. For example, you could run the entire process by hand by invoking /lib/cpp and then cc -S and then /bin/as and finally ld. If you think this is easy, try compiling a simple program in this way.

Running a Program

When you type a.out at the command line, a whole bunch of things must happen before your program is actually run. The loader magically does these things for you. On UNIX systems, the loader creates a process. This involves reading the file and creating an address space for the process. Page table entries for the instructions, data and program stack are created and the register set is initialized. Then the loader executes a jump instruction to the first instruction in the program. This generally causes a page fault and the first page of your instructions is brought into memory. On some systems the loader is a little more interesting. For example, on systems like Windows NT that provide support for dynamically loaded libraries (DLLs), the loader must resolve references to such libraries similar to the way a linker does.


Figure 2 illustrates a typical layout for program memory. It is the job of the loader to map the program, static data (including globals and strings) and the stack to physical addresses. Notice that the stack is mapped to the high addresses and grows down and the program and data are mapped to the low addresses. The area labeled heap is where the data you allocate via malloc is placed. A call to malloc may use the sbrk system call to add more physical pages to the program's address space (for more information on malloc, free and sbrk, see the man pages).

Memory layout

Figure 2: Memory layout.

Procedure Call Conventions

A call to a procedure is a context switch in your program. Just like any other context switch, some state must be saved by the calling procedure, or caller, so that when the called procedure, or callee, returns the caller may continue execution without distraction. To enable separate compilation, a compiler must follow a set of rules for use of the registers when calling procedures. This procedure call convention may be different across compilers (does cc and gcc use the same calling convention?) which is why object files created by one compiler cannot always be linked with that of another compiler.

A typical calling convention involves action on the part of the caller and the callee. The caller places the arguments to the callee in some agreed upon place; this place is usually a few registers and the extras are passed on the stack (the stack pointer may need to be updated). Then the caller saves the value of any registers it will need after the call and jumps to the callee's first instruction. The callee then allocates memory for its stack frame and saves any registers who's values are guaranteed to be unaltered through a procedure call, e.g. return address. When the callee is ready to return, it places the return value, if any, in a special register and restores the callee-saved registers. It then pops the stack frame and jumps to the return address.

Original version from CSE451, Autumn 1996.
Modified by, Autumn 1997.