The instructions that a CPU fetches from memory and executes are not at all understandable to human beings. They are machine codes which tell the computer precisely what to do. The hexadecimal number 0x89E5 is an Intel 80486 instruction which copies the contents of the ESP register to the EBP register. One of the first software tools invented for the earliest computers was an assembler, a program which takes a human readable source file and assembles it into machine code. Assembly languages explicitly handle registers and operations on data and they are specific to a particular microprocessor. The assembly language for an Intel X86 microprocessor is very different from the assembly language for an Alpha AXP microprocessor. The following Alpha AXP assembly code shows the sort of operations that a program can perform:
ldr r16, (r15) ; Line 1
ldr r17, 4(r15) ; Line 2
beq r16,r17,100 ; Line 3
str r17, (r15) ; Line 4
100: ; Line 5
The first statement (on line 1) loads register 16 from the address held in register 15. The next instruction loads register 17 from the next location in memory. Line 3 compares the contents of register 16 with that of register 17 and, if they are equal, branches to label 100. If the registers do not contain the same value then the program continues to line 4 where the contents of r17 are saved into memory. If the registers do contain the same value then no data needs to be saved. Assembly level programs are tedious and tricky to write and prone to errors. Very little of the Linux kernel is written in assembly language and those parts that are are written only for efficiency and they are specific to particular microprocessors.
The C Programming Language and Compiler
Writing large programs in assembly language is a difficult and time consuming task. It is prone to error and the resulting program is not portable, being tied to one particular processor family. It is far better to use a machine independent language like C. C allows you to describe programs in terms of their logical algorithms and the data that they operate on. Special programs called compilers read the C program and translate it into assembly language, generating machine specific code from it. A good compiler can generate assembly instructions that are very nearly as efficient as those written by a good assembly programmer. Most of the Linux kernel is written in the C language. The following C fragment:
if (x != y)
x = y ;
performs exactly the same operations as the previous example assembly code. If the contents of the variable x are not the same as the contents of variable y then the contents of y will be copied to x. C code is organized into routines, each of which perform a task. Routines may return any value or data type supported by C. Large programs like the Linux kernel comprise many separate C source modules each with its own routines and data structures. These C source code modules group logical functions such as filesystem handling code.
C supports many types of variables, or locations in memory which can be referenced by a symbolic name. In the above C fragment x and y refer to locations in memory. The programmer does not care where in memory the variables are put, it is the linker (see below) that has to worry about that. Some variables contain different sorts of data, integer and floating point and others are pointers.
Pointers are variables that contain the address, the location in memory of other data. Consider a variable called x, it might live in memory at address 0x80010000. You could have a pointer, called px, which points at x. px might live at address 0x80010030. The value of px would be 0x80010000: the address of the variable x.
C allows you to bundle together related variables into data structures. For example,
struct {
int i ;
char b ;
} my_struct ;
is a data structure called my_struct which contains two elements, an integer (32 bits of data storage) called i and a character (8 bits of data) called b.
Linkers are programs that link together several object modules and libraries to form a single, coherent, program. Object modules are the machine code output from an assembler or compiler and contain executable machine code and data together with information that allows the linker to combine the modules together to form a program. For example one module might contain all of a program’s database functions and another module its command line argument handling functions. Linkers fix up references between these object modules, where a routine or data structure referenced in one module actually exists in another module. The Linux kernel is a single, large program linked together from its many constituent object modules.