Basic Computation and Programming with C

INTRODUCTION TO ASSEMBLER

Initially when computer was invented, people used to communicate with them by on and off switches denoting primitive instructions. Then machine language was introduced, where programmer have to mention instructions as well as operands and addresses in binary format, i.e., using 0 and 1. Both writing and understanding a machine language program is difficult. The functionality of this program is difficult to understand, and a person going through it may not be sure of what can be achieved through the program.

Assembly language program is one in which symbols such as letters, digits and special characters, are used for operation part, address part and other parts of instruction code. Both machine language and assembly language are referred as low level languages as the coding a problem is at the individual instruction level, i.e., for each line of a program written in these languages one and only one machine instruction is executed. Assembly language is considered as a second generation language. Computers based on different processors have got their own assembly languages which depend on the architecture of the processor. An assembler translates a program written in assembly language to machine language code.

Output of an assembler is an object file with extension ‘.obj’. It is the role of linker (another system-software) to make executable .exe file from it. Another system-software called loader, present in operating systems, loads the executable on memory when the program is required to run. The process is illustrated in Fig. 3.1. Process for producing executable file.

Typically, assemblers make two passes over the assembly file, i.e., reads the assembly program twice. (One pass assemblers with more complicated design are also available.)