Interaction Between Assembly and Higher Level Language

Interaction Between Assembly and Higher Level Language

• There are three language levels that can be used to write a program.

1. Machine level language 2. Assembly level language 3. High level language.

Machine Level Programs

• A program which has simply a sequence of the binary codes for the instructions is called machine level language program. This binary form of the program is referred to as machine language because it is the form required by the machine. However, to write a program in machine language, programmer has to memorize the thousands of binary instruction codes for a processor. This task is difficult and error prone.

Assembly Language Programs

• To make programming easier, usually programmers write programs in assembly language. They translate the assembly language program to machine language so that it can be loaded into memory and executed. Assembly language uses two, three or four letter words to represent each instruction types. These words are referred to as mnemonics.

• The letters in an assembly language mnemonic are usually initials or a shortened form of the English word(s) for the operation performed by the instruction. For example, the mnemonic for addition is ADD, the mnemonic for logic AND

operation is AND, and the mnemonic for the instruction for copy data from one location to another is MOV. Therefore, the meaning expressed by mnemonics help us to remember the operation performed by the instruction.

• Assembly language statements are usually written in a standard form and assembly language has its own unique syntactical structure, such as requiring upper case or lower case, or requiring colons after label definitions. Here we discuss the common features that assembler shares.

• The assembly text is usually divided into fields, separated by spaces and tabs. A format for a typical line from assembly language program can be given as

Label: Mnemonic Operand1, Operand2; Comment

• The first field, which is optional, is the label field, used to specify symbolic labels. A label is an identifier that is assigned to the address of the first byte of the instruction in which it appears. As mentioned earlier, the presence of a label is optional, but if present, the label provides a symbolic name that can be used in branch instructions to branch to the instruction.

• The second field is mnemonic, which is compulsory. All instructions must contain a mnemonic. The third and following fields are operands. The presence of the operands depends on the instruction. Some instructions have no operands, some have one, and some have two. If there are two operands, they are separated by a

comma.

• The last field is a comment field. It begins with a delimiter such as the semicolon and continues to the end of the line. The comments are for our benefits, they tell us what the program is trying to accomplish. Fig. 6.12.1 shows a typical 8086 assembly language instruction.

High-level Language Programs

•Another way of writing a program for a microcomputer is with a high-level language, such as Basic, Pascal, C, or C++. The program statements used in such languages are even more English-like than those of assembly language. Each program statement in high-level language represents one or more than one machine code instructions. The program statements in the high-level language are converted into machines codes with the help of interpreter or compiler.

•Since one program statement in high-level language has many machine code instructions, high-level language programs are written faster than assembly or machine language programs. However, programs written in a high-level language needs interpretation or compilation and hence they always execute more slowly and require more memory than the same programs written in assembly language. The Table 6.12.1 shows the comparison between machine, assembly and high-level languages.

Compiler and Interpreter

• The computer understands only machine language. So, a translator is needed to translate the symbolic statements of a high level language into computer executable machine language.

• The programs that translate high-level programs into machine language are called interpreters and compilers.

• Compiler: A compiler is a program which translates the source code written in a high-level language into computer-readable machine language that can be directly loaded and executed.

• Interpreter: Unlike compiler, an interpreter translates a high-level language statement in a source program to a machine code and executes it immediately, before translating the next source language statement. When an error is found, the execution of the program is halted and an error message is displayed on the screen of the computer.

Comparison between Interpreter and Compiler

Review Questions

1. What do you mean by machine language program?

2. What do you mean by assembly language program?

3. Give comparison between machine level, assembly level and high level languages.

4. What is compiler?

5. What is interpreter ?

6. Give comparison between compiler and interpreter.