Assembling, Linking and Executing

Assembling, Linking and Executing
1) Assembling:
- Assembling converts source program into object program if syntactically correct and generates an intermediate .obj file or module.
- It calculates the offset address for every data item in data segment and every instruction in code segment.
- A header is created which contains the incomplete address in front of the generated obj module during the assembling.
- Assembler complains about the syntax error if any and does not generate the object module.
- Assembler creates .obj .lst and .crf files and last two are optional files that can be created at run time.
- For short programs, assembling can be done manually where the programmer translates each mnemonic into the machine language using lookup table.
- Assembler reads each assembly instruction of a program as ASCII character and translates them into respective machine code.
Assembler Types:
There are two types of assemblers:
a) One pass assembler:
- This assembler scans the assembly language program once and converts to object code at the same time.1) Assembling:
- Assembling converts source program into object program if syntactically correct and generates an intermediate .obj file or module.
- It calculates the offset address for every data item in data segment and every instruction in code segment.

- A header is created which contains the incomplete address in front of the generated obj module during the assembling.
- Assembler complains about the syntax error if any and does not generate the object module.
- Assembler creates .obj .lst and .crf files and last two are optional files that can be created at run time.
- For short programs, assembling can be done manually where the programmer translates each mnemonic into the machine language using lookup table.
- Assembler reads each assembly instruction of a program as ASCII character and translates them into respective machine code.
Assembler Types:
There are two types of assemblers:

a) One pass assembler:
- This assembler scans the assembly language program once and converts to object code at the same time.

This assembler has the program of defining forward references only.
- The jump instruction uses an address that appears later in the program during scan, for that case the programmer defines such addresses after the program is assembled.
b) Two pass assembler
- This type of assembler scans the assembly language twice.
- First pass generates symbol table of names and labels used in the program and calculates their relative address.
- This table can be seen at the end of the list file and here user need not define anything.
- Second pass uses the table constructed in first pass and completes the object code creation.
- This assembler is more efficient and easier than earlier.