Assembly language, addressing modes and system software

Why assembly still appears

Assembly language is a one-to-one symbolic form of machine code: LDD, ADD, STO, JMP, CMP, JPE. It is used where speed or direct hardware control matters — device drivers and embedded systems — and in the exam because it shows whether you really understand what the processor does.

Four addressing modes

Given LDx n:

• Immediate (LDM #n): load the value $n$ itself.
• Direct (LDD n): load the contents of address $n$.
• Indirect (LDI n): address $n$ holds another address; load the contents of that.
• Indexed (LDX n): load the contents of address $n$ plus the index register — the mode that makes array processing possible in a loop.

Assemblers and translators

An assembler converts assembly to machine code, usually in two passes: the first builds a symbol table of labels, the second generates the code now that every label's address is known — which is why forward references work. A compiler translates a high-level program once, producing a fast standalone executable but reporting all errors together. An interpreter translates and runs line by line: slower and it needs the interpreter present, but it stops at the first error, which suits development.

The operating system

Memory management, file management, process scheduling, device management via drivers, security and the user interface. It exists to hide hardware detail from applications, so the same program runs on different machines.

Utility software

Defragmenter, backup, compression, antivirus, disk repair — housekeeping tools, distinct from the OS itself and from application software.