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Introduction to C  ›  Lesson

C vs Assembly

C vs Assembly 10 minAssembly is a readable form of one processor's instructions, while C is portable source that a compiler translates into the instructions of whichever processor you target.

C vs Assembly

Early operating systems were often written in assembly, one line per machine instruction, for one specific processor. When UNIX was rewritten in C in 1973, it became possible to move the whole system to new machines by building it again rather than rewriting it. That move is the whole story of C versus assembly.

The rule

Assembly speaks one chip's language; C lets a compiler translate for any chip.

Assembly is a readable spelling of a processor's own instructions. Each processor family has its own instruction set, so assembly written for an ARM chip means nothing to an x86-64 chip or a RISC-V chip.

C describes what you want. A compiler for the target processor decides which instructions do it. Take one line from our logger:

total = total + reading;

For a 32-bit ARM chip, a compiler might produce something like this (the exact output depends on the compiler and its settings):

ldr  r0, [sp, #4]    @ load total from memory
ldr  r1, [sp, #8]    @ load reading
add  r0, r0, r1      @ add them
str  r0, [sp, #4]    @ store the new total

A compiler for x86-64 would produce different instructions from the same line. With optimisation turned on, it might keep total in a register and skip the loads and stores entirely.

Assembly C
Runs on One processor family Any processor with a C compiler
One line means One instruction Zero or more instructions
Who picks registers You The compiler
Control Total Nearly total

Assembly has not disappeared. It still appears in the first instructions a device runs at power-on, in special instructions C has no words for (such as switching interrupts off), and in a few hand-tuned inner loops. Even then it is usually a small piece inside a C project, and most compilers offer an extension for embedding a few lines of it.

You can watch the translation yourself: gcc -S logger.c stops after compiling and writes the assembly to logger.s.

The shortcut

Ask one question of each piece of code: "Does this need a specific instruction on a specific chip?" If yes, write that small piece in assembly. If no, write it in C and let the compiler choose.

Moving the logger to a new boardStep 1 of 4

  1. The new board

    The greenhouse project switches from an ARM board to a RISC-V board.

Drills

Answer fast — trust the rule.

quick drill

The logger was written in C for ARM. What is usually needed to run it on an x86-64 machine?

quick drill

Which GCC flag stops after compiling and writes the assembly to a .s file?

quick drill

Which task is most likely to still need a few lines of assembly?

quick drill

How many machine instructions does one C statement become?

Remember

  • Assembly speaks one chip's language; C lets a compiler translate for any chip.
  • Shortcut: "does this need a specific instruction on a specific chip?" Only then reach for assembly, and keep it small.
  • Trap: assuming a C line maps to one fixed set of instructions. The compiler and its settings decide.

Practice — level up

0/1 solved

Write your answer, then compare it with a rubric and a model answer.

ExplainPorting the logger

The greenhouse logger must move from an ARM board to a RISC-V board. Explain what the move costs if the logger is written in C and what it costs if it is written in assembly, and say when a few lines of assembly would still be justified.