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69

What is the difference between object code, machine code and assembly code?

Can you give a visual example of their difference?

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I'm also curious about where did the "object code" name came from? What does the "object" word supposed to mean in it? Is it somehow related to object-oriented programming or just a coincidence of names? – SasQ Apr 4 at 1:52
    
@SasQ: Object code. – Jesse Good Aug 22 at 1:18
    
I'm not asking about what is an object code, Captain Obvious. I'm asking about where did the name come from and why is it called "object" code. – BarbaraKwarc Aug 23 at 8:14
up vote 172 down vote accepted

Machine code is binary (1's and 0's) code that can be executed directly by the cpu. If you were to open a "machine code" file in a text editor you would see garbage, including unprintable characters (no, not those unprintable characters ;).

Object code is a portion of machine code that hasn't yet been linked into a complete program. It's the machine code for one particular library or module that will make up the completed product. It may also contain placeholders or offsets not found in the machine code of a completed program that the linker will use to connect everything together.

Assembly code is plain-text and (somewhat) human read-able source code that has a mostly direct 1:1 analog with machine instructions. This is accomplished using mnemonics for the actual instructions/registers/other resources. Examples include things like JMP or MULT for the jump and multiplication instructions. Unlike Machine Code, the CPU does not understand Assembly Code.

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13  
+1: nice, but somewhat simplifying answer - not all assembly instructions are translated 1:1 to machine instructions, and object files may also contain other data (relocation information, symbol tables, ...) – Christoph Jan 21 '09 at 20:25
1  
Added a weasel word for your first issue, edited to make the 2nd clearer. – Joel Coehoorn Jan 21 '09 at 20:35
2  
@Olof: RISC architectures sometimes provide an assembly-level virtual instruction set - eg MIPS pseudo-instructions ( en.wikipedia.org/wiki/MIPS_architecture#Pseudo_instructions ) – Christoph Feb 23 '11 at 19:39
1  
@Panzercrisis Machine code is the only code that ever runs. Before assembly code can run on a computer, an assembler program must first convert it to machine code. In that context, your question has no meaning, because at run-time they are the same thing. – Joel Coehoorn Mar 22 '13 at 14:53
2  
@Panzercrisis Nothing get's added by the assembler. It's a directly translation of what you wrote to actual machine instructions. And I wouldn't call the extra code put in by compilers "unecessary" – Joel Coehoorn Mar 22 '13 at 20:54
up vote 74 down vote

The other answers gave a good description of the difference, but you asked for a visual also. Here is a diagram showing they journey from C code to an executable.

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I find this really helpful, but it is missing the "Machine code" label – Alexx Roche Aug 6 '13 at 9:12
    
So when it's at the executable code level, is that equivalent to machine code? – CMCDragonkai Dec 14 '14 at 6:39
1  
In the context of this diagram, the "object code" is the machine code. – Graphics Noob Dec 14 '14 at 23:36
    
Actually, both the object code and executable code are machine codes. the difference is that object code is not the completed program. It needs to be combined with other helper library/module codes as indicated in the diagram to form a complete executable program/code. – okeyxyz Mar 26 at 20:24
    
@okeyxyz at what level would it be correct to say it is directly executed by the processor? After the assembler, after the linker, after the loader, after it gets converted to microcontroller? – Celeritas Jul 27 at 3:45
up vote 36 down vote

Assembly code is a human readable representation of machine code:

mov eax, 77
jmp anywhere

Machine code is pure hexadecimal code:

5F 3A E3 F1

I assume you mean object code as in an object file. This is a variant of machine code, with a difference that the jumps are sort of parameterized such that a linker can fill them in.

An assembler is used to convert assembly code into machine code (object code) A linker links several object (and library) files to generate an executable.

I have once written an assembler program in pure hex (no assembler available) luckily this was way back on the good old (ancient) 6502. But I'm glad there are assemblers for the pentium opcodes.

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42  
No no no no. Machine code is not hex code. it's pure binary. Hex code is just a convenient representation of binary. – Breton Jan 21 '09 at 21:05
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If we are really going into extremes, its not binary, it is an amount of stored electricity in a circuit. ;-) – Toon Krijthe Jan 21 '09 at 21:13
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Yes of course. There is a relationship between the hexidecimal, and what you would call "Machine Code", but it's not quite accurate to say hexidecimal is machine code. That's all I'm trying to say. – Breton Feb 23 '11 at 3:41
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@Breton In that sense, there is no such thing as "hex code" right? "Hex code" is just a way of viewing the machine code. You can view the machine code in hexadecimal, binary, octal, decimal, or however you like. Also again in that sense, there is no "binary code" as well. Again, "binary code" is just a way of viewing the machine code. – Utku Nov 15 '15 at 11:22
2  
@Breton What you say does not really make much sense.. Binary is a way of representation, just like hex. If it is not hex, it is not binary either. – Koray Tugay Mar 9 at 6:12
up vote 11 down vote

8B 5D 32 is machine code

mov ebx, [ebp+32h] is assembly

lmylib.so containing 8B 5D 32 is object code

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up vote 6 down vote

One point not yet mentioned is that there are a few different types of assembly code. In the most basic form, all numbers used in instructions must be specified as constants. For example:

$1902: BD 37 14 : LDA $1437,X
$1905: 85 03    : STA $03
$1907: 85 09    : STA $09
$1909: CA       : DEX
$190A: 10       : BPL $1902

The above bit of code, if stored at address $1900 in an Atari 2600 cartridge, will display a number of lines in different colors fetched from a table which starts at address $1437. On some tools, typing in an address, along with the rightmost part of the line above, would store to memory the values shown in the middle column, and start the next line with the following address. Typing code in that form was much more convenient than typing in hex, but one had to know the precise addresses of everything.

Most assemblers allow one to use symbolic addresses. The above code would be written more like:

rainbow_lp:
  lda ColorTbl,x
  sta WSYNC
  sta COLUBK
  dex
  bpl rainbow_lp

The assembler would automatically adjust the LDA instruction so it would refer to whatever address was mapped to the label ColorTbl. Using this style of assembler makes it much easier to write and edit code than would be possible if one had to hand-key and hand-maintain all addresses.

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1  
+1. One more additional point: there are also different assembly language syntaxes, most famous being Intel and AT&T. – informatik01 Jan 7 '14 at 15:40
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@informatik01: How about Intel 8080 mnemonics vs Zilog Z80? I would guess that predates the Intel vs AT&T syntax war. – supercat Jan 7 '14 at 16:39
    
Not arguing, I just mentioned that aspect (different syntax) and gave an example of two most popular/well known/famous syntaxes. – informatik01 Jan 7 '14 at 16:46
up vote 1 down vote

Assembly code is discussed here.

"An assembly language is a low-level language for programming computers. It implements a symbolic representation of the numeric machine codes and other constants needed to program a particular CPU architecture."

Machine code is discussed here.

"Machine code or machine language is a system of instructions and data executed directly by a computer's central processing unit."

Basically, assembler code is the language and it is translated to object code (the native code that the CPU runs) by an assembler (analogous to a compiler).

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up vote 1 down vote

I think these are the main differences

  • readability of the code
  • control over what is your code doing

Readability can make the code improved or substituted 6 months after it was created with litte effort, on the other hand, if performance is critical you may want to use a low level language to target the specific hardware you will have in production, so to get faster execution.

IMO today computers are fast enough to let a programmer gain fast execution with OOP.

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up vote 0 down vote

Assembly is short descriptive terms humans can understand that can be directly translated into the machine code that a CPU actually uses.

While somewhat understandable by humans, Assembler is still low level. It takes a lot of code to do anything useful.

So instead we use higher level languages such as C, BASIC, FORTAN (OK I know I've dated myself). When compiled these produce object code. Early languages had machine language as their object code.

Many languages today such a JAVA and C# usually compile into a bytecode that is not machine code, but one that easily be interpreted at run time to produce machine code.

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Your comment about Java and C# - both use Just In Time compilation so that bytecodes are not interpretted. C# (.NET generally) compiles to Intermediate Language (IL) which is then JITed into native machine language for the target CPU. – Craig Shearer Jan 21 '09 at 20:52

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