Here, the memory at address 1000h gets the value of ax. If this example is executed right after the previous one, the 2 bytes at 1000h and 1001h will be a 2 byte integer with the value of 20.
Some instructions do things like subtraction or logical operations like "not":
The machine code example earlier in this article would be this in assembly language:
Here, 42 and ax are added together and the result is stored back in ax. In x86 assembly it is also possible to combine a memory access and mathematical operation like this:
This instruction adds the value of the 2 byte integer stored at 1000h to ax and stores the answer in ax.
This instruction computes the or of the contents of the registers ax and bx and stores the result back into ax.
Usually, instructions are executed in the order they appear in memory, which is the order they are typed in the assembly code. The processor just executes them one after another. However, in order for processors to do complicated things, they need to execute different instructions based on what the data they were given is. The ability of processors to execute different instructions depending on something's outcome is called "branching". Instructions that decide what the next instruction should be are called "branch instructions".
In this example, suppose someone wants to calculate the amount of paint they will need to paint a square with a certain side length. However, due to economy of scale the paint store will not sell them any less than amount of paint needed to paint a 100 x 100 square.
To figure out the amount of paint they will need to get based on the length of the square they want to paint, they come up with this set of steps:
That algorithm can be expressed in the following code where ax is the side length.
continue:
This example introduces several new things, but the first two instructions are familiar. They copy the value of ax into bx and then subtract 100 from bx.
One of the new things in this example is called a label, a concept found in assembly languages in general. Labels can be anything the programmer wants (unless it is the name of an instruction, which would confuse the assembler). In this example, the label is 'continue'. It is interpreted by the assembler as the address of an instruction. In this case, it is the address of mult ax.
Another new concept is that of flags. On x86 processors, many instructions set 'flags' in the processor that can be used by the next instruction to decide what to do. In this case, if bx was less than 100, sub will set a flag that says the result was less than zero.
The next instruction is jge which is short for 'jump if greater than or equal to'. It is a branch instruction. If the flags in the processor specify that the result was greater than or equal to zero, instead of just going to the next instruction the processor will jump to the instruction at the continue label, which is "mul ax".
This example works fine, but it is not what most programmers would write. The subtract instruction set the flag correctly, but it also changes the value it operates on, which required the ax to be copied into bx. Most assembly languages allow for comparison instruction that do not change any of the arguments they are passed, but still set the flags properly and x86 assembly is no exception.
continue:
Now, instead of subtracting 100 from ax, seeing if that number is less than zero, and assigning it back to ax, ax is left unchanged. The flags are still set the same way, and the jump is still taken in the same situations.
While input and output are a fundamental part of computing, there is no one way they are done in assembly language. This is because the way I/O works depends on the set up of the computer and the operating system its running, not just what kind of processor it has. In the example section the Hello World example uses MS-DOS operating system calls and the example after it uses BIOS calls.
It is possible to do I/O in assembly language. Indeed, assembly language can generally express anything that a computer is capable of doing. However, even though there are instructions to add and branch in assembly language that will always do the same thing there are no instructions in assembly language that always do I/O.
The important thing to note is that the way that I/O works is not part of any assembly language because it is not part of how the processor works.
Even though assembly language is not directly run by the processor - machine code is, it still has a lot to do with it. Each processor family supports different features, instructions, rules for what the instructions can do, and rules for what combination of instructions are allowed where. Because of this, different types of processors still need different assembly languages.
Because each version of assembly language is tied to a processor family, it lacks something called "portability". Something that has portability or is portable can be easily transferred from one type of computer to another. While other types of programming languages are portable, assembly language, in general, is not.
Although assembly language allows for an easy way to use all the processor's features, it is not used for modern software projects for several reasons:
As a result of these drawbacks, high-level languages like Pascal, C, and C++ are used for most projects instead. They allow programmers to express their ideas more directly instead of having to worry about telling the processor what to do every step of the way. They're called high-level because the ideas the programmer can express in the same amount code are more complicated.
Programmers writing code in compiled high level languages use a program called a compiler to transform their code into assembly language. Compilers are much harder to write than assemblers are. Also, high-level languages do not always allow programmers to use all the features of the processor. This is because high-level languages are designed to support all processor families. Unlike assembly languages, that only support one type of processor, high-level languages are portable.
Even though compilers are more complicated than assemblers, decades of making and researching compilers has made them very good. Now, there is not much reason to use assembly language anymore for most projects, because compilers can usually figure out how to express programs in assembly language as well or better than programmers.
A "Hello, world!" program written in x86 assembly:
adosseg
.model small
.stack 100h
.data
hello_message db 'Hello, World!',0dh,0ah,'$'
.code
main proc
main endp
end main.
A function that prints a number to the screen using BIOS interrupts written in NASM x86 assembly. Modular code is possible to write in assembly, but it takes extra effort. Note that anything that comes after a semicolon on a line is a comment and is ignored by the assembler. Putting comments in assembly language code is very important because large assembly language programs are so hard to understand.
printn:
gloop: inc si ; length of string
num: add dx, ('A'- 10) ; hex value, add 'A' to dx - 10.
anum: push dx ; put dx onto stack.
tloop: pop ax ; get its value
Toffee
Toffee is a candy which is made by boiling sugar with butter, and then cooling the mixture so that it becomes hard. Sometimes it is made with treacle.
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Chop (Ukraine)
Chop (, , ) is a city found in the Zakarpattia Oblast (province) of western Ukraine, near the borders of Slovakia and Hungary. It is separated from the Hungarian town of Záhony by the river Tisza.
According to the Ukrainian national census, Chop had a population in 2001 of 8,919, of whom 40% were Ukrainians, 39.2% ethnic Hungarians and 20.8% Gypsies, Russians, Slovaks, Belarusians, Jews .