Building an OS - 1 - Hello world

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Hello and welcome! In this tutorial, I will show you what it takes to build an operating system from scratch, so let's jump straight into it!

First, let's see what tools we will need.

For editing text, any text editor will do; I will be using the micro editor, which I really like because it uses common keyboard shortcuts. However, you can use anything you like.

We will use make to build our project. nasm will be our assembler, which we will use to assemble our assembly code.

We will also need some virtualization software. I will use qemu, but you can use any virtualization software you like such as VirtualBox, VMware etc.

I will be using Ubuntu in this tutorial; if you want to follow on Windows, your best option would be to use the Windows Subsystem for Linux. You can find a tutorial on how to set that up in the video description. The second option would be Cygwin which has most of the tools that we need. On MacOS you shouldn't have any trouble finding these tools using Homebrew.

Let's create a src folder in our project, and inside we'll make a file called main.asm. The first part of our operating system will be written in a programming language called assembly. Later, we will be using C, but right now we don't really have a choice and we really have to use assembly.

So what exactly is this assembly thing? Shortly, the assembly language is the human readable interpretation of machine code. When you compile a program in a programming language such as C or C++, it gets converted into machine code which is the language that the processor understands. Assembly makes it easier for us, humans, to read and write machine code. Higher level programming languages, such as C or C++, need to be translated by the compiler into machine code. This involves many steps, like building an abstract syntax tree, and a huge amount of optimization. Assembly is much simpler, because the instructions simply need to be converted into their machine code representation by a tool called an assembler.

An assembly instruction is made up of a mnemonic, or a keyword, and a number of parameters which are called operands, typically zero, one or two operands.

add ax, 7
mov bx, ax
inc ax
 
mnemonic operand1, operand2, ...

An important thing I'd like to mention about the assembly language is that there are differences between different processors. The instructions supported on an x86 processor, that you find in laptops and desktops, are different than instructions supported on an ARM CPU, which is found in smartphones or tablets. Even different processors with the same architecture might have differences; for example SSE is a feature introduced in the Pentium 3 processor line that didn't exist in the Pentium 2 line. However, newer processors easily keep backwards compatibility so that all the programs can still run without any modification. Backwards compatibility for the x86 architecture goes so far back that it can still run software on a modern computer that was designed for the 8086 CPU, the first CPU ever made, using the x86 architecture which was at least 40 years ago.

So, to be clear, we will be writing our operating system for the x86 architecture. This means that we'll be using the x86 assembly language.

So, what happens when you turn on your computer? First the BIOS sticks in performing all sorts of tests showing a fancy logo and then the part that's the most important to us it stars the operating system so how does it do that there are actually two ways in which the BIOS can load an operating system in the How the BIOS finds an OS first method which is now called legacy booting the BIOS loads the first block of data or the first sector from each boot device into memory until it finds a certain signature once it finds that signature it jumps to the first instruction in the loaded block and this is where our operating system starts the second method is called efi which works a bit differently in this mode the bios looks for a certain efi partition on each device which contains on special efi programs for the moment we won't be covering efi will only look at legacy mode now that we know how the bios loads our operating system here's what we need to do we will write some code assemble it and then we will put it in the first sector of a floppy disk we also need to somehow add that signature that the bios requires after that we can test our operating system so let's begin coding we know that the bios always puts our operating system at address 7000 so the first thing we need to do is give our sender this information this is done using the org directive which tells the assembler to calculate all memory offset starting at 7000 changing this line to another number won't make the bios load a different address it will only tell the assembler that the variables and labels from our code should be calculated with the offset 7000 before we continue I need to explain the difference between a directive and an instruction a directive is a way of giving the assembler a clue about how to interpret our code when instruction is translated into a machine code instruction a directive won't get translated it is only giving a clue to the assembler next we tell our assembler to emit 16-bit code as I mentioned before any x86 CPU must be backwards compatible with the original 8086 CPU so if an operating system that was designed for the 8086 is run on a modern CPU it still needs to think that it's running on an 8086 because of this the CPU always starts in 16-bit mode bits is also a directive which tells the assembler to emit 16-bit code writing bits 32 won't make the processor running 32-bit mode it is only directive which tells the assembler to emit 32-bit code now I'll define the main labels to mark where our code begins for now we just want to note that the BIOS loads our operating system correctly so I'll only write a halt instruction which holds the processor in certain cases the CPU can start executing again so I'll just create another hold label and then jump to it this way if the CPU ever starts again it will be stuck in an infinite loop it's not a good idea to allow the processor to continue executing beyond the end of our program our program is almost done all that's left to do is add that signature that the BIOS requires the BIOS expects that the last two bytes of the first sector are a a 5 5 we will be putting our program on a standard 1.44 megabytes floppy disk where one sector has 512 bytes the BIOS requires that the last 2 bytes of the first sector are a a 5 5 we can ask Nasim to emit bytes directly by using the DB directive which stands for declare constant byte the times directive can be used to repeat instructions or data here we use it to pad our program so that it fills up to 510 bytes after which we declare the two byte signature in NASM the dollar sign can be used to obtain the assembly position of the beginning of the current line and the double dollar sign gives us the position of the beginning of the current section in our case dollar - dollar other is about the length of the program so far measured in bytes finally we declared a signature DW is a directive similar to DB but it declares the two byte constant which is generally referred to as a word with this we have successfully written our first operating system so far it doesn't really do anything but stop the processor let's test it if it works I created a bill directory to keep things organized for building the project I created a make file I added a rule to build the main dot ASM code using NASM an output in a binary format I added a rule to build Kimmage where I simply took the binary file previously built and padded it with zeros until it has 1.44 megabytes finally we can test our little operating system you can use any virtualization software you want such as VirtualBox VM where I use camel because it's really easy to setup and it can be used from a command line as you can see the system boots from floppy and then it does nothing exactly as we expected so far our operating system does nothing and does it perfectly now that we know it works let's go back to the code and print a hello world message to screen before I start explaining how you can do that I need to explain some basic concepts about the x86 architectures x86 CPU Registers all processors have a number of registers which are really small pieces of memory that can be written and read very fast and are built into the CPU here's a diagram of holder registers on an x86 CPU there are several types of registers the general-purpose registers can be used for almost any purpose the index registers are usually used for keeping indices and pointers they can also be used for other purposes the program counter is a special register which keeps track of which memory location the current instruction begins the segment registers are used to keep track of the currently active memory segments which I will explain in just a moment there is also a Flags register which contains some special flags which are set by various instructions there are a few more special purpose registers but will only introduce them when we need them now we'll talk a bit about RAM memory the 8086 CPU had a 20-bit address bus this meant that you could access up to 2 to the power of 20 which means about one megabyte of memory at the time typical computers had around 64 to 128 kilobytes of memory so the engineers at Intel thought this limit was huge for various reasons they decided to use a segment and offset addressing scheme for Memory segmentation memory in this scheme you address memory by using two 16-bit values the segment and the offset each segment contains 64 kilobytes of memory where each byte can be accessed using the offset value segments overlap every 16 bytes this means that you can convert a segment offset address to an absolute address by shifting the segment four bits to the left were multiplying it by 16 and then adding the offset this also means that there are multiple ways of addressing the same location in memory for example the absolute address 7000 which is where the BIOS flows our operating system can be written as any combination that you can see on the screen there are some special registers which are used to specify the actively used segments CS contain the code segment which is the segment the processor executes code from the IP or program counter register only gives us the offset the D s and D s registers are data segments newer processors introduced additional data segments FS and GS SS contains the current stack register in order to access the outside one of these active statements we need to load that Simon into one of these registers the code segment can only be modified by performing a jump now how do Referencing a memory location you reference a memory location from assembly you use this syntax a segment register followed by a colon followed by an expression which gives the offset put between in brackets the segment register can be omitted in which case the DSL register will be used the processor is capable of doing some arithmetic for us as long as we use this expression the base and index operands can be any general-purpose processor registers in 16-bit mode there are a few limitations however only B P and B X can be used as base registers and only Si and di can be used as index registers these limitations exist because of how the 8086 CPU was originally designed where they had to put such limitations to keep the complexity down another example of one such limitation is that we can't write constants to the segment registers directly we have to use an intermediary register with the introduction of the 386 processor just a few years later 32-bit mode was introduced which pretty much rendered 16-bit mode obsolete a lot of newer cpu features were simply not added to the 16-bit mode because it is absolute and it only exists for backwards compatibility it is still useful to learn because most of the things that apply to a 16-bit mode apply to a 32-bit or 62 bit mode and it is much simpler its main use today is in the startup sequence most operating systems switch to 32 or 64-bit mode immediately after starting up we will do the same thing in a future video but we can't just yet for now we are limited to the first sector of a floppy disk that is 512 bytes which is very little space once we are able to load a from the floppy disk we can do a lot more all operating systems have to do the same thing in order to boot but until we get there let's get back to referencing our memory locations so I already talked about the base and index operands the scale and displacement operands are numerical constants the scale can only be used in 32 and 64-bit modes and it can only have a value of one to four or eight the displacement can be any signed integer constant all the operands in a memory reference expression are optional so you only have to use whatever you need so here's an example first I defined a label which points to a word having the value 100 the first instruction puts the offset of the label into the ax register the second instruction puts the memory contents per our label point set since we didn't specify a segment register D s is going to be used we haven't used the base index or scale but only a constant which is the offset for label points to in assembly labels are simply constant which points to a specific offset here's a more complicated example where we want to read this third element in an array in this example we put the offset of the array into BX and the index of the third element in Si since we use zero-based indexing the third element is array of two and each element in the array is a word which is a two bytes wide so we put in si the value 4 you can see here that we use the multiplication symbol the assembler is capable of calculating the result of constant expressions and putting the result in the resulting machine code however you can try to move BX ax times 2 ax is not known at compile time so it is not a constant for that you have to be used the multiply instruction referencing memory is the only place where you can put registers in an expression finally we put into ax the third element in the array by referencing the memory location at BX plus si BX is our base register and si is our in this register now back to our operating system the code segment register has been set up for us by the BIOS and it points to segment 0 there are some biases out there which actually jump to our code using a difference I meant an offset such as segment 7c 0 offset 0 but the standard behavior is to use segment 0 offset 7000 we don't know if the data segment an extra segment are properly initialized so this is what we have to do next since we can't write a constant directly to a segment register we have to use an intermediary register we will use a X the move instruction copies a reader from the source on the left side to the destination on the right side we also set up the stack segment to 0 and a stack pointer to the beginning of our program so what exactly is this stack basically the stack is a piece of memory that we can accessed in a first in first out manner using the push and pop instructions the stack also has a special purpose when using functions when you call a function the return address is added to the stack when you return from a function the processor will read the return address from the stack and then jump to it another thing to note about the stack is that it grows downwards SP points to the The stack top of the stack when you push something SP is decremented by the number of bytes pushed and then the data is written to memory this is why we set up the stack to point to the start of our operating system because it grows downwards if we set it up to the end of our program it would overwrite our program we don't want that so we just put it somewhere where it won't overwrite anything the beginning of our operating system is a pretty safe spot now we'll start coding a Buddhist function which prints a string to the screen always document your assembly functions so our function will receive a pointer to a string in DSS i and it will print characters until it encounters a null character because I decided to write the function above main I have to add a jump instruction above so main is still the entry point to our program first I push the registers that I'm going to modify to the stack after which we enter the main loop the load SB instruction loads apart from the address DSS I into the AL register and then increments si next I wrote the loop exit condition the or instruction performs a bitwise or and store the result in the left hand side operand in this case al orange a value to itself will modify the value at all but what it will modify is the Flex register if there is multi zero the zero flag will be set the next instruction is the conditional jump which jumps to the down label if the zero flag is set so essentially if the next character is null we jump outside the loop there's something I forgot when I recorded the video or jump instruction to the loop label so that the code will loop after exiting the loop we pop the registers we previously pushed in reverse order and then we'll return from this function so far our function takes a string iterates every character until it encounters the null character and then exits what's left to do is to print the character to the screen the way we can do that is using the BIOS as the name suggests the BIOS or the basic input/output system does more than just start a system it also provides some very basic functions which allow us to do some very basic stuff such as writing text to the screen so how exactly do we call the BIOS to print the character for us the answer is that we use interrupts so what are interrupt an interrupt is basically a signal which makes the processor stop whatever it is doing to handle that event there are three possible ways of triggering an interrupt the first way it is through an exception an exception is generated by the CPU if a critical error is encountered and it cannot continue executing for example dividing by zero will trigger an interrupt operating systems can use these interrupts to stop the misbehaving process or to attempt to restore it to working order hardware can also trigger interrupts for example when a key is pressed on the keyboard or when the disk controller finished performing enough synchronous read the third way in which interrupts can be triggered is through the int instruct shun interrupts are numbered from 0 to 255 so the instruction requires a parameter indicating the interrupt number to trigger the BIOS install some interrupt handlers for us so that we can use its functionality Examples of BIOS interrupts typically the BIOS reserves an interrupt number for a category of functions and the value in the aah register is used to choose between the available functions in that category to print text to the screen we will need to call interrupts 10 hexadecimal which contains the video services category by setting eh to 0 a hexadecimal will call the right text in teletype mode function here's a detailed description of this function so what we need to do in order to call this function is to set the aah registered to 0e hexadecimal al to the ASCII character that we want to print and B H to the page number the build parameter is only used in graphics mode so we can ignore it because you're currently running in text mode when I recorded the video I forgot to set the page number to zero after that we call interrupts one zero hexadecimal finally let's add a string containing the text hello world followed by a new line to add a new line you need to print both the line feed and the carriage return characters I created an awesome macro so that I don't have to remember the hex codes for these characters every time to declare string we use the DB directive which conveniently allows us to write as many characters as we want all that left to do is to set the SSI to address of the string and then call the Podesta let's now test our program you because I forgot to put that jump instruction I only go to the age after fixing the issue the message helloworld is displayed great so we have successfully written a tiny apartment system which can print text to the screen this was a lot of work and we learned a lot of new stuff about how computers work we'll continue the next time when we will improve our assembly skills and learn some new stuff by extending our operating system to print numbers to the screen after that we get into the complex task of loading stuff from the disk thank you for watching and see you the next time bye bye