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Hello and welcome in this lecture, and we will check to see if the cpu support long mode. 

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The modern processors should support long mode.  Since checking it is really simple, we will see how to do it 

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.

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Another check we need to perform is 1g page support.

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Our memory module allocates pages using this feature. So our kernel requires the processor support 1g page

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.

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The first thing we are going to do is we are going to save the drive id for later use. 

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Remember we have saved the drive id in dl register before we jump to the loader. 

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we define the variable 

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drive id 

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Just like we did in the boot file

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and save the value of dl to the variable.

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Ok let’s check long mode. 

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There is a special instruction called cpuid 

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which returns processor identification and feature information. We pass the input value 

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80000001 to eax 

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and execute cpuid. By passing different number to eax, we will get different information about processor returned by cpuid.  

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Passing 80000001 to eax will return processor features. 

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The information about long mode support is saved in edx. 

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We test bit 29 in edx. 

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If it is set, it means that long mode is supported. 

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Otherwise long mode is not available and we jump to the label not support. 

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So we use test instruction to test.

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the bit 29 in edx

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We use jz instruction.

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If zero flag is set, it means that the bit we checked is not set and we jump to not support. 

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Also, we check 1g page support which is at bit 26, so we test edx

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and the bit 26.

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If it is set, this feature is supported. 

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If zero flag is set, it means that this feature is not supported and we jump to not support. 

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There is one thing left to do.  Before we use value 80000001, 

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we need to check whether it supports this input value.  In order to check that, we pass value

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80000000 to eax 

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and execute cpuid instruction. If the return value in eax 

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is less than 

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80000001,  we know that it does not support input value

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So we use jb instruction, which means jump if below and we simply jump to not support.

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Because our operating system is designed for running on long mode, we place not support at the end of the code 

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,

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and stop here if the processor does not support it.  Otherwise, we print message long mode is supported. 

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so we change the message

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to long mode is supported.

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OK, let's build the project and test it out in virtual machine.

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We open the terminal.

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and to navigate to the boot folder.

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and run the build script.

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OK, let's open the bochs, we double click it.

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As you see long mode is supported is printed on screen?

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OK, this is what it's showing in real machine.

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That's it for this lecture.

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See you in the next video.