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To create the fat16 image, we need free dos image and then the free dos is used to format our image to fat16 format

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.

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So let’s go ahead and download free dos image.  We search free dos and click the free dos project, 

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in the free dos page, 

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we click download free dos 1.2. 

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In the download page, 

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we choose standard cd rom. 

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As you can see, I have downloaded the free dos image here, after you download the file, we can

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copy it to the new folder.

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So here we create a new folder.

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called test and we move free dos image to it.

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Next, we create another image

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os image file, which is used as the image for the operating system. 

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The boot image file that we used in the previous lectures is discarded. 

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To create an image file, we opened the command prompt.

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And we navigate to the test folder.

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We run the command bximage, 

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the settings are the same as we did with boot image. The difference is that

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we set the image file to 100mb. 

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100mb is enough to store and test our projects in this section. And the name of the image file

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is called OS.image.

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OK, we have a new image.

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The last file we need is bochs configuration file

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where we add the free dos image and os image. 

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We open bochs and edit disk and boot options. 

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In the channel 0, we add the two image files here.  We navigate to the first device and select disk

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.

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The name of the image file is os.img we just created. 

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As for the chs value, we use the value here

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.

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We type 203 16 63. 

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OK, we move to the next one.

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As for the second device, we choose cdrom

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add the dos image we just downloaded. 

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Notice that the status here is ejected, we change it to inserted. 

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Since we want to boot the dos image and format the os image file, so in the boot option, 

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we select cd rom to boot first.

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OK.

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And save the file.

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Click save.

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Alright, the preparation is finished.

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We exit it out.

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Now we can run bochs.

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we double click it.

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As you see, free dos is running, 

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select the language and continue with the installation. 

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Partition drive c and press enter. 

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OK, now we can reboot the computer.

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At this point, we return to dos.

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and in the dos prompt, we format the os.img. So we type format c drive and press enter. 

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And we proceed with the format.

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The volume label, we name it, for example, os and press enter. 

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Ok now the os image is finally created. 

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We exit out the bochs.

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In windows environment, we can mount the fat16 image using osfmount.  

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So we search osfmount 

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and in the page, we go to download section and download 64-bit version. 

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After we download it, we open the package to install it. 

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The installation is simple. Click next and install. 

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When the installation is done, we can mount the image.
So we open 

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osfmount

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To mount the image, we first click mount new. 

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In this dialogue, we select the os image we just created. 

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and click next.

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Here you can see the type of image is fat16 and size is 100mb, which is the correct image

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.

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So we select the image and click next.

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If you want to copy the file in the image, you need to uncheck read only drive. 

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Then we mount the image. 

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Right now, we can see the image is os.img and the drive of the image is d

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.

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In the Windows Explorer.

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the drive D is showing here. 

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Since we don’t copy files into it after we format the image, there is no files in the drive D.

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To unmount the partition is simple, all we need to do is select the drive and click dismount.  

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Alright, after we create the image file, let's see what we have in os image.

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The software we use to inspect the image file is hex editor. So first off, we download the hxd and install it. 

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So we search

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hxd 

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and go to the download section. 

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we click download page.

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As you see, the download links are listed here. Once the package is downloaded, you can install it

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.

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The process of installation is pretty much the same. 

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Click next and install.

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After you install it, we open hxd, 

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and in this window we click file and open

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Then we choose os file.

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click open.

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As you see, the image is pretty much filled with 0s. The data which is important to us 

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is the data located in the offset 0x1be.  Remember when we write the boot loader in the lecture the first program, 

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we talked about the partition entries and constructed a valid looking partition entr

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.

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At that time, we don’t want to mount the image, 

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so the partition entry we set doesn’t correctly represent the partitions.

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But, in this section, because we want to mount this partition and transfer data between the operating systems, 

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we need correct partition entry info, which is the data here. 

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As you can see, we only have one partition entry.  The starting and ending chs values are included 

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in these 6 bytes. 

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The starting sector is stored here.  They will be written in the boot file and then write into the image 

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which will represent the correct partition.

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The starting sector is 3f 

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and each sector assumes to be 512 in our system. So the first partition is located 

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at the offset 3f*512. We get the result 7e00. 

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If we go to the address 7e00, 

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we can see we have a small block of data. 

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In the right column, you see the characters fat16. 

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Ok, the next thing we are going to do is we are going to change the boot code and write it in the image. 

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So we open the boot folder.

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in boot.asm, we change the value of the partition entry accordingly. 

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The data is starting chs value, 

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the partition type, 

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the ending chs value, 

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the starting lba 

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and the sectors of the partition.  Note that the data is 4 bytes stored in the little-endian order 

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which means the low byte of the data is stored in the low memory address. 

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So the value is 03 1f 11.  

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We will also change the loader file which loads the image in the memory. 

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The size of loader file is increased to 15 sectors. 

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and we will change the loader file in the next video. Since the boot file is still used to do the same task, 

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we leave other parts of the file unchanged. 

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OK, that's it.

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Now, we write the boot file in the os image to see if it works. Because we want to test the boot code, 

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we change the build script to assemble only boot.asm and write it into the image file. 

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So we comment out other commands.

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OK, let's test it out.

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We open the terminal.

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Since we want to write the boot code into os.img, we copy the os image file to the boot folder

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.

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And also, we change the boot.img to os.img.

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Now, let's run the build script.

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Let's reexamine the image.

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As you see, the boot code is written into the first sector and the partition entry remains unchanged.

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OK, let's try to mount the image to see if it works.

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In the Explorer, we can see the partition is showing here.

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So, the image file is still recognized by Windows after we writes boot code into the image.

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OK, what we are going to do next is we are going to write the image to the USB flash drive, so we insert usb

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flash drive and open rufus.

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Choose the image we want to write to the disk, the os.img in this case, checks the device is the correct one

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and we click start.

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If everything goes well, you will see that USB flash drive is recognized as a 100mb fat16 partition

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.

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So let's open Windows Explorer and a 100mb fat16 partition is showing here. In the following lectures,

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We will parse the data and implement a file module.

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See you in the next video