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Ok, let’s first create the fat16 image. In this example, we use the free dos to format the image to fat16 image

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.

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So we need free dos. To download free dos image, we search free dos and click free dos project, 

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in the free dos page, we go to download section and click download free dos 1.2.

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In the download page, we choose standard cd rom. 

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Once the file is downloaded, we can go to next step.

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As you see, I havedownloaded free dos image here, now we create a new folder called test.

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And copy free dos image to this folder.

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Next we create another image, os image file, which is used as the image for the operating system in the following lectures. 

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The boot image file in the previous lectures is discarded. 

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So in the terminal,

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we navigate to the test folder.

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and create an image file by the command bximage, 

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the settings are the same as we did with boot image. 

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The difference is that we set the image file to 100mb.  

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100mb is enough to store and test our projects in this section. 

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And the name of the image is called os.img

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.

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Alright,

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We have created the os.img

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The last file we need is bochs configuration file where we add the free dos image and os image.  So we run bochs

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and select edit options.

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The option is 12 edit disk and boot options. In the channel 0, 

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we add the two image files. We choose 4 to set the first device 

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and the type of device is disk. 

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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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So we type  203 16 63. 

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We use default settings.

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OK, move to the second device.

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So we choose option 5.

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The type of device is cd rom and the name of the file is  dos image we just downloaded. 

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we change the status to inserted, and we leave other settings unchanged. 

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Since we want to boot the dos image and format the os image file, in the boot option, we set the boot drive from floppy drive to cd rom. 

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OK, return to the top menu.

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Select option f4 to save options.

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Alright,

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The preparation is finished, we run bochs.  We type c to continue.

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As you can see, the free dos is running.

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select the language 

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and continue with the installation. 

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Partition drive c, 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. 

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So we format c drive and press enter. 

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We proceed with the format.

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The volume label in this example is set to os.

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Ok the os image is finally created. We close it out. 

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In ubuntu, we can simply mount the fat16 image by typing 

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gnome disk image mounter writable 

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and the image file, os.img in this case, press enter. 

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As you can see  the os folder is showing here.   Because we didn’t copy files into it after we format the image, 

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there is no files in the folder. To dismount the partition is simple, 

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all we need to do is click unmount here

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.

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After we create the image file, we can inspect the os image. The command we use is hexdump 

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the option we need here is -C which means we want to see the characters, and the name of the image file os.img

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.

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we hit enter.

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as you see, the image is pretty much filled with 0s. 

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The data which is important to us is the data located in the offset 1be in the first sector. 

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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 entry.

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At that time,

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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,  we want to mount this partition and transfer data between the operating systems, 

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so 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. They will be written in the boot file 

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and then write it into the image which will represent the correct partition.

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As you can see here, the starting sector is 3f and each sector assumes to be 512 in our system

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,

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so the first partition is located at the offset 3f*512. We get the result 7e00

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.

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If we go to the address 7e00, we can see we have a small block of data. In the right column, 

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the characters fat16 is here.

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Ok, once we create the fat16 image, the next thing we are going to do is we are going to change the boot code

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and write it in the image. 

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So in the boot folder,

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we open boot.asm

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In this file, we change the value of the partition entry accordingly. The data here is 

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the starting chs value

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,

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the partition type, 

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the ending chs value, 

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the starting lba value 

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and the sectors of the partition.  

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Note that the data is 4 bytes stored in the little-endian order which means the low byte is in the low memory address.

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so the count of the partition is 03 1f 11

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Also, we will change the size of the loader to 15 sectors.

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in the next video we will change the loader file.

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That’s it for the boot.asm.

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With the partition entry adjusted to the correct value, we can build product in the terminal. Before we build the project,

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in the build script,

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we need to comment out other commands because we only want to test the boot code.

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The output file is changed to os.img.

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OK.

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Now we copy os.img

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to the boot folder.

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Let's build the project.

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OK, let's rexamine the os.img

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As you can see, the boot code is written into the first sector of the image and the partition entry remains

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unchanged.

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

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So we mount the image.

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Alright, you can see the image is still recognized as 100mb fat16 partition.

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The next thing we are going to do is we are going to write the image file into the USB flash drive.

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Just as we did in the previous lectures, we insert the USB flash drive and in the terminal, we type

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sudo fdisk

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-l to list the partitions.

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The name of device in my computer is sdb. We write sudo 

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dd, the input file 

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is os.img and the name of the output file

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is dev sdb

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we need to check out the output file is correct one and press enter.

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You can see the usb flash drive is recognized as 100mb fat16 partition.

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In the following lectures, we will parse the data and implement the file module. 

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See you in the next video.