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With the c module loaded, we can parse the fat16 structure. What we are going to do in the loader

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is we are going to find the kernel file and user program. Once we find them, we will load them in the memory 

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and then we transfer the control to the kernel. 

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So the file module in the loader includes two basic functions, search file and read file. 

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OK, let's get started.

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As you can see, this is the general structure of fat16 format. 

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We have bpb bios parameter block, file allocation tables, 

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root directory and data sections. 

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The bios parameter block includes information such as bytes per sector, sectors per cluster, 

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total sectors, etc. 

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The file allocation table is used to locate the clusters.  In the fat16 system, 

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the cluster is the basic allocation unit which is a set of consecutive sectors.

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The sectors per cluster in the BPB will give us this info. 

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Now let's move to the file allocation table, this table contains the chains of links for a file,

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that is, the value in the table indicates the next cluster number. Then you use that value as an index to 

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find the value in the table which will give you the next cluster number. 

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To find all the clusters of a file, you continue the process until you reach the end of the file. 

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The number of the file allocation tables we have in the image is usually 2. 

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We can get this info in the bios parameter block as well.

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The next section is root directory which follows the file allocation tables. We can calculate the size of root directory 

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with the info in bios parameter block. 

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In the root directory section, we have root directory entries with each one being 32 bytes. 

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And these entries represent the files we have in the root directory of the fat16 partition. 

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If we have files and folders in the root directory, we will see them stored as directory entries here

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.

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The data section stores the data of the files and folders. If we have subfolders,

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the files and folders within the subfolders will be stored as directory entries in the data section. 

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So the root directory section, as its name implies, stores the files and folders in the root directory.

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What we are going to do next is we are going to inspect the fat16 image so that we can see 

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how to find a file manually. 

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First off,  we mount os image. So we open osfmount

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and choose  os.img.

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Then we uncheck read only drive, because we want to add files in this image and then we click mount.

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As you can see right now, the os image is empty.

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So here we add the loader folder and kernel.bin file.

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Now, we dismount this partition.

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So we click dismount.

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OK, now we can open the image with hex editor.

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If you use Linux or Mac OS, you can inspect the image with hexdump command.

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Alright, this is what we got in the hex editor. The first 512 bytes are the mbr code we wrote in the previous videos. 

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The next few sectors are the loader file 

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we wrote in the last lecture. 

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Ok, let’s find the fat16 partition. In the partition entry, we retrieve the starting lba here, 

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which is 3f 00 00 00. 

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The data stored in the little-endian order. 

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So the starting lba is 3f. 

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We multiply it by 512 which produces the result 7e00. 

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This is the address of the fat partition. 

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So we go to

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7e00

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Now let’s interpret the data with the bios parameter block info. The oem name is free dos 4.1. 

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The bytes per sector is 200 in hexadecimal which is 512 in decimal. 

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The sectors per cluster is 4, so each cluster is 2kb. And we have 1 sector reserved. 

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The next field shows the number of fat tables we have, 2 fat tables in this case. 

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The root entries represent the number of entries we have in the root directory section, 

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which is 512 in this example. 

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The total sectors indicate the number of sectors in this partition. Here we have value 0. 

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So if it is 0, we will use extended sectors 

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located at the offset 20.

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As you can see, the total sectors is 31f11. 

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Let's move on.

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The media descriptor is f8 which means it’s a fixed media.

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The sectors per fat table indicates the number of sectors for a file allocation table, 

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so we have c8 sectors for each fat table. 

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OK, we skip the sectors per track and head.

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Then we get to hidden sectors which represent how many sectors we have before the partition. 

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Remember we got the value 3f in the lba field of partition entry. 

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The value is equal to what we have in this field. 

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We have talked about the extended sectors. The next field we care about is label. Since we format the partition with the label os, 

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here we can see os in this field.

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The last one is system type. We got fat16 in this field. 

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OK, with all the information in the bpb, we can locate the fat allocation table 

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and root directory section. We start off with fat allocation table. To locate the table, 

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we use the value in the reserved sectors which represent the number of sectors we have 

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starting from the bpb to the file allocation table. 

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In this case, we have 1.

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So the allocation table is at the next sector. 

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As you can see, this is the start of fat allocation table. In the fat16 system, 

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we use 16-bit value for the cluster number. 

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And note that the first two value are reserved. 

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Therefore, we start with the third entry in the fat table. We will see how to use this table in a minute.

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Following the fat tables, we have root directory section. 

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To locate the section, 

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we need the start address of fat table and the size of fat tables.  In the bios parameter block, 

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we know that we have 2 fat tables and each table occupies c8 sectors. 

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So we add the size of fat tables c8*2 to the start of the fat table 

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which is sector 1, 

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we get result 191 sectors. 

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Then we multiply it by 512, the size is 32200 in hexadecimal

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.

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This is the offset from the start of the partition.  So we add 7e00 to the result. 

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The root directory section should be at this location. We go to 

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3a000.

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As you can see, we have a couple of root directory entries here.  Since each entry is 32 bytes, 

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we have three entries. 

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The first one is for the volume label, 

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we named it os when we format the partition.  The next entry is kernel.bin file 

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and the last one is loader folder we added ourselves.

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Ok let’s take a look at the entry structure. In the offset 0, 

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we have file name which is 8 bytes. 

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The extension name is stored in the next three bytes. 

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The label has no extension name. So we only got os. The next entry contains file name and extension name.

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Let’s continue. The next byte stores the file attributes. Bit 3 means volume id. So we have 8 in the first entry

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.

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Bit 4 indicates folder, so we have hexadecimal value 10 in the loader entry. 

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and we know that loader is a folder. Bit 5 represents archive, 

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We have 20 in the second entry.

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So kernel.bin is a file.

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Then we get 10 bytes data reserved.  Let’s move on.  The next four bytes are for the date and time.

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We won’t interpret the data and time in our system. 

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Let's continue. The starting cluster is the info we need to locate the file data in the data section.

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It’s two bytes data. The last 4 bytes are used to store the file size.

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Alright,

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that's it for the directory entries.

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Let’s see how to find the file data with the entry. 

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To do it, we use the starting cluster, 2 in this case. 

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and you can see this is the file size and the result is file size in bytes.

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Note that the starting cluster number for the data section is 2.  

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Therefore, we will subtract 2 from the starting cluster of the entry to get the correct number.

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In this example, it’s 0 which is the start of the data section. 

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The data section follows the root directory section. 

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We have calculated the address of root directory section which is 3a000 in this example

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.

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The size of root directory section is the number of entries * 32

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.

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We know that we have 512 entries in the bios parameter block, 

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so the size of the section is 512*32 

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equals 4000 in hexadecimal. 

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We add it to 3a000 and address is 3e000.

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And this is the address we are looking for.

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So we go to

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3e000

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You can see we have lots of data here.

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We get one cluster of data for this file, which is 2kb of data, how do we find the following

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data since the size of the file is more than 2kb?

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To do it, we find the next cluster using fat table. Remember the starting cluster number is 2. 

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So we use 2 as an index to locate the item in the table. T

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So let's go back to the fat table.

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You can see at the index 2 of the table, we get index 3, then we use 3 as an index into this table

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.

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and we get the value 4. 

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Next we use 4 as an index to do the same process. 

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As you see, in this example, we get consecutive numbers. We continue doing it until we reach the end of the sectors

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which is the value ffff. 

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now we collect all the clusters for the file data. 

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And all the data is stored in these clusters.

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To get the data using the clusters,  we use the formula, as we have talked about, 

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the start address of data section + the cluster number – 2 and then times the size of the cluster. 

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If you like, you can check the data in those clusters.

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As for the subfolders, the data of the subfolders is the 32-byte directory entries 

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as we saw in the root directory section. 

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The difference is that they are stored in the data section. For example, in the root directory entry, 

153
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we have one folder loader. The file size is 0 

154
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if this is a directory. 

155
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And the starting cluster is c. 

156
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ok we use the formula to locate its data. the data section plus the cluster number c – 2 

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times the size of clusters. 

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The data section is at 3e000 and we add it to the result 5000 in hexadecimal. 

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So we go to

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43000.

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Now you can see we have 13 entries build.h debug.c etc, which are the files in the loader folder.

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Also we have 2 more entries. 

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The . and .. 

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The . is pointing to itself. And .. is pointing to its parent directory. 

165
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Alright, that’s it for inspecting the image. 

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In the next video, we will start building the file module. 

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See you in the next lecture.