📄 write your own operating system - lesson 2.htm
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<DIV class=Section1>
<P class=MsoTitle><A name=_Toc534120514>Write Your Own Operating System
Tutorial</A></P>
<DIV
style="BORDER-RIGHT: medium none; PADDING-RIGHT: 0in; BORDER-TOP: medium none; PADDING-LEFT: 0in; PADDING-BOTTOM: 1pt; BORDER-LEFT: medium none; PADDING-TOP: 0in; BORDER-BOTTOM: windowtext 0.75pt solid">
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style="mso-bookmark: _Toc534120514"></SPAN><AHREF="LESSON1.HTM"><SPAN
style="mso-bookmark: _Toc534120514">Previous
Lesson</SPAN><SPANSTYLE='MSO-BOOKMARK:_TOC534120514'></SPAN></A><SPAN
style="mso-bookmark: _Toc534120514"><SPANSTYLE='MSO-TAB-COUNT:1'>
</SPAN></SPAN><A
href="http://www.cse.unl.edu/~jgompert/OS/TableOfContents.htm"><SPANSTYLE='MSO-BOOKMARK:_TOC534120514'>Table
of Contents</SPAN><SPANSTYLE='MSO-BOOKMARK:_TOC534120514'></SPAN></A><SPAN
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</SPAN></SPAN><A
href="http://www.cse.unl.edu/~jgompert/OS/lesson3.htm"><SPANSTYLE='MSO-BOOKMARK:_TOC534120514'>Next
Lesson</SPAN><SPAN style="mso-bookmark: _Toc534120514"></SPAN></A><SPAN
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<H1><A name=_Toc534120515>Lesson 2: Making Our First Bootable Disk</A></H1>
<P class=MsoNormal>In this lesson, we will learn how to create a boot program on
a floppy disk.<SPAN style="mso-spacerun: yes"> </SPAN>We will start by
modifying the Microsoft DOS Boot Record.</P>
<P class=MsoNormal>For our purposes, we want to replace the boot loader code
without changing the other data in the boot sector.<SPAN
style="mso-spacerun: yes"> </SPAN>If we change the data to something
invalid, then DOS and Windows will not recognize the disk as being valid.<SPAN
style="mso-spacerun: yes"> </SPAN>Windows will give an error saying the
disk is not formatted.<SPANSTYLE="MSO-SPACERUN: yes?> </SPAN>This will
cause you to be unable to access any of the files on the disk.<SPAN
style="mso-spacerun: yes"> </SPAN>However, we can change the boot program
code all we want and, as long as we don抰 mess with the other data, DOS and
Windows will be able to read and write the files on the disk just fine.</P>
<P class=MsoNormal>We will leave the first instruction (<SPAN
style="FONT-FAMILY: 'Courier New'">jmp 0x3E</SPAN>) alone, because we need to
jump over the Boot Record data.<SPAN style="mso-spacerun: yes">
</SPAN>Thus we can begin modifying the code at 0x3E.<SPAN
style="mso-spacerun: yes"> </SPAN>Run the DOS DEBUG program and load the
first sector of a formatted floppy disk into memory at address 0.<SPAN
style="mso-spacerun: yes"> </SPAN>Then type the command</P>
<P class=ComputerText>-u 3E</P>
<P class=ComputerText><![if !supportEmptyParas]><![endif]> <o:p></o:p></P>
<P class=MsoNormal>to view the instructions there.<SPAN
style="mso-spacerun: yes"> </SPAN>Now, we will begin modifying the
code.<SPAN style="mso-spacerun: yes"> </SPAN>Type the command</P>
<P class=ComputerText>-a 3E</P>
<P class=ComputerText><![if !supportEmptyParas]><![endif]> <o:p></o:p></P>
<P class=MsoNormal>to begin assembling instructions.<SPAN
style="mso-spacerun: yes"> </SPAN>The prompt changes from a hyphen to the
address at the location that we gave.<SPAN style="mso-spacerun: yes">
</SPAN>Type the following instruction and press enter.</P>
<P class=ComputerText>jmp 3E</P>
<P class=ComputerText><![if !supportEmptyParas]><![endif]> <o:p></o:p></P>
<P class=MsoNormal>The instruction is assembled to machine code and placed into
memory, and the following prompt is the next available memory after the
instruction you just entered.<SPAN style="mso-spacerun: yes"> </SPAN>Press
Enter once more to exit the assembly mode.<SPAN style="mso-spacerun: yes">
</SPAN>The whole procedure on my computer looked like this.</P>
<P class=ComputerText>-a 3E</P>
<P class=ComputerText>0AFC:003E jmp 3E</P>
<P class=ComputerText>0AFC:0040</P>
<P class=ComputerText>-</P>
<P class=MsoNormal style="MARGIN-BOTTOM: 0pt"><SPANSTYLE="FONT-FAMILY:'COURIER
New??><![if !supportEmptyParas]><![endif]> <o:p></o:p></SPAN></P>
<P class=MsoNormal>The segment address (0x0AFC, in my case) can (an probably
will be) different on your computer, or even between different sessions of
DEBUG.<SPAN style="mso-spacerun: yes"> </SPAN>Now view the instruction you
just entered by giving the unassembled command.</P>
<P class=ComputerText>-u 3E</P>
<P class=ComputerText><![if !supportEmptyParas]><![endif]> <o:p></o:p></P>
<P class=MsoNormal>As you can see, the first instruction is now our jump
instruction.<SPAN style="mso-spacerun: yes"> </SPAN>This will create an
infinite loop.<SPAN style="mso-spacerun: yes"> </SPAN>If we quit DEBUG
now, no changes will be saved, but we can now write our modified boot sector
back to the disk (overwriting the previous one) by typing this command.</P>
<P class=ComputerText>-w 0 0 0 1</P>
<P class=ComputerText><![if !supportEmptyParas]><![endif]> <o:p></o:p></P>
<P class=MsoNormal>This "write" command uses the same syntax as the "load"
command.<SPAN style="mso-spacerun: yes"> </SPAN>This writes the data found
at memory address 0 to disk 0, starting with sector 0 and writing 1 sector.<SPAN
style="mso-spacerun: yes"> </SPAN><I>Be very careful when using the write
command.<SPAN style="mso-spacerun: yes"> </SPAN>This command can be used
to overwrite sectors on any drive, and cause loss of data.</I></P>
<P class=MsoNormal>You can now boot with this floppy.<SPAN
style="mso-spacerun: yes"> </SPAN>When you boot, BIOS will load the first
sector off the disk into memory and begin execution at the beginning of the
sector.<SPAN style="mso-spacerun: yes"> </SPAN>This will be the jump to
0x3E instruction.<SPANSTYLE="MSO-SPACERUN: yes?> </SPAN>The instruction
there is one to jump to 0x3E, so this will continue forever.<SPAN
style="mso-spacerun: yes"> </SPAN>Try it.<SPAN
style="mso-spacerun: yes"> </SPAN>Boot up a computer with this disk.<SPAN
style="mso-spacerun: yes"> </SPAN>Nothing appears to happen.<SPAN
style="mso-spacerun: yes"> </SPAN>The computer will just sit there and do
nothing.<SPAN style="mso-spacerun: yes"> </SPAN>But your new 搊perating
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