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POabout linux v0.01 bootsect.S
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copied!<p>Recently I'm looking at the linux 0.01 source code, since the bootsect.S in 2.6.11 and upper version is useless and it is a good place to start learning linux code, therefore I choose to trace the first version of linux. :P</p> <p>I have some question in bootsect.S. The following is some of the code in bootsect.S linux v 0.01.</p> <p>P.S the first version assembly code is using intel syntax instead of at&t.</p> <pre><code>mov ax,#0x0001 | protected mode (PE) bit lmsw ax | This is it! jmpi 0,8 | jmp offset 0 of segment 8 (cs) which is the second entry of the gdt. </code></pre> <p>gdt: </p> <pre><code>.word 0,0,0,0 | dummy .word 0x07FF | 8Mb - limit=2047 (2048*4096=8Mb) .word 0x0000 | base address=0 .word 0x9A00 | code read/exec .word 0x00C0 | granularity=4096, 386 .word 0x07FF | 8Mb - limit=2047 (2048*4096=8Mb) .word 0x0000 | base address=0 .word 0x9200 | data read/write .word 0x00C0 | granularity=4096, 386 </code></pre> <p>The booting process seems to be like the following:</p> <ul> <li><p>move the bootloader code from 0x7c00 to 0x9000</p></li> <li><p>jumps to 0x9000</p></li> <li><p>set the segment registers.</p></li> <li><p>load the system code to 0x10000 (the system code contains boot/head.S and init/main.c according to the Makefile)</p></li> <li><p>load temporary gdt and idt with lgdt and lidt</p></li> <li><p>enable A20 to access the 16mb physical memory.</p></li> <li><p>set cr0 PE bit to go to protected mode</p></li> <li><p>jump to 0x000000</p></li> </ul> <p>the following is the Makefile for system:</p> <pre><code>tools/system: boot/head.o init/main.o \ $(ARCHIVES) $(LIBS) $(LD) $(LDFLAGS) boot/head.o init/main.o \ $(ARCHIVES) \ $(LIBS) \ -o tools/system > System.map </code></pre> <p>It seems like that the head.S and main.c is link together as the system binary which the bootsect loads into memory.</p> <p>My question is if the system code(which entry is head.S/startup_32 ) is loaded in 0x10000 than why not jumps to 0x10000 instead jumps to 0x000000? Isn't it weird to jump to 0x0 since there is no code loaded inside there??</p> <p>the following is the link to download the source code: <a href="https://docs.google.com/viewer?a=v&pid=explorer&chrome=true&srcid=0B1F0m2rUn8BYMjQ4ZDQxZTUtODI5My00MGZiLTgwZDQtM2ZiZWQ2ZWQxYzIx" rel="nofollow">https://docs.google.com/viewer?a=v&pid=explorer&chrome=true&srcid=0B1F0m2rUn8BYMjQ4ZDQxZTUtODI5My00MGZiLTgwZDQtM2ZiZWQ2ZWQxYzIx</a></p>

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