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<DIV><FONT face=Arial size=2>Hi,</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>X86 is a little endian machine so <FONT
face="Times New Roman" size=3>e9 bd ff -> JMP relative 0xFFBD or JMP
relative -0x43 (i might be of by one ...) = -67 decimal (64 for the
nops and 3 for the e9 bd ff ) its is relative to the next instruction if i
remember right ...</FONT></FONT></DIV>
<DIV> </DIV>
<DIV><FONT face=Arial size=2>greetings,</FONT></DIV>
<DIV><FONT face=Arial size=2>todthgie</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<BLOCKQUOTE
style="PADDING-RIGHT: 0px; PADDING-LEFT: 5px; MARGIN-LEFT: 5px; BORDER-LEFT: #000000 2px solid; MARGIN-RIGHT: 0px">
<DIV style="FONT: 10pt arial">----- Original Message ----- </DIV>
<DIV
style="BACKGROUND: #e4e4e4; FONT: 10pt arial; font-color: black"><B>From:</B>
<A title=shadravan_f@yahoo.com href="mailto:shadravan_f@yahoo.com">Shadravan
Fontanov</A> </DIV>
<DIV style="FONT: 10pt arial"><B>To:</B> <A title=linuxbios@linuxbios.org
href="mailto:linuxbios@linuxbios.org">linuxbios@linuxbios.org</A> </DIV>
<DIV style="FONT: 10pt arial"><B>Sent:</B> Friday, June 29, 2007 19:03</DIV>
<DIV style="FONT: 10pt arial"><B>Subject:</B> [LinuxBIOS] General question
abouts jumps in machine code</DIV>
<DIV><BR></DIV>Hello,<BR><BR>I wanted to verify that the relative jump in my
program is properly transalated into machine code.<BR>Consider following
example:<BR><code begins><BR>extern char handler[],
endhandler[]; /* C-code glue for the asm insert
*/<BR>asm(<BR> ".data\n"<BR>
".code16\n"<BR> ".globl handler,
endhandler\n"<BR> "\n"<BR>
"handler:\n"<BR> "nop\n" "nop\n" "nop\n" "nop\n" "nop\n"
"nop\n" "nop\n" "nop\n"<BR> "nop\n" "nop\n" "nop\n" "nop\n"
"nop\n" "nop\n" "nop\n" "nop\n"<BR> "nop\n" "nop\n" "nop\n"
"nop\n" "nop\n" "nop\n" "nop\n" "nop\n"<BR> "nop\n" "nop\n"
"nop\n" "nop\n" "nop\n" "nop\n" "nop\n" "nop\n"<BR> "nop\n"
"nop\n" "nop\n" "nop\n" "nop\n" "nop\n" "nop\n" "nop\n"<BR>
"nop\n" "nop\n" "nop\n" "nop\n" "nop\n" "nop\n" "nop\n"
"nop\n"<BR> "nop\n" "nop\n" "nop\n" "nop\n" "nop\n" "nop\n"
"nop\n" "nop\n"<BR> "nop\n" "nop\n" "nop\n" "nop\n" "nop\n"
"nop\n" "nop\n" "nop\n"<BR> "jmp
handler\n"<BR> "endhandler:\n"<BR>
"\n"<BR> ".text\n"<BR>
".code32\n"<BR>);<BR><BR>int main(void) <BR>{<BR> return
0;<BR>}<BR><code ends><BR><BR>There are 64 NOPs, on the basis of these I
want to locate my jmp instruction after compiling. I am interested about "jmp
handler\n"-instruction.<BR><BR>I compile the example above, then
start<BR> hexdump -vC compiled_example | less<BR>and look for 64 times of
0x90 (opcode for NOP). I find them:<BR><snippet of hexdump
starts><BR>00005740 00 00 00 00 00 00 00 00 84 e5 04 08 90 90
90 90 |................|<BR>00005750 90 90 90 90 90 90 90 90
90 90 90 90 90 90 90 90 |................|<BR>00005760 90 90 90 90
90 90 90 90 90 90 90 90 90 90 90 90
|................|<BR>00005770 90 90 90 90 90 90 90 90 90 90 90 90
90 90 90 90 |................|<BR>00005780 90 90 90 90 90 90 90
90 90 90 90 90 e9 bd ff 00 |................|<BR>00005790 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
|................|<BR><snippet of hexdump end><BR>Exactly 64 times 0x90,
fine... The next code after the last 0x90 is 0xe9. Look at "Intel Architecture
Software Developers Manual Vol2" tells: its jump instruction, the next two
bytes (bd ff) specify the relative address to jump. bdff must be in
second complement and represent minus 4201. But offset 4201 does
not jumps to the start of the NOP sequence, it jumps to a very smaller
address...<BR><BR>Do you have any
hints?<BR><BR>Regards<BR><BR>Shadravan<BR><BR>
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