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GDT - Revision history
2026-07-04T15:52:21Z
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Vegard: new page
2019-12-14T22:32:51Z
<p>new page</p>
<p><b>New page</b></p><div>You can get the (kernel mapped) address of the GDT using the <tt>sgdt</tt> instruction from userspace:<br />
<br />
<source lang="C"><br />
#include <stdio.h><br />
#include <stdlib.h><br />
#include <unistd.h><br />
<br />
struct desc_ptr {<br />
unsigned short size;<br />
unsigned long address;<br />
} __attribute__((packed)) ;<br />
<br />
static inline void sgdt(struct desc_ptr *dtr)<br />
{<br />
asm volatile("sgdt %0" : "=m" (*dtr));<br />
}<br />
<br />
int main(int argc, char *argv[])<br />
{<br />
while (1) {<br />
struct desc_ptr gdt_descr;<br />
sgdt(&gdt_descr);<br />
<br />
printf("%lx %u\n", gdt_descr.address, gdt_descr.size);<br />
<br />
usleep(100000);<br />
}<br />
<br />
return 0;<br />
}<br />
</source><br />
<br />
This program will typically bounce between CPUs and therefore print different addresses depending on which CPU the <tt>sgdt</tt> instructions runs on.<br />
<br />
[[Category:Programming]]<br />
[[Category:x86]]</div>
Vegard