syscalls

1 files changed, 44 insertions, 0 deletions

diff --git a/asm/gdt.s b/asm/gdt.s
new file mode 100644
index 0000000..698dcf6
--- /dev/null
+++ b/asm/gdt.s
@@ -0,0 +1,44 @@
+//http://wiki.osdev.org/GDT_Tutorial
+.global asm_setup_gdt
+//.global tss_flush
+
+// call as setup_gdt(GDT,sizeof(GDT))
+asm_setup_gdt:
+
+	// re-fill gdt_descriptor with new GDT location and size
+	movl 4(%esp),%eax
+	movl %eax, gdt_descriptor+2
+
+	movw 8(%esp),%ax
+	movw %ax, gdt_descriptor
+	//
+
+	lgdt gdt_descriptor	#load new descriptor table!
+
+	// reload to take effect
+	reloadSegments:
+
+	    #Reload CS register containing code selector:
+	    jmp   $0x08,$reload_CS # 0x08 points at the new code selector
+
+	reload_CS:
+	    mov $0x10, %ax #0x10 points at the new data selector
+	    mov %ax, %ds
+	    mov %ax, %es
+	    mov %ax, %fs
+	    mov %ax, %gs
+	    mov %ax, %ss
+
+	tss_flush:
+
+	   movw $0x2B,%ax      # Load the index of our TSS structure - The index is
+			       # 0x28, as it is the 5th selector and each is 8 bytes
+			       # long, but we set the bottom two bits (making 0x2B)
+			       # so that it has an RPL of 3, not zero.
+	   ltr %ax             # Load 0x2B into the task state register.
+
+	   ret
+
+	   
+
+ret