some docs about the interrupt handling

doc/interrupt.md

@@ -0,0 +1,21 @@
+# Interrupt handling
+
+When an interrupt is raised, the x86 cpu will
+push some of the current working states into
+current user stack. (At least for GE ints) in
+the following order:
+
+push  (EFLAGS)
+push  %cs
+pushl %eip
+push  (ERROR_CODE)
+
+The eip should points to the instruction that
+caused the exception.
+
+ERROR_CODE can be segment selector index (if
+the exception is segment related) or simply
+zero
+
+Notice that the error code doesn't always 
+exists. In some cases, it can be absent.