update README for amd64

README.amd64

@@ -27,3 +27,25 @@ run
 OR
 qemu-system-x86_64 -m 512 -hda obj/kernel.img -drive file=obj/sfs.img,media=disk,cache=writeback,index=2 -s -serial file:obj/serial.log -monitor stdio
 
+------------------
+debug
+------------------
+* qemu+gdb
+
+  qemu and gdb do not properly handle mode-switching between 32- and
+  64-bit code: once gdb connects in 32-bit mode, it will not switch to
+  64-bit mode.  Thus, debugging 64-bit code requires attaching gdb after
+  qemu is already executing 64-bit code.  Do not use -S, since that will
+  attach in 16-/32-bit mode.
+
+  //In a terminal..
+  $ uCore_run -d obj 
+
+  //In a different terminal
+  $ cp gdbinit.amd64 .gdbinit
+  $ gdb
+  ...
+  The target architecture is assumed to be i386:x86-64
+  0xffffffffc011b5fc in ?? ()
+  (gdb) continue
+

ucore/gdbinit.amd64

@@ -1,5 +1,7 @@
-target remote :1234
 set architecture i386:x86-64:intel
+target remote :1234
+symbol-file obj/kernel/kernel-amd64.elf
 define hook-stop
 x/i $pc
 end
+br  __panic