Low-level Mach debugging.

1 files changed, 13 insertions, 0 deletions

diff --git a/microkernel/mach/gnumach/debugging.mdwn b/microkernel/mach/gnumach/debugging.mdwn
index 7f4fc6ad..94fdb461 100644
--- a/microkernel/mach/gnumach/debugging.mdwn
+++ b/microkernel/mach/gnumach/debugging.mdwn
@@ -51,3 +51,16 @@ Then boot your system and do something like this:
 
 This is especially useful if you need to manually trigger some stuff inside the
 running kernel, as with the *D1* example.
+
+
+If you're doing real low level debugging, you might want to put variations of
+the following snipped into the code, this code will write a `#` character at
+line `[LINE]`, column `[COLUMN]` on the screen:
+
+    *((char *) 0xb8000 + 2 * ([LINE] * 80 + [COLUMN])) = '#';
+    halt_cpu ();
+
+The call of `halt_cpu` will -- as the name suggests -- halt the system
+afterwards.  This might be what you want or it might not, but it is needed at
+some place when running the kernel inside QEMU, as QEMU somehow decides not to
+update its display buffer anymore under certain conditions.