On Wed, Aug 10, 2011 at 11:54 AM, Andreas Färber <andreas.faerber@web.de> wrote:

Am 10.08.2011 um 17:48 schrieb William Hahne:

On Wed, Aug 10, 2011 at 6:00 AM, Mark Cave-Ayland <mark.cave-ayland@siriusit.co.uk> wrote:

On 09/08/11 22:55, William Hahne wrote:

It is possible for BootX to purposefully divide by zero expecting 0 as
the result.

Index: kernel/forth.c
===================================================================
--- kernel/forth.c (revision 1041)
+++ kernel/forth.c (working copy)
@@ -1157,6 +1157,12 @@
{
const ucell b = POP();
const ducell a = DPOP();
+
+ if (b == 0) { // can't divide by zero
+ PUSH(0);
+ DPUSH(0);
+ return;
+ }
#ifdef NEED_FAKE_INT128_T
if (a.hi != 0) {
fprintf(stderr, "mudivmod called (0x%016llx %016llx /
0x%016llx)\n",

There is something wrong here: nothing will purposefully divide by zero as it will invoke a trap somewhere. Again, I think this is hiding another bug somewhere and is not necessarily the correct fix.

You would be surprised then, if you type "0 0 /" into a OF mac you will get 0. I thought it was a bug as well at first but after carefully looking at the source code for BootX I found that it wasn't. This is probably just a peculiarity of Apple's OF implementation.

I'd rather think this is an architectural difference: On ppc, divwo sets the OV bit in XER but divw doesn't, PowerISA 2.06B saying that the result of the division is undefined in both cases.

On POWER5 I do get zero as well, so it's not just Apple.

0 > 0 0 / ok
1 > . 0 ok

I'd suggest to stick the new code fragment into an appropriate ppc #ifdef if sparc does it differently.

Andreas

That makes sense. Here is a patch with an appropriate #ifdef added.

Index: kernel/forth.c

===================================================================

--- kernel/forth.c (revision 1046)

+++ kernel/forth.c (working copy)

@@ -1157,6 +1157,17 @@

{

const ucell b = POP();

const ducell a = DPOP();

+#if defined(CONFIG_PPC) || defined(CONFIG_PPC64)

+ /* PPC OF provides a result of 0 after a division by zero

+ rather than an exception */

+ if (b == 0) {

+ PUSH(0);

+ DPUSH(0);

+ return;

+ }

+#endif

#ifdef NEED_FAKE_INT128_T

if (a.hi != 0) {

fprintf(stderr, "mudivmod called (0x%016llx %016llx / 0x%016llx)\n",

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William Hahne