From 3bad5547c2389c9752f89278e096f45a3c6c6c24 Mon Sep 17 00:00:00 2001
From: Jeff Parsons <Jeff@pcjs.org>
Date: Tue, 1 Dec 2015 11:44:57 -0800
Subject: [PATCH] Notes on FPU stack faults (which only affects the 80387 and
 up, which isn't fully supported yet)

---
 modules/pcjs/lib/x86.js    |  2 +-
 modules/pcjs/lib/x86fpu.js | 14 +++++++++++++-
 2 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/modules/pcjs/lib/x86.js b/modules/pcjs/lib/x86.js
index ce5b128c7..00acca715 100644
--- a/modules/pcjs/lib/x86.js
+++ b/modules/pcjs/lib/x86.js
@@ -554,7 +554,7 @@ var X86 = {
             OE:     0x0008,     // bit 3: Overflow
             UE:     0x0010,     // bit 4: Underflow
             PE:     0x0020,     // bit 5: Precision
-            SF:     0x0040,     // bit 6: Stack Fault (80387 and later)
+            SF:     0x0040,     // bit 6: Stack Fault (80387 and later; triggers an Invalid Operation exception)
             EXC:    0x007F,     // all of the above exceptions
             ES:     0x0080,     // bit 7: Error/Exception Status/Summary (Interrupt Request on 8087)
             C0:     0x0100,     // bit 8: Condition Code 0
diff --git a/modules/pcjs/lib/x86fpu.js b/modules/pcjs/lib/x86fpu.js
index d434acc23..befb6237c 100644
--- a/modules/pcjs/lib/x86fpu.js
+++ b/modules/pcjs/lib/x86fpu.js
@@ -427,7 +427,16 @@ X86FPU.prototype.opUnimplemented = function()
 X86FPU.prototype.checkException = function()
 {
     this.regStatus &= ~X86.FPU.STATUS.ES;
-    if (this.regStatus & (~this.regControl & X86.FPU.STATUS.EXC)) {
+    /*
+     * NOTE: The "Stack Fault" (SF) status bit wasn't introduced until the 80387, so it triggers the pre-existing
+     * "Invalid Operation" (IE) exception; there is no corresponding "Stack Fault" (SE) exception, and the matching
+     * control bit is still reserved.  Consequently, X86.FPU.CONTROL.EXC is a *subset* of X86.FPU.STATUS.EXC (0x3F
+     * instead of 0x7F).
+     *
+     * However, we shouldn't have to do anything special when SF is set, because any setException() call that sets
+     * SF should ALSO set IE.
+     */
+    if (this.regStatus & (~this.regControl & X86.FPU.CONTROL.EXC)) {
         this.regStatus |= X86.FPU.STATUS.ES;    // set ES whenever one or more unmasked EXC bits are set
     }
     if ((this.regStatus & X86.FPU.STATUS.ES) && !(this.regControl & X86.FPU.CONTROL.IEM)) {
@@ -451,6 +460,9 @@ X86FPU.prototype.checkException = function()
  *      PE (0x0020 bit 5: Precision)
  *      SF (0x0040 bit 6: Stack Fault; 80387 and later)
  *
+ * Also, as noted in checkException(), any time you set the SF bit, you should also set the IE bit, because
+ * Stack Fault is a subset of Invalid Operation.  TODO: We should include a test for that in the assertion below.
+ *
  * @this {X86FPU}
  * @param {number} n (one or more of the above error status bits)
  * @return {boolean} (true if unmasked exception exists, false if not)