v1.20.0: Limited stepping support and a couple signed/unsigned division fixes

blog/2015/10/27/README.md

@@ -104,7 +104,7 @@ XBTS (Extract Bit String) instruction -- an instruction that existed only on B0
 	&0654:12E8 CF              IRET    
 
 If the above function returns 0xB0, then the 80386 is a B0 or earlier stepping, so Windows 95 displays the
-following message and exits:
+following message and aborts:
 
 	Windows may not run correctly with the 80386 processor that is installed in this computer.
 	Upgrade your 80386 processor.
@@ -168,5 +168,40 @@ Windows 95 using a newer 80386, and then later "downgraded" the CPU to a B1, Win
 suffered from the multiplication flaw, you would see the 32-bit multiplication warning on start-up, but you could
 still continue to run, and if there was no multiplication problem, you would not see any message at all.
 
+---
+
+PCjs v1.20.0 now supports a "stepping" attribute on the <cpu> element, which you can use to simulate specific
+stepping behavior.  For example, a *machine.xml* file with the following CPU definition: 
+
+	<cpu id="cpu386" model="80386" stepping="b0"/>
+
+will cause Windows 95 to abort exactly as described as above.  Similarly, selecting a 80386 B1 stepping:
+
+	<cpu id="cpu386" model="80386" stepping="b1"/>
+
+will cause Windows 95 to display the 32-bit multiplication warning shown above (PCjs deliberately fails the exact
+multiplication test that Windows 95 performs).
+
+If you want to simulate a B1 stepping that does *not* have the 32-bit multiplication flaw, set the stepping to B2:
+
+	<cpu id="cpu386" model="80386" stepping="b2"/>
+
+B2 was not an actual 80386 stepping; it is a *pseudo-stepping* that provides a simple way of specifying a B1 80386 that
+passes all 32-bit multiplication tests.
+
+As previously discussed, Windows 95 SETUP will refuse to install on any "A" or "B" stepping, but if it's already been
+installed, it *will* start up.
+
+PCjs stepping support is extremely limited at this point.  Here's a summary:
+
+1. 80386 steppings A0-B0 provide *limited* support for the short-lived XBTS and IBTS instructions
+2. 80386 steppings A0-B1 enable [Errata #7](/blog/2015/02/23/) for STOSB (as tested by Windows 95; see above)
+3. 80386 stepping B1 enables 32-bit multiplication errors (as tested by Windows 95; see above)
+4. 80386 stepping B2 includes all supported B1 errata, but without 32-bit multiplication errors
+
+In addition, on 80386 reset, we set the CPU revision number in DX to the appropriate value for the specified stepping.
+
+Support for additional 80286 and 80386 errata may be added over time, as interesting scenarios or test cases are discovered.
+
 *[@jeffpar](http://twitter.com/jeffpar)*  
 *October 27, 2015*

modules/pcjs/lib/x86cpu.js

@@ -957,7 +957,7 @@ X86CPU.prototype.setReg = function(i, reg)
  *
  * All other 80386 registers are undefined after a reset (ie, Intel did not document how or if they are set).
  *
- * We've elected to set DX to 0x0304 on a reset, which is consistent with a 80386-C0, since we have no desire to
+ * We've elected to set DX to 0x0308 on a reset, the highest known 80386 revision, since we have no desire to
  * try to emulate all the bugs in older (eg, B1) steppings -- at least not initially.  We leave stepping-accurate
  * emulation for another day.  It's also known that the B1 (and possibly B0) reported 0x0303 in DX, and that
  * the D0 stepping reported 0x0305; beyond that, it's not known exactly what revision numbers Intel used for all
@@ -1079,9 +1079,10 @@ X86CPU.prototype.resetRegs = function()
             break;
         case X86.STEPPING_80386_D1:
         case X86.STEPPING_80386_D2:
-        default:
-            this.regEDX = 0x0308;       // in the absence of a specific stepping, default to the highest known revision
+            this.regEDX = 0x0308;
             break;
+        default:
+            break;                      // in the absence of a specific stepping, we leave DX set to zero
         }
         this.regCR0 = X86.CR0.ET;       // formerly MSW
         this.regCR1 = 0;                // reserved

modules/pcjs/lib/x86ops.js

@@ -2955,7 +2955,7 @@ X86.opSTOSb = function()
          * TODO: Extend this errata to STOSW, as well as MOVSB, MOVSW, INSB, and INSW.  Also, scope out the
          * extent to which this errara also existed on earlier steppings.
          */
-        if (this.stepping == X86.STEPPING_80386_B1) {
+        if (this.stepping >= X86.STEPPING_80386_A0 && this.stepping <= X86.STEPPING_80386_B1) {
             if (!(this.opPrefixes & X86.OPFLAG.ADDRSIZE) != (this.getByte(this.regLIP) != X86.OPCODE.AS)) {
                 maskAddr ^= (0xffff0000|0);
             }
@@ -4169,9 +4169,7 @@ X86.opGRP3b = function()
 {
     this.fMDSet = false;
     this.aOpModGrpByte[this.getIPByte()].call(this, X86.aOpGrp3b, X86.fnSRCNone);
-    if (this.fMDSet) {
-        this.regEAX = (this.regEAX & ~this.maskData) | (this.regMDLo & this.maskData);
-    }
+    if (this.fMDSet) this.regEAX = (this.regEAX & ~this.maskData) | (this.regMDLo & this.maskData);
 };
 
 /**