Fixed high 8-bit register MOVs, restricted ACCESSED bit updates to SEGMENT descriptors, and prevented the Debugger from looping endlessly on bad instructions

devices/pc/machine/compaq/deskpro386/vga/4096kb/machine.xml

@@ -11,7 +11,7 @@
 	<video ref="/devices/pc/video/ibm/vga/ibm-vga-lockfs.xml"/>
 	<keyboard id="keyboard"/>
 	<fdc ref="/disks/pc/library.xml" automount='{B: {name: "Win95 Build 499 (Disk 1)", path: "/disks/pc/windows/win95/build499/WIN95-DISK01.json"}}'/>
-	<debugger id="debugger" messages="fault|tss|int" commands='m dos off;bp 1ED4:16B4 "let fn=ah;dos;if fn!=3f||cx!=24"'/>
+	<debugger id="debugger" messages="fault|tss|int" commands='m dos off'/>
 	<panel ref="/devices/pc/panel/wide386.xml"/>
 	<hdc id="hdcAT" type="at" drives='[{name:"68Mb Hard Disk",type:4,path:"http://static.pcjs.org/devices/pc/machine/compaq/deskpro386/vga/4096kb/WDEB386-68Mb.json"}]'/>
 	<chipset id="chipset" model="deskpro386" floppies="[1200,1200]" monitor="vga"/>

modules/pcjs/lib/debugger.js

@@ -1538,13 +1538,15 @@ if (DEBUGGER) {
              */
             if (this.nSuppressBreaks && fProt || !this.segDebugger) return null;
         }
-
+        var seg = this.segDebugger;
         if (!fProt) {
-            this.segDebugger.loadReal(sel);
+            seg.loadReal(sel);
+            seg.limit = 0xffff;         // although an ACTUAL real-mode segment load would not modify the limit,
+            seg.offMax = 0x10000;       // proper segDebugger operation requires that we update the limit ourselves
         } else {
-            this.segDebugger.loadProt(sel);
+            seg.loadProt(sel);
         }
-        return this.segDebugger;
+        return seg;
     };
 
     /**
@@ -2929,7 +2931,7 @@ if (DEBUGGER) {
         if (!fRegs || this.nStep == 1)
             this.doUnassemble();
         else {
-            this.doRegisters(null);
+            this.doRegisters();
         }
     };
 
@@ -3841,6 +3843,7 @@ if (DEBUGGER) {
         if (dbgAddrIns.addr != X86.ADDR_INVALID && dbgAddr.addr != X86.ADDR_INVALID) {
             do {
                 sBytes += str.toHex(this.getByte(dbgAddrIns, 1), 2);
+                if (dbgAddrIns.addr == null) break;
             } while (dbgAddrIns.addr != dbgAddr.addr);
         }
 

modules/pcjs/lib/x86ops.js

@@ -3079,7 +3079,7 @@ X86.opMOVBLb = function MOVBLb()
  */
 X86.opMOVAHb = function MOVAHb()
 {
-    this.regEAX = (this.regEAX & 0xff) | (this.getIPByte() << 8);
+    this.regEAX = (this.regEAX & ~0xff00) | (this.getIPByte() << 8);
     if (BACKTRACK) this.backTrack.btiAH = this.backTrack.btiMem0;
     this.nStepCycles -= this.cycleCounts.nOpCyclesLAHF;
 };
@@ -3091,7 +3091,7 @@ X86.opMOVAHb = function MOVAHb()
  */
 X86.opMOVCHb = function MOVCHb()
 {
-    this.regECX = (this.regECX & 0xff) | (this.getIPByte() << 8);
+    this.regECX = (this.regECX & ~0xff00) | (this.getIPByte() << 8);
     if (BACKTRACK) this.backTrack.btiCH = this.backTrack.btiMem0;
     this.nStepCycles -= this.cycleCounts.nOpCyclesLAHF;
 };
@@ -3103,7 +3103,7 @@ X86.opMOVCHb = function MOVCHb()
  */
 X86.opMOVDHb = function MOVDHb()
 {
-    this.regEDX = (this.regEDX & 0xff) | (this.getIPByte() << 8);
+    this.regEDX = (this.regEDX & ~0xff00) | (this.getIPByte() << 8);
     if (BACKTRACK) this.backTrack.btiDH = this.backTrack.btiMem0;
     this.nStepCycles -= this.cycleCounts.nOpCyclesLAHF;
 };
@@ -3115,7 +3115,7 @@ X86.opMOVDHb = function MOVDHb()
  */
 X86.opMOVBHb = function MOVBHb()
 {
-    this.regEBX = (this.regEBX & 0xff) | (this.getIPByte() << 8);
+    this.regEBX = (this.regEBX & ~0xff00) | (this.getIPByte() << 8);
     if (BACKTRACK) this.backTrack.btiBH = this.backTrack.btiMem0;
     this.nStepCycles -= this.cycleCounts.nOpCyclesLAHF;
 };

modules/pcjs/lib/x86seg.js

@@ -1194,6 +1194,26 @@ X86Seg.prototype.updateMode = function(fLoad, fProt, fV86)
                 if (this.checkWrite == this.checkWriteProt) this.checkWrite = this.checkWriteProtDown;
                 this.fExpDown = true;
             }
+            if (fLoad && this.id < X86Seg.ID.VER) {
+                /*
+                 * We must update the descriptor's ACCESSED bit whenever the segment is "accessed" (ie,
+                 * loaded); unlike the ACCESSED and DIRTY bits in PTEs, a descriptor ACCESSED bit is only
+                 * updated on loads, not on every memory access.
+                 *
+                 * We compute address of the descriptor byte containing the ACCESSED bit (offset 0x5);
+                 * note that it's perfectly normal for addrDesc to occasionally be invalid (eg, when the CPU
+                 * is creating protected-mode-only segment registers like LDT and TSS, or when the CPU has
+                 * transitioned from real-mode to protected-mode and new selector(s) have not been loaded yet).
+                 *
+                 * TODO: Note I do NOT update the ACCESSED bit for null GDT selectors, because I assume the
+                 * hardware does not update it either.  In fact, I've seen code that uses the null GDT descriptor
+                 * for other purposes, on the assumption that that descriptor is completely unused.
+                 */
+                if ((this.sel & ~X86.SEL.RPL) && this.addrDesc !== X86.ADDR_INVALID) {
+                    var addrType = this.addrDesc + X86.DESC.ACC.TYPE.OFFSET;
+                    this.cpu.setByte(addrType, this.cpu.getByte(addrType) | (X86.DESC.ACC.TYPE.ACCESSED >> 8));
+                }
+            }
         }
         /*
          * TODO: For non-SEG descriptors, are there other checks or functions we should establish?
@@ -1204,24 +1224,6 @@ X86Seg.prototype.updateMode = function(fLoad, fProt, fV86)
          * we're updating segment registers as part of a mode change.
          */
         if (fLoad) {
-            /*
-             * We must update the descriptor's ACCESSED bit whenever the segment is "accessed" (ie,
-             * loaded); unlike the ACCESSED and DIRTY bits in PTEs, a descriptor ACCESSED bit is only
-             * updated on loads, not on every memory access.
-             *
-             * We compute address of the descriptor byte containing the ACCESSED bit (offset 0x5);
-             * note that it's perfectly normal for addrDesc to occasionally be invalid (eg, when the CPU
-             * is creating protected-mode-only segment registers like LDT and TSS, or when the CPU has
-             * transitioned from real-mode to protected-mode and new selector(s) have not been loaded yet).
-             *
-             * TODO: Note I do NOT update the ACCESSED bit for null GDT selectors, because I assume the
-             * hardware does not update it either.  In fact, I've seen code that uses the null GDT descriptor
-             * for other purposes, on the assumption that that descriptor is completely unused.
-             */
-            if ((this.sel & ~X86.SEL.RPL) && this.addrDesc !== X86.ADDR_INVALID) {
-                var addrType = this.addrDesc + X86.DESC.ACC.TYPE.OFFSET;
-                this.cpu.setByte(addrType, this.cpu.getByte(addrType) | (X86.DESC.ACC.TYPE.ACCESSED >> 8));
-            }
             this.cpl = this.sel & X86.SEL.RPL;
             this.dpl = (this.acc & X86.DESC.ACC.DPL.MASK) >> X86.DESC.ACC.DPL.SHIFT;
             if (this.cpu.model < X86.MODEL_80386 || !(this.ext & X86.DESC.EXT.BIG)) {