Added preliminary PDP-10 opcode decoding

modules/pdp10/lib/cpustate.js

@@ -124,6 +124,9 @@ class CPUStatePDP10 extends CPUPDP10 {
         this.model = model;
         this.addrReset = +parmsCPU['addrReset'] || 0;
 
+        this.opDecode = PDP10.opKA10.bind(this);
+        this.opUndefined = PDP10.opUndefined.bind(this);
+
         /** @type {IRQ|null} */
         this.irqNext = null;        // the head of the active IRQ list, in priority order
 
@@ -171,7 +174,8 @@ class CPUStatePDP10 extends CPUPDP10 {
      */
     initCPU()
     {
-        this.regPC = this.pcLast = this.addrReset;
+        this.regEA = 0;
+        this.regPC = this.lastPC = this.addrReset;
 
         /*
          * This is queried and displayed by the Panel when it's not displaying its own ADDRESS register
@@ -180,7 +184,7 @@ class CPUStatePDP10 extends CPUPDP10 {
          *
          * We initialize it to the current PC.
          */
-        this.addrLast = this.regPC;
+        this.lastAddr = this.regPC;
 
         /*
          * opFlags contains various conditions that stepCPU() needs to be aware of.
@@ -272,9 +276,10 @@ class CPUStatePDP10 extends CPUPDP10 {
     {
         var state = new State(this);
         state.set(0, [
+            this.regEA,
             this.regPC,
-            this.pcLast,
-            this.addrLast,
+            this.lastPC,
+            this.lastAddr,
             this.opFlags
         ]);
         state.set(1, []);
@@ -298,9 +303,10 @@ class CPUStatePDP10 extends CPUPDP10 {
          * of what save() does when it collects a bunch of object properties into an array.
          */
         [
+            this.regEA,
             this.regPC,
-            this.pcLast,
-            this.addrLast,
+            this.lastPC,
+            this.lastAddr,
             this.opFlags
         ] = data[0];
 
@@ -317,17 +323,44 @@ class CPUStatePDP10 extends CPUPDP10 {
     /**
      * getOpcode()
      *
-     * NOTE: This function is nothing more than a convenience, and we fully expect it to be inlined at runtime.
+     * This fetches the next opcode, decodes the low 23 bits (I,X,Y), sets regEA, and returns the
+     * the high 18 bits for further decoding.  While it's true that, in general, only the high 13 bits
+     * are relevant to the operation, it's cleaner to treat the original opcode as two half-words.
      *
      * @this {CPUStatePDP10}
      * @return {number}
      */
     getOpcode()
     {
-        var pc = this.pcLast = this.regPC;
+        var pc = this.lastPC = this.regPC;
         var opCode = this.readWord(pc);
         this.regPC = (pc + 1) % PDP10.ADDR_LIMIT;
-        return opCode;
+        var e, x, r = opCode, nMaxIndirect = 4;
+        while (true) {
+            /*
+             * Bits 0-22 (I,X,Y) contain what we call a "reference address" (R), which is used to calculate the
+             * "effective address" (E).  To determine E from R, we must extract I, X, and Y from R, set E to Y,
+             * then add [X] to E if X is non-zero.  If I is zero, then we're done; otherwise, we must set R to [E]
+             * and repeat the process.
+             */
+            e = r & PDP10.OPCODE.Y_MASK;
+            x = (r >> PDP10.OPCODE.X_SHIFT) & PDP10.OPCODE.X_MASK;
+            if (x) e = (e + this.readWord(x)) & PDP10.ADDR_MASK;
+            if (!(r & PDP10.OPCODE.I_BIT)) break;
+            /*
+             * We allow this process to repeat only a few times, because more than that suggests we're executing
+             * garbage.  Also, allowing this to loop endlessly would hang the simulation; if it turns out there is
+             * real-world PDP-10 code that requires a long series of indirections, we should factor this logic out
+             * as a separate operation.  Note that would mean passing the full opcode on to the next level of decoding.
+             */
+            if (--nMaxIndirect < 0) {
+                this.bus.fault(r);
+                break;
+            }
+            r = this.readWord(e);
+        }
+        this.regEA = e;
+        return (opCode / PDP10.ADDR_LIMIT)|0;
     }
 
     /**
@@ -367,7 +400,7 @@ class CPUStatePDP10 extends CPUPDP10 {
      */
     getLastAddr()
     {
-        return this.addrLast;
+        return this.lastAddr;
     }
 
     /**
@@ -378,7 +411,7 @@ class CPUStatePDP10 extends CPUPDP10 {
      */
     getLastPC()
     {
-        return this.pcLast;
+        return this.lastPC;
     }
 
     /**
@@ -659,7 +692,7 @@ class CPUStatePDP10 extends CPUPDP10 {
      */
     readWordFromPhysical(addr)
     {
-        return this.bus.getWord(this.addrLast = addr);
+        return this.bus.getWord(this.lastAddr = addr);
     }
 
     /**
@@ -673,7 +706,7 @@ class CPUStatePDP10 extends CPUPDP10 {
      */
     writeWordToPhysical(addr, data)
     {
-        this.bus.setWord(this.addrLast = addr, data);
+        this.bus.setWord(this.lastAddr = addr, data);
     }
 
     /**
@@ -775,8 +808,7 @@ class CPUStatePDP10 extends CPUPDP10 {
             this.opFlags &= PDP10.OPFLAG.PRESERVE;
 
             var opCode = this.getOpcode();
-
-            // this.decode(opCode);
+            this.opDecode(opCode);
 
         } while (this.nStepCycles > 0);