diff --git a/modules/pdp10/lib/debugger.js b/modules/pdp10/lib/debugger.js
index 737946e29..16a33eca1 100644
--- a/modules/pdp10/lib/debugger.js
+++ b/modules/pdp10/lib/debugger.js
@@ -1786,11 +1786,27 @@ class DebuggerPDP10 extends Debugger {
                             opCode = -1;
                             break;
                         }
-                        if (operand < 0 || operand > PDP10.OPCODE.X_MASK) {
-                            operand &= PDP10.OPCODE.X_MASK;
-                            if (MAXDEBUG) this.println("index (" + sOperand + ") truncated to " + this.toStrBase(operand));
-                        }
-                        opCode += operand << PDP10.OPCODE.X_SHIFT;
+                        /*
+                         * Here's a fun tidbit from the April 1978 MACRO-10 manual, p. 4-5:
+                         *
+                         *      NOTE: To assemble the index, MACRO places the index register address in a fullword of storage,
+                         *      swaps its halfwords, and then adds the swapped word to the instruction word.
+                         *
+                         * Which means that an instruction like this (where AC is zero):
+                         *
+                         *        8839	037653	205 00 0 00 400000 		MOVSI	AC,(1B<^O<AC>>)	;INITIALIZE AC
+                         *
+                         * produces an instruction that does NOT use indexing at all, even though it is coded as such.  So my
+                         * simplistic masking of the index operand with PDP10.OPCODE.X_MASK, while logical, was completely wrong:
+                         *
+                         *      if (operand < 0 || operand > PDP10.OPCODE.X_MASK) {
+                         *          operand &= PDP10.OPCODE.X_MASK;
+                         *          if (MAXDEBUG) this.println("index (" + sOperand + ") truncated to " + this.toStrBase(operand));
+                         *      }
+                         *      opCode += operand << PDP10.OPCODE.X_SHIFT;
+                         */
+                        operand = PDP10.SWAP(this.truncate(operand, 36, true));
+                        opCode += operand;
                     }
 
                     sOperand = match[2];
diff --git a/versions/pdpjs/1.34.3/pdp10-dbg.js b/versions/pdpjs/1.34.3/pdp10-dbg.js
index b2fc07501..386640c43 100644
--- a/versions/pdpjs/1.34.3/pdp10-dbg.js
+++ b/versions/pdpjs/1.34.3/pdp10-dbg.js
@@ -204,8 +204,8 @@ function xd(a,b,c){var d=-1;c||(d=a.v.f(b),2756==d>>23&&a.v.oa==b&&(b=ud(a.v,1))
 function Jf(a,b,c,d){var e=Y(b.B),f=a.Na(b,1),g,h,k,l=0,p=f/Zb|0,q;for(q in Kf)if(g=Kf[q][p&q]){h=+q;p>>=6;switch(h){case 32512:k=Lf;l=p&3;break;case 32256:k=Mf;l=p&7;break;case 29248:k=Nf,l=(p&48)>>2|(p&6)>>1}break}k=k&&k[l]||"";"S"==k&&g>Of&&(k="B");k=Pf[g||0]+k;if(g){if(28700==h)h=f/$b&127,l=L(a,h,-1);else for(h=f>>23&15,l=L(a,h,-1),p=0;l&&p<Qf.length;p++)if(g==Qf[p][0]&&(q=Qf[p][h])){k=Pf[q];l="";break}k=Da(k,8)+(l?l+",":"");f&4194304&&(k+="@");k+=L(a,f&262143,-1);(f=f>>18&15)&&(k+="("+L(a,f,
 -1)+")")}else k=Da(k,8)+qf(a,f);f=k;g="";h=L(a,e.B,18)+":";if(-1!==e.B&&-1!==b.B){do if(k=a.Na(e,1),g+=" "+qf(a,k),null==e.B)break;while(e.B!=b.B)}h+=Da(g,16)+f;c&&(h=Da(h,48)+";"+(c||""),h=a.v.A.Ua?h+("cycles="+hd(a.v).toString()+" cs="+n(a.v.Ha)):h+(null!=d?"="+d.toString():""));return h}
 function Rf(a,b,c,d,e){var f=-1,g,h;if(b){for(var k=b.toUpperCase(),l=0;l<Qf.length;l++)for(var p in Qf[l])if(+p&&(h=Qf[l][p],k==Pf[h])){k=Pf[Qf[l][0]];c&&(c=L(a,+p)+","+c);break}for(var q in Kf){g=+q;p=Kf[q];switch(g){case 32512:l=Lf;break;case 32256:l=Mf;break;case 29248:l=Nf;break;default:l=[""]}for(var D in p){h=p[D];for(var F=0;F<l.length;F++){var J=l[F];"S"==J&&h>Of&&(J="B");if(k==Pf[h]+J){f=29248!=g?F:(F&3)<<1|(F&12)<<2;f=(D|f<<6)*Zb;break}}if(0<=f)break}if(0<=f)break}!(0>f)||c&&c.match(/^[0-9A-Z$%.?]/i)||
-(c=b+c,b="",f=0)}else c&&(f=g=0);if(0<=f&&c)for(k=c.split(","),2<k.length&&(e||a.u("too many operands: "+c),k.length=0,f=-1),q=0;q<k.length;q++)if(h=k[q].trim()){D=h.match(/(@?)([^(]*)\(?([^)]*)\)?/);if(!D){e||a.u("unknown operand: "+h);f=-1;break}D[1]&&(f+=4194304);if(h=D[3]){h=$e(a,h,e);if(void 0==h){f=-1;break}if(0>h||15<h)h&=15;f+=h<<18}h=D[2];if(q||1==k.length)h=h?h.replace(/(^|[^0-9])\./g,"$1"+L(a,d,18)):"0";h=$e(a,h,e);if(void 0==h){f=-1;break}if(!q&&1<k.length)if(28700==g){if(0>h||127<h)h&=
-127;f+=h*$b}else{if(0>h||15<h)h&=15;f+=h<<23}else{if(b||q)if(0>h||262143<h)h&=Tb;f+=h}}0>f&&!e&&a.u("unknown instruction: "+b+" "+c);return f}function ef(a){var b,c;a.i=["bp"];if(a.N)for(b=1;b<a.N.length;b++){c=a.N[b];c=hf(c);var d=a.G;Yc(d.b[c>>>d.i],!1)}a.N=["br"];if(a.K)for(b=1;b<a.K.length;b++)c=a.K[b],c=hf(c),d=a.G,Yc(d.b[c>>>d.i],!0);a.K=["bw"];a.xa=0;a.pa=0}
+(c=b+c,b="",f=0)}else c&&(f=g=0);if(0<=f&&c)for(k=c.split(","),2<k.length&&(e||a.u("too many operands: "+c),k.length=0,f=-1),q=0;q<k.length;q++)if(h=k[q].trim()){D=h.match(/(@?)([^(]*)\(?([^)]*)\)?/);if(!D){e||a.u("unknown operand: "+h);f=-1;break}D[1]&&(f+=4194304);if(h=D[3]){h=$e(a,h,e);if(void 0==h){f=-1;break}h=W(X(a,h,36,!0));f+=h}h=D[2];if(q||1==k.length)h=h?h.replace(/(^|[^0-9])\./g,"$1"+L(a,d,18)):"0";h=$e(a,h,e);if(void 0==h){f=-1;break}if(!q&&1<k.length)if(28700==g){if(0>h||127<h)h&=127;
+f+=h*$b}else{if(0>h||15<h)h&=15;f+=h<<23}else{if(b||q)if(0>h||262143<h)h&=Tb;f+=h}}0>f&&!e&&a.u("unknown instruction: "+b+" "+c);return f}function ef(a){var b,c;a.i=["bp"];if(a.N)for(b=1;b<a.N.length;b++){c=a.N[b];c=hf(c);var d=a.G;Yc(d.b[c>>>d.i],!1)}a.N=["br"];if(a.K)for(b=1;b<a.K.length;b++)c=a.K[b],c=hf(c),d=a.G,Yc(d.b[c>>>d.i],!0);a.K=["bw"];a.xa=0;a.pa=0}
 m.Ra=function(a,b,c){var d=!0;c||Sf(this,a,b,!1,!0);if(a!=this.i){var e=hf(b);if(-1===e)this.u("invalid address: "+L(this,b.B,18)),d=!1;else{var f=this.G;f.b[e>>>f.i].Ra(e&16383,a==this.K)}}d&&(a.push(b),c?b.qa=!0:(Tf(this,a,a.length-1,"set"),ff(this)));return d};function Sf(a,b,c,d,e){var f=!1;c=hf(c);for(var g=1;g<b.length;g++){var h=b[g];if(c==hf(h)&&(!d||h.qa)){f=!0;h.qa||e||Tf(a,b,g,"cleared");b.splice(g,1);b!=a.i&&(d=a.G,Yc(d.b[c>>>d.i],b==a.K));h.qa||ff(a);break}}return f}
 function Uf(a,b){for(var c=1;c<b.length;c++)Tf(a,b,c);return b.length-1}function Tf(a,b,c,d){c=b[c];a.u(b[0]+" "+L(a,c.B,18)+(d?" "+d:c.hb?' "'+c.hb+'"':""))}function If(a,b){if(void 0!==b)Zc(a,b,1,a.i,!0),a.O=0;else for(b=1;b<a.i.length;b++){var c=a.i[b];if(c.qa){if(!Sf(a,a.i,c,!0))break;b=0}}}
 function Zc(a,b,c,d,e){var f=!1;if(!a.xa++)for(var g=1;!f&&g<d.length;g++){var h=d[g];if(!e||h.qa)for(var k=hf(h)&(d==a.i?65535:-1),l=0;l<c;l++)if(b+l==k){var p,f=!0;h.qa&&(Sf(a,d,h,!0),e=!0);if(p=h.zb){for(var f=!1,q=0;q<p.length;q++)if(!Vf(a,p[q],!0)){if(p[q].indexOf("if")){f=!0;break}for(var D=q+1;D<p.length&&p[D].indexOf("else");D++)q++;if(D==p.length){f=!0;break}}a.v.A.P||(f=!0)}if(f){e||Tf(a,d,g,"hit");break}}}a.xa--;return f}