diff --git a/src/cmd/5a/lex.c b/src/cmd/5a/lex.c
index 252a282a05..e569fe62d3 100644
--- a/src/cmd/5a/lex.c
+++ b/src/cmd/5a/lex.c
@@ -405,6 +405,7 @@ struct
 	"MRC",		LTYPEJ, 1,
 
 	"PLD",		LTYPEPLD, APLD,
+	"UNDEF",	LTYPEE,	AUNDEF,
 	0
 };
 
diff --git a/src/cmd/5l/5.out.h b/src/cmd/5l/5.out.h
index 08a60d0642..3c726e924b 100644
--- a/src/cmd/5l/5.out.h
+++ b/src/cmd/5l/5.out.h
@@ -185,6 +185,8 @@ enum	as
 	ASTREXD,
 
 	APLD,
+	
+	AUNDEF,
 
 	ALAST,
 };
diff --git a/src/cmd/5l/asm.c b/src/cmd/5l/asm.c
index c8e50305c6..22695b0716 100644
--- a/src/cmd/5l/asm.c
+++ b/src/cmd/5l/asm.c
@@ -1791,6 +1791,15 @@ if(debug['G']) print("%ux: %s: arm %d\n", (uint32)(p->pc), p->from.sym->name, p-
 			o1 |= (-p->from.offset) & 0xfff;
 		} else
 			o1 |= p->from.offset & 0xfff;
+	case 96:	/* UNDEF */
+		// This is supposed to be something that stops execution.
+		// It's not supposed to be reached, ever, but if it is, we'd
+		// like to be able to tell how we got there.  Assemble as
+		//	BL $0
+		v = (0 - pc) - 8;
+		o1 = opbra(ABL, C_SCOND_NONE);
+		o1 |= (v >> 2) & 0xffffff;
+		break;
 	}
 	
 	out[0] = o1;
diff --git a/src/cmd/5l/optab.c b/src/cmd/5l/optab.c
index 76f2d4dda5..be25b6ed61 100644
--- a/src/cmd/5l/optab.c
+++ b/src/cmd/5l/optab.c
@@ -233,6 +233,8 @@ Optab	optab[] =
 	{ ASTREXD,	C_SOREG,C_REG,	C_REG,		92, 4, 0 },
 
 	{ APLD,		C_SOREG,C_NONE,	C_NONE,		95, 4, 0 },
+	
+	{ AUNDEF,		C_NONE,	C_NONE,	C_NONE,		96, 4, 0 },
 
 	{ AXXX,		C_NONE,	C_NONE,	C_NONE,		 0, 4, 0 },
 };
diff --git a/src/cmd/5l/pass.c b/src/cmd/5l/pass.c
index 34932fd4a0..50593ced97 100644
--- a/src/cmd/5l/pass.c
+++ b/src/cmd/5l/pass.c
@@ -119,7 +119,7 @@ loop:
 				i--;
 				continue;
 			}
-			if(a == AB || (a == ARET && q->scond == 14) || a == ARFE)
+			if(a == AB || (a == ARET && q->scond == 14) || a == ARFE || a == AUNDEF)
 				goto copy;
 			if(q->cond == P || (q->cond->mark&FOLL))
 				continue;
@@ -140,7 +140,7 @@ loop:
 				}
 				(*last)->link = r;
 				*last = r;
-				if(a == AB || (a == ARET && q->scond == 14) || a == ARFE)
+				if(a == AB || (a == ARET && q->scond == 14) || a == ARFE || a == AUNDEF)
 					return;
 				r->as = ABNE;
 				if(a == ABNE)
@@ -166,7 +166,7 @@ loop:
 	p->mark |= FOLL;
 	(*last)->link = p;
 	*last = p;
-	if(a == AB || (a == ARET && p->scond == 14) || a == ARFE){
+	if(a == AB || (a == ARET && p->scond == 14) || a == ARFE || a == AUNDEF){
 		return;
 	}
 	if(p->cond != P)
diff --git a/src/cmd/5l/span.c b/src/cmd/5l/span.c
index 242ba1603d..acacb66bb0 100644
--- a/src/cmd/5l/span.c
+++ b/src/cmd/5l/span.c
@@ -847,6 +847,7 @@ buildop(void)
 		case ASTREXD:
 		case ATST:
 		case APLD:
+		case AUNDEF:
 			break;
 		}
 	}
diff --git a/src/cmd/6a/lex.c b/src/cmd/6a/lex.c
index 23f4637e10..6a1c652654 100644
--- a/src/cmd/6a/lex.c
+++ b/src/cmd/6a/lex.c
@@ -1007,6 +1007,7 @@ struct
 	"PREFETCHT1",		LTYPE2,	APREFETCHT1,
 	"PREFETCHT2",		LTYPE2,	APREFETCHT2,
 	"PREFETCHNTA",		LTYPE2,	APREFETCHNTA,
+	"UNDEF",	LTYPE0,	AUNDEF,
 
 	0
 };
diff --git a/src/cmd/6l/6.out.h b/src/cmd/6l/6.out.h
index 4271944ce4..cd861c038e 100644
--- a/src/cmd/6l/6.out.h
+++ b/src/cmd/6l/6.out.h
@@ -745,6 +745,8 @@ enum	as
 	AMOVQL,
 	ABSWAPL,
 	ABSWAPQ,
+	
+	AUNDEF,
 
 	ALAST
 };
diff --git a/src/cmd/6l/optab.c b/src/cmd/6l/optab.c
index 717d083e99..0716fa4453 100644
--- a/src/cmd/6l/optab.c
+++ b/src/cmd/6l/optab.c
@@ -1294,6 +1294,8 @@ Optab optab[] =
 	
 	{ AMOVQL,	yrl_ml,	Px, 0x89 },
 
+	{ AUNDEF,		ynone,	Px, 0x0f, 0x0b },
+
 	{ AEND },
 	0
 };
diff --git a/src/cmd/6l/pass.c b/src/cmd/6l/pass.c
index 758f61d651..fc89fd8fc3 100644
--- a/src/cmd/6l/pass.c
+++ b/src/cmd/6l/pass.c
@@ -79,6 +79,7 @@ nofollow(int a)
 	case ARETFL:
 	case ARETFQ:
 	case ARETFW:
+	case AUNDEF:
 		return 1;
 	}
 	return 0;
diff --git a/src/cmd/8a/lex.c b/src/cmd/8a/lex.c
index d3a635cdcb..4c3b0e6a32 100644
--- a/src/cmd/8a/lex.c
+++ b/src/cmd/8a/lex.c
@@ -672,6 +672,7 @@ struct
 	"PREFETCHT1",		LTYPE2,	APREFETCHT1,
 	"PREFETCHT2",		LTYPE2,	APREFETCHT2,
 	"PREFETCHNTA",		LTYPE2,	APREFETCHNTA,
+	"UNDEF",	LTYPE0,	AUNDEF,
 
 	0
 };
diff --git a/src/cmd/8l/8.out.h b/src/cmd/8l/8.out.h
index 8329f4a6da..924ba93901 100644
--- a/src/cmd/8l/8.out.h
+++ b/src/cmd/8l/8.out.h
@@ -458,6 +458,8 @@ enum	as
 	APREFETCHNTA,
 	
 	ABSWAPL,
+	
+	AUNDEF,
 
 	ALAST
 };
diff --git a/src/cmd/8l/optab.c b/src/cmd/8l/optab.c
index 81fe25d042..7a588fca49 100644
--- a/src/cmd/8l/optab.c
+++ b/src/cmd/8l/optab.c
@@ -779,6 +779,8 @@ Optab optab[] =
 	{ APREFETCHNTA,	yprefetch,	Pm,	0x18,(00) },
 
 	{ ABSWAPL,	ybswap,	Pm,	0xc8 },
+	
+	{ AUNDEF,		ynone,	Px,	0x0f, 0x0b },
 
 	0
 };
diff --git a/src/cmd/8l/pass.c b/src/cmd/8l/pass.c
index 9704e3530e..27d8d4ee2a 100644
--- a/src/cmd/8l/pass.c
+++ b/src/cmd/8l/pass.c
@@ -75,6 +75,7 @@ nofollow(int a)
 	case ARET:
 	case AIRETL:
 	case AIRETW:
+	case AUNDEF:
 		return 1;
 	}
 	return 0;