diff --git a/asm/assemble.c b/asm/assemble.c
index ecf5c093..46ce9303 100644
--- a/asm/assemble.c
+++ b/asm/assemble.c
@@ -935,15 +935,13 @@ int64_t assemble(int32_t segment, int64_t start, int bits, insn *instruction)
                 nasm_nonfatal("instruction not supported in %d-bit mode", bits);
                 break;
             case MERR_ENCMISMATCH:
-                nasm_nonfatal("specific encoding scheme not available");
+                nasm_nonfatal("instruction not encodable with %s prefix",
+                              prefix_name(instruction->prefixes[PPS_REX]));
                 break;
             case MERR_BADBND:
-                nasm_nonfatal("bnd prefix is not allowed");
-                break;
             case MERR_BADREPNE:
                 nasm_nonfatal("%s prefix is not allowed",
-                              (has_prefix(instruction, PPS_REP, P_REPNE) ?
-                               "repne" : "repnz"));
+                              prefix_name(instruction->prefixes[PPS_REP]));
                 break;
             case MERR_REGSETSIZE:
                 nasm_nonfatal("invalid register set size");
@@ -1644,16 +1642,22 @@ static int64_t calcsize(int32_t segment, int64_t offset, int bits,
         ins->rex &= ~REX_P;        /* Don't force REX prefix due to high reg */
     }
 
-    switch (ins->prefixes[PPS_VEX]) {
+    switch (ins->prefixes[PPS_REX]) {
     case P_EVEX:
         if (!(ins->rex & REX_EV))
             return -1;
         break;
+    case P_VEX:
     case P_VEX3:
     case P_VEX2:
         if (!(ins->rex & REX_V))
             return -1;
         break;
+    case P_REX:
+        if (ins->rex & (REX_V|REX_EV))
+            return -1;
+        ins->rex |= REX_P;      /* Force REX prefix */
+        break;
     default:
         break;
     }
@@ -1687,16 +1691,19 @@ static int64_t calcsize(int32_t segment, int64_t offset, int bits,
             nasm_nonfatal("invalid high-16 register in non-AVX-512");
             return -1;
         }
-        if (ins->rex & REX_EV)
+        if (ins->rex & REX_EV) {
             length += 4;
-        else if (ins->vex_cm != 1 || (ins->rex & (REX_W|REX_X|REX_B)) ||
-                 ins->prefixes[PPS_VEX] == P_VEX3)
+        } else if (ins->vex_cm != 1 || (ins->rex & (REX_W|REX_X|REX_B)) ||
+                 ins->prefixes[PPS_REX] == P_VEX3) {
+            if (ins->prefixes[PPS_REX] == P_VEX2)
+                nasm_nonfatal("instruction not encodable with {vex2} prefix");
             length += 3;
-        else
+        } else {
             length += 2;
+        }
     } else if (ins->rex & REX_MASK) {
         if (ins->rex & REX_H) {
-            nasm_nonfatal("cannot use high register in rex instruction");
+            nasm_nonfatal("cannot use high byte register in rex instruction");
             return -1;
         } else if (bits == 64) {
             length++;
@@ -1849,6 +1856,8 @@ static int emit_prefix(struct out_data *data, const int bits, insn *ins)
         case P_OSP:
             c = 0x66;
             break;
+        case P_REX:
+        case P_VEX:
         case P_EVEX:
         case P_VEX3:
         case P_VEX2:
@@ -1994,7 +2003,7 @@ static void gencode(struct out_data *data, insn *ins)
 
         case 0172:
         {
-            int mask = ins->prefixes[PPS_VEX] == P_EVEX ? 7 : 15;
+            int mask = ins->prefixes[PPS_REX] == P_EVEX ? 7 : 15;
             const struct operand *opy;
 
             c = *codes++;
@@ -2054,7 +2063,7 @@ static void gencode(struct out_data *data, insn *ins)
         case 0270:
             codes += 2;
             if (ins->vex_cm != 1 || (ins->rex & (REX_W|REX_X|REX_B)) ||
-                ins->prefixes[PPS_VEX] == P_VEX3) {
+                ins->prefixes[PPS_REX] == P_VEX3) {
                 bytes[0] = (ins->vex_cm >> 6) ? 0x8f : 0xc4;
                 bytes[1] = (ins->vex_cm & 31) | ((~ins->rex & 7) << 5);
                 bytes[2] = ((ins->rex & REX_W) << (7-3)) |
@@ -2383,11 +2392,12 @@ static enum match_result find_match(const struct itemplate **tempp,
     int i;
 
     /* broadcasting uses a different data element size */
-    for (i = 0; i < instruction->operands; i++)
+    for (i = 0; i < instruction->operands; i++) {
         if (i == broadcast)
             xsizeflags[i] = instruction->oprs[i].decoflags & BRSIZE_MASK;
         else
             xsizeflags[i] = instruction->oprs[i].type & SIZE_MASK;
+    }
 
     merr = MERR_INVALOP;
 
@@ -2507,18 +2517,24 @@ static enum match_result matches(const struct itemplate *itemp,
 	return MERR_INVALOP;
 
     /*
-     * {evex} available?
+     * {rex/vexn/evex} available?
      */
-    switch (instruction->prefixes[PPS_VEX]) {
+    switch (instruction->prefixes[PPS_REX]) {
     case P_EVEX:
         if (!itemp_has(itemp, IF_EVEX))
             return MERR_ENCMISMATCH;
         break;
+    case P_VEX:
     case P_VEX3:
     case P_VEX2:
         if (!itemp_has(itemp, IF_VEX))
             return MERR_ENCMISMATCH;
         break;
+    case P_REX:
+        if (itemp_has(itemp, IF_VEX) || itemp_has(itemp, IF_EVEX) ||
+            bits != 64)
+            return MERR_ENCMISMATCH;
+        break;
     default:
         break;
     }
@@ -2667,6 +2683,9 @@ static enum match_result matches(const struct itemplate *itemp,
                      * considered a wildcard match rather than an error.
                      */
                     opsizemissing = true;
+                } else if (is_class(REG_HIGH, type) &&
+                           instruction->prefixes[PPS_REX]) {
+                    return MERR_ENCMISMATCH;
                 }
             } else if (is_broadcast &&
                        (brcast_num !=
@@ -2764,13 +2783,14 @@ static enum match_result matches(const struct itemplate *itemp,
 
 static enum ea_type process_ea(operand *input, ea *output, int bits,
                                int rfield, opflags_t rflags, insn *ins,
-                               const char **errmsg)
+                               const char **errmsgp)
 {
     bool forw_ref = !!(input->opflags & OPFLAG_UNKNOWN);
     int addrbits = ins->addr_size;
     int eaflags = input->eaflags;
+    const char *errmsg = NULL;
 
-    *errmsg = "invalid effective address"; /* Default error message */
+    errmsg = NULL;
 
     output->type    = EA_SCALAR;
     output->rip     = false;
@@ -2793,7 +2813,7 @@ static enum ea_type process_ea(operand *input, ea *output, int bits,
 
         /* broadcasting is not available with a direct register operand. */
         if (input->decoflags & BRDCAST_MASK) {
-            *errmsg = "broadcast not allowed with register operand";
+            errmsg = "broadcast not allowed with register operand";
             goto err;
         }
 
@@ -2809,7 +2829,7 @@ static enum ea_type process_ea(operand *input, ea *output, int bits,
 
         /* Embedded rounding or SAE is not available with a mem ref operand. */
         if (input->decoflags & (ER | SAE)) {
-            *errmsg = "embedded rounding is available only with "
+            errmsg = "embedded rounding is available only with "
                 "register-register operations";
             goto err;
         }
@@ -2838,7 +2858,7 @@ static enum ea_type process_ea(operand *input, ea *output, int bits,
             }
 
             if (bits == 64 && !(IP_REL & ~input->type) && (eaflags & EAF_SIB)) {
-                *errmsg = "instruction requires SIB encoding, cannot be RIP-relative";
+                errmsg = "instruction requires SIB encoding, cannot be RIP-relative";
                 goto err;
             }
 
@@ -3224,6 +3244,14 @@ static enum ea_type process_ea(operand *input, ea *output, int bits,
     return output->type;
 
 err:
+    if (!errmsg) {
+        /* Default error message */
+        static char invalid_address_msg[40];
+        snprintf(invalid_address_msg, sizeof invalid_address_msg,
+                 "invalid %d-bit effective address", bits);
+        errmsg = invalid_address_msg;
+    }
+    *errmsgp = errmsg;
     return output->type = EA_INVALID;
 }
 
diff --git a/asm/parser.c b/asm/parser.c
index 584e40c9..daafa920 100644
--- a/asm/parser.c
+++ b/asm/parser.c
@@ -55,50 +55,6 @@ static int end_expression_next(void);
 
 static struct tokenval tokval;
 
-static int prefix_slot(int prefix)
-{
-    switch (prefix) {
-    case P_WAIT:
-        return PPS_WAIT;
-    case R_CS:
-    case R_DS:
-    case R_SS:
-    case R_ES:
-    case R_FS:
-    case R_GS:
-        return PPS_SEG;
-    case P_LOCK:
-        return PPS_LOCK;
-    case P_REP:
-    case P_REPE:
-    case P_REPZ:
-    case P_REPNE:
-    case P_REPNZ:
-    case P_XACQUIRE:
-    case P_XRELEASE:
-    case P_BND:
-    case P_NOBND:
-        return PPS_REP;
-    case P_O16:
-    case P_O32:
-    case P_O64:
-    case P_OSP:
-        return PPS_OSIZE;
-    case P_A16:
-    case P_A32:
-    case P_A64:
-    case P_ASP:
-        return PPS_ASIZE;
-    case P_EVEX:
-    case P_VEX3:
-    case P_VEX2:
-        return PPS_VEX;
-    default:
-        nasm_panic("Invalid value %d passed to prefix_slot()", prefix);
-        return -1;
-    }
-}
-
 static void process_size_override(insn *result, operand *op)
 {
     if (tasm_compatible_mode) {
@@ -185,7 +141,7 @@ static void process_size_override(insn *result, operand *op)
 }
 
 /*
- * Brace decorators are are parsed here.  opmask and zeroing
+ * Braced keywords are are parsed here.  opmask and zeroing
  * decorators can be placed in any order.  e.g. zmm1 {k2}{z} or zmm2
  * {z}{k3} decorator(s) are placed at the end of an operand.
  */
@@ -715,42 +671,51 @@ restart_parse:
     if (i == TOKEN_EOS)
         goto fail;
 
-    while (i == TOKEN_PREFIX ||
-           (i == TOKEN_REG && IS_SREG(tokval.t_integer))) {
-        first = false;
+    while (i) {
+        int slot = PPS_SEG;
 
-        /*
-         * Handle special case: the TIMES prefix.
-         */
-        if (i == TOKEN_PREFIX && tokval.t_integer == P_TIMES) {
-            expr *value;
+        if (i == TOKEN_PREFIX) {
+            slot = tokval.t_inttwo;
 
-            i = stdscan(NULL, &tokval);
-            value = evaluate(stdscan, NULL, &tokval, NULL, pass_stable(), NULL);
-            i = tokval.t_type;
-            if (!value)                  /* Error in evaluator */
-                goto fail;
-            if (!is_simple(value)) {
-                nasm_nonfatal("non-constant argument supplied to TIMES");
-                result->times = 1L;
-            } else {
-                result->times = value->value;
-                if (value->value < 0) {
-                    nasm_nonfatalf(ERR_PASS2, "TIMES value %"PRId64" is negative", value->value);
-                    result->times = 0;
+            if (slot == PPS_TIMES) {
+                /* TIMES is a very special prefix */
+                expr *value;
+
+                i = stdscan(NULL, &tokval);
+                value = evaluate(stdscan, NULL, &tokval, NULL,
+                                 pass_stable(), NULL);
+                i = tokval.t_type;
+                if (!value)                  /* Error in evaluator */
+                    goto fail;
+                if (!is_simple(value)) {
+                    nasm_nonfatal("non-constant argument supplied to TIMES");
+                    result->times = 1;
+                } else {
+                    result->times = value->value;
+                    if (value->value < 0) {
+                        nasm_nonfatalf(ERR_PASS2, "TIMES value %"PRId64" is negative", value->value);
+                        result->times = 0;
+                    }
                 }
+                first = false;
+                continue;
             }
+        } else if (i == TOKEN_REG && IS_SREG(tokval.t_integer)) {
+            slot = PPS_SEG;
+            first = false;
         } else {
-            int slot = prefix_slot(tokval.t_integer);
-            if (result->prefixes[slot]) {
-               if (result->prefixes[slot] == tokval.t_integer)
-                    nasm_warn(WARN_OTHER, "instruction has redundant prefixes");
-               else
-                    nasm_nonfatal("instruction has conflicting prefixes");
-            }
-            result->prefixes[slot] = tokval.t_integer;
-            i = stdscan(NULL, &tokval);
+            break;              /* Not a prefix */
         }
+
+        if (result->prefixes[slot]) {
+            if (result->prefixes[slot] == tokval.t_integer)
+                nasm_warn(WARN_OTHER, "instruction has redundant prefixes");
+            else
+                nasm_nonfatal("instruction has conflicting prefixes");
+        }
+        result->prefixes[slot] = tokval.t_integer;
+        i = stdscan(NULL, &tokval);
+        first = false;
     }
 
     if (i != TOKEN_INSN) {
diff --git a/asm/tokens.dat b/asm/tokens.dat
index 356b39a2..0a782464 100644
--- a/asm/tokens.dat
+++ b/asm/tokens.dat
@@ -1,6 +1,6 @@
 ## --------------------------------------------------------------------------
 ##
-##   Copyright 1996-2016 The NASM Authors - All Rights Reserved
+##   Copyright 1996-2021 The NASM Authors - All Rights Reserved
 ##   See the file AUTHORS included with the NASM distribution for
 ##   the specific copyright holders.
 ##
@@ -46,28 +46,45 @@
 % TOKEN_QMARK, 0, 0, 0
 ?
 
-% TOKEN_PREFIX, 0, 0, P_*
+% TOKEN_PREFIX, PPS_ASIZE, 0, P_*
 a16
 a32
 a64
 asp
+
+% TOKEN_PREFIX, PPS_LOCK, 0, P_*
 lock
+
+% TOKEN_PREFIX, PPS_OSIZE, 0, P_*
 o16
 o32
 o64
 osp
+
+% TOKEN_PREFIX, PPS_REP, 0, P_*
 rep
 repe
 repne
 repnz
 repz
-times
-wait
 xacquire
 xrelease
 bnd
 nobnd
 
+% TOKEN_PREFIX, PPS_TIMES, 0, P_*
+times
+
+% TOKEN_PREFIX, PPS_WAIT, 0, P_*
+wait
+
+% TOKEN_PREFIX, PPS_REX, TFLAG_BRC, P_*
+rex
+evex
+vex
+vex3
+vex2
+
 % TOKEN_SIZE, SIZE_*, 0, S_*
 byte
 word
@@ -154,11 +171,6 @@ rz-sae
 sae
 z
 
-% TOKEN_PREFIX, 0, TFLAG_BRC, P_*
-evex
-vex3
-vex2
-
 # Multi-character operators. Used in ppscan().
 % TOKEN_SHR, 0, 0, 0
 >>
diff --git a/common/common.c b/common/common.c
index 13237994..aa778055 100644
--- a/common/common.c
+++ b/common/common.c
@@ -1,6 +1,6 @@
 /* ----------------------------------------------------------------------- *
  *   
- *   Copyright 1996-2016 The NASM Authors - All Rights Reserved
+ *   Copyright 1996-2021 The NASM Authors - All Rights Reserved
  *   See the file AUTHORS included with the NASM distribution for
  *   the specific copyright holders.
  *
@@ -46,14 +46,15 @@
 int globalbits = 0;
 /*
  * Common list of prefix names; ideally should be auto-generated
- * from tokens.dat
+ * from tokens.dat. This MUST match the enum in include/nasm.h.
  */
 const char *prefix_name(int token)
 {
     static const char *prefix_names[] = {
         "a16", "a32", "a64", "asp", "lock", "o16", "o32", "o64", "osp",
         "rep", "repe", "repne", "repnz", "repz", "times", "wait",
-        "xacquire", "xrelease", "bnd"
+        "xacquire", "xrelease", "bnd", "nobnd", "{rex}", "{evex}", "{vex}",
+        "{vex3}", "{vex2}"
     };
     unsigned int prefix = token-PREFIX_ENUM_START;
 
diff --git a/include/nasm.h b/include/nasm.h
index 922a1bb2..4d0dcafd 100644
--- a/include/nasm.h
+++ b/include/nasm.h
@@ -619,7 +619,9 @@ enum prefixes { /* instruction prefixes */
     P_XRELEASE,
     P_BND,
     P_NOBND,
+    P_REX,
     P_EVEX,
+    P_VEX,
     P_VEX3,
     P_VEX2,
     PREFIX_ENUM_LIMIT
@@ -715,14 +717,15 @@ enum ea_type {
  * the introduction of HLE.
  */
 enum prefix_pos {
-    PPS_WAIT,   /* WAIT (technically not a prefix!) */
-    PPS_REP,    /* REP/HLE prefix */
-    PPS_LOCK,   /* LOCK prefix */
-    PPS_SEG,    /* Segment override prefix */
-    PPS_OSIZE,  /* Operand size prefix */
-    PPS_ASIZE,  /* Address size prefix */
-    PPS_VEX,    /* VEX type */
-    MAXPREFIX   /* Total number of prefix slots */
+    PPS_TIMES = -1,     /* TIMES (not a slot, handled separately) */
+    PPS_WAIT  =  0,   	/* WAIT (technically not a prefix!) */
+    PPS_REP,    	/* REP/HLE prefix */
+    PPS_LOCK,   	/* LOCK prefix */
+    PPS_SEG,    	/* Segment override prefix */
+    PPS_OSIZE,  	/* Operand size prefix */
+    PPS_ASIZE,  	/* Address size prefix */
+    PPS_REX,    	/* REX/VEX type */
+    MAXPREFIX   	/* Total number of prefix slots */
 };
 
 /*
diff --git a/test/vex.asm b/test/vex.asm
index 6772c7ce..7c1b3f82 100644
--- a/test/vex.asm
+++ b/test/vex.asm
@@ -1,9 +1,39 @@
 	bits 64
+	add eax,edx
+	{rex} add eax,edx
+	add al,dl
+	{rex} add al,dl
+	add ah,dl
+	comisd xmm0,xmm1
+	{rex} comisd xmm0,xmm1
 	vcomisd xmm0,xmm31
 	vcomisd xmm0,xmm1
+	{vex} vcomisd xmm0,xmm1
 	{vex2} vcomisd xmm0,xmm1
 	{vex3} vcomisd xmm0,xmm1
 	{evex} vcomisd xmm0,xmm1
+	{vex2} vcomisd xmm0,xmm1
+	{vex3} vcomisd xmm0,xmm1
+	{evex} vcomisd xmm0,xmm1
+	{vex} vcomisd xmm0,[r8+rax*1]
+	{vex3} vcomisd xmm0,[r8+rax*1]
+	{evex} vcomisd xmm0,[r8+rax*1]
+	{vex} vcomisd xmm0,[rax+r8*2]
+	{vex3} vcomisd xmm0,[rax+r8*2]
+	{evex} vcomisd xmm0,[rax+r8*2]
+
+	;; These errors may be caught in different passes, so
+	;; some shadows the others...
 %ifdef ERROR
+  %if ERROR <= 1
+	{vex2} vcomisd xmm0,[rax+r8*2]
+	{rex} add ah,dl
+	bits 32
+	mov eax,[r8d]
+  %endif
+  %if ERROR <= 2
+	{rex} vcomisd xmm0,xmm1
+	{vex} add eax,edx
 	{vex3} add eax,edx
+  %endif
 %endif
diff --git a/travis/test/vex.asm b/travis/test/vex.asm
index 6772c7ce..7c1b3f82 100644
--- a/travis/test/vex.asm
+++ b/travis/test/vex.asm
@@ -1,9 +1,39 @@
 	bits 64
+	add eax,edx
+	{rex} add eax,edx
+	add al,dl
+	{rex} add al,dl
+	add ah,dl
+	comisd xmm0,xmm1
+	{rex} comisd xmm0,xmm1
 	vcomisd xmm0,xmm31
 	vcomisd xmm0,xmm1
+	{vex} vcomisd xmm0,xmm1
 	{vex2} vcomisd xmm0,xmm1
 	{vex3} vcomisd xmm0,xmm1
 	{evex} vcomisd xmm0,xmm1
+	{vex2} vcomisd xmm0,xmm1
+	{vex3} vcomisd xmm0,xmm1
+	{evex} vcomisd xmm0,xmm1
+	{vex} vcomisd xmm0,[r8+rax*1]
+	{vex3} vcomisd xmm0,[r8+rax*1]
+	{evex} vcomisd xmm0,[r8+rax*1]
+	{vex} vcomisd xmm0,[rax+r8*2]
+	{vex3} vcomisd xmm0,[rax+r8*2]
+	{evex} vcomisd xmm0,[rax+r8*2]
+
+	;; These errors may be caught in different passes, so
+	;; some shadows the others...
 %ifdef ERROR
+  %if ERROR <= 1
+	{vex2} vcomisd xmm0,[rax+r8*2]
+	{rex} add ah,dl
+	bits 32
+	mov eax,[r8d]
+  %endif
+  %if ERROR <= 2
+	{rex} vcomisd xmm0,xmm1
+	{vex} add eax,edx
 	{vex3} add eax,edx
+  %endif
 %endif
diff --git a/travis/test/vex.bin.t b/travis/test/vex.bin.t
index 2145d4cf..4f015064 100644
Binary files a/travis/test/vex.bin.t and b/travis/test/vex.bin.t differ
diff --git a/travis/test/vex.json b/travis/test/vex.json
index e1db1923..ff5337d4 100644
--- a/travis/test/vex.json
+++ b/travis/test/vex.json
@@ -1,6 +1,6 @@
 [
 	{
-		"description": "Test VEX2/VEX3/EVEX prefix",
+		"description": "Test explicit REX/VEX2/VEX3/EVEX prefix",
 		"id": "vex",
 		"format": "bin",
 		"source": "vex.asm",
@@ -10,11 +10,20 @@
 		]
 	},
 	{
-		"description": "Test VEX3 prefix error",
+		"description": "Test early REX/VEX prefix errors",
 		"ref": "vex",
-		"option": "-Ox -DERROR -o vex.bin.err",
+		"option": "-Ox -DERROR=1 -o vex1.bin.err",
 		"target": [
-			{ "stderr": "vex.stderr" }
+			{ "stderr": "vex1.stderr" }
+		],
+		"error": "expected"
+	},
+	{
+		"description": "Test late REX/VEX prefix errors",
+		"ref": "vex",
+		"option": "-Ox -DERROR=2 -o vex2.bin.err",
+		"target": [
+			{ "stderr": "vex2.stderr" }
 		],
 		"error": "expected"
 	}
diff --git a/travis/test/vex.stderr b/travis/test/vex.stderr
deleted file mode 100644
index b7645c38..00000000
--- a/travis/test/vex.stderr
+++ /dev/null
@@ -1 +0,0 @@
-./travis/test/vex.asm:8: error: specific encoding scheme not available
diff --git a/travis/test/vex1.stderr b/travis/test/vex1.stderr
new file mode 100644
index 00000000..0e9425f6
--- /dev/null
+++ b/travis/test/vex1.stderr
@@ -0,0 +1,3 @@
+./travis/test/vex.asm:29: error: instruction not encodable with {vex2} prefix
+./travis/test/vex.asm:30: error: cannot use high byte register in rex instruction
+./travis/test/vex.asm:32: error: invalid operands in non-64-bit mode
diff --git a/travis/test/vex2.stderr b/travis/test/vex2.stderr
new file mode 100644
index 00000000..f50b5406
--- /dev/null
+++ b/travis/test/vex2.stderr
@@ -0,0 +1,3 @@
+./travis/test/vex.asm:35: error: instruction not encodable with {rex} prefix
+./travis/test/vex.asm:36: error: instruction not encodable with {vex} prefix
+./travis/test/vex.asm:37: error: instruction not encodable with {vex3} prefix