diff --git a/decode.c b/decode.c
index ef799da..d806080 100644
--- a/decode.c
+++ b/decode.c
@@ -91,6 +91,7 @@ struct InstrDesc
 #define DESC_LOCK(desc) (((desc)->operand_indices >> 11) & 1)
 #define DESC_VSIB(desc) (((desc)->operand_indices >> 15) & 1)
 #define DESC_OPSIZE(desc) (((desc)->reg_types >> 11) & 7)
+#define DESC_LEGACY(desc) (((desc)->operand_sizes >> 8) & 1)
 #define DESC_SIZE_FIX1(desc) (((desc)->operand_sizes >> 10) & 7)
 #define DESC_SIZE_FIX2(desc) (((desc)->operand_sizes >> 13) & 3)
 #define DESC_INSTR_WIDTH(desc) (((desc)->operand_sizes >> 15) & 1)
@@ -400,9 +401,7 @@ direct:
     unsigned op_size;
     unsigned op_size_alt = 0;
     if (!(DESC_OPSIZE(desc) & 4)) {
-        if (DESC_OPSIZE(desc) == 1)
-            op_size = 1;
-        else if (mode == DECODE_64)
+        if (mode == DECODE_64)
             op_size = ((prefix_rex & PREFIX_REXW) || DESC_OPSIZE(desc) == 3) ? 4 :
                               UNLIKELY(prefixes[PF_66] && !DESC_IGN66(desc)) ? 2 :
                                                            DESC_OPSIZE(desc) ? 4 :
@@ -632,7 +631,7 @@ direct:
         // 2 = memory, address-sized, used for mov with moffs operand
         FdOp* operand = &instr->operands[DESC_IMM_IDX(desc)];
         operand->type = FD_OT_MEM;
-        operand->size = op_size;
+        operand->size = operand_sizes[(desc->operand_sizes >> 6) & 3];
         operand->reg = FD_REG_NONE;
         operand->misc = FD_REG_NONE;
 
@@ -722,31 +721,6 @@ direct:
         }
     }
 
-    if (instr->type == FDI_XCHG_NOP)
-    {
-        // Only 4890, 90, and 6690 are true NOPs.
-        if (instr->operands[0].reg == 0 && instr->operands[1].reg == 0)
-        {
-            instr->operands[0].type = FD_OT_NONE;
-            instr->operands[1].type = FD_OT_NONE;
-            instr->type = FDI_NOP;
-        }
-        else
-        {
-            instr->type = FDI_XCHG;
-        }
-    }
-
-    if (UNLIKELY(instr->type == FDI_3DNOW))
-    {
-        unsigned opc3dn = instr->imm;
-        if (opc3dn & 0x40)
-            return FD_ERR_UD;
-        uint64_t msk = opc3dn & 0x80 ? 0x88d144d144d14400 : 0x30003000;
-        if (!(msk >> (opc3dn & 0x3f) & 1))
-            return FD_ERR_UD;
-    }
-
 skip_modrm:
     if (UNLIKELY(prefixes[PF_LOCK])) {
         if (!DESC_LOCK(desc) || instr->operands[0].type != FD_OT_MEM)
@@ -754,7 +728,10 @@ skip_modrm:
         instr->flags |= FD_FLAG_LOCK;
     }
 
-    if (UNLIKELY(op_size == 1 || instr->type == FDI_MOVSX || instr->type == FDI_MOVZX)) {
+    if (UNLIKELY(DESC_LEGACY(desc))) {
+        // Without REX prefix, convert one-byte GP regs to high-byte regs
+        // This actually only applies to SZ8/MOVSX/MOVZX; but no VEX-encoded
+        // instructions have a byte-sized GP register in the first two operands.
         if (!(prefix_rex & PREFIX_REX)) {
             for (int i = 0; i < 2; i++) {
                 FdOp* operand = &instr->operands[i];
@@ -765,10 +742,33 @@ skip_modrm:
                     operand->misc = FD_RT_GPH;
             }
         }
+
+        if (instr->type == FDI_XCHG_NOP) {
+            // Only 4890, 90, and 6690 are true NOPs.
+            if (instr->operands[0].reg == 0 && instr->operands[1].reg == 0) {
+                instr->operands[0].type = FD_OT_NONE;
+                instr->operands[1].type = FD_OT_NONE;
+                instr->type = FDI_NOP;
+            } else {
+                instr->type = FDI_XCHG;
+            }
+        }
+
+        if (UNLIKELY(instr->type == FDI_3DNOW)) {
+            unsigned opc3dn = instr->imm;
+            if (opc3dn & 0x40)
+                return FD_ERR_UD;
+            uint64_t msk = opc3dn & 0x80 ? 0x88d144d144d14400 : 0x30003000;
+            if (!(msk >> (opc3dn & 0x3f) & 1))
+                return FD_ERR_UD;
+        }
+
+        instr->operandsz = 0;
+    } else {
+        instr->operandsz = UNLIKELY(DESC_INSTR_WIDTH(desc)) ? op_size - 1 : 0;
     }
 
     instr->size = off;
-    instr->operandsz = UNLIKELY(DESC_INSTR_WIDTH(desc)) ? op_size - 1 : 0;
 
     return off;
 }
diff --git a/parseinstrs.py b/parseinstrs.py
index 9e726db..fb02815 100644
--- a/parseinstrs.py
+++ b/parseinstrs.py
@@ -24,7 +24,8 @@ INSTR_FLAGS_FIELDS, INSTR_FLAGS_SIZES = zip(*[
     ("modreg_size", 2),
     ("vexreg_size", 2),
     ("imm_size", 2),
-    ("unused2", 2),
+    ("legacy", 1),
+    ("unused2", 1),
     ("size_fix1", 3),
     ("size_fix2", 2),
     ("instr_width", 1),
@@ -274,12 +275,19 @@ class InstrDesc(NamedTuple):
                 raise Exception(f"conflicting vector operand sizes in {self}")
         else: # either empty or GP operand size
             dynsizes = [OpKind.SZ_OP]
-            if "SZ8" in self.flags: extraflags["opsize"] = 1
+            if "SZ8" in self.flags:
+                extraflags["opsize"] = 1
+                dynsizes = []
             if "D64" in self.flags: extraflags["opsize"] = 2
             if "F64" in self.flags: extraflags["opsize"] = 3
             extraflags["instr_width"] = "INSTR_WIDTH" in self.flags
             extraflags["lock"] = "LOCK" in self.flags
 
+        if "SZ8" in self.flags or mnem in ("MOVSX", "MOVZX", "XCHG_NOP", "3DNOW"):
+            if "SZ8" not in self.flags and "INSTR_WIDTH" in self.flags:
+                raise Exception("legacy instr with +w without SZ8")
+            extraflags["legacy"] = 1
+
         imm_byte = self.imm_size(4) == 1
         extraflags["imm_control"] = flags.imm_control | imm_byte
 
@@ -297,7 +305,7 @@ class InstrDesc(NamedTuple):
         for i, opkind in enumerate(self.operands):
             sz = self.OPKIND_SIZES[opkind.size] if opkind.size >= 0 else opkind.size
             if opkind.kind == "IMM":
-                if imm_byte and sz not in (dynsizes[0], 1):
+                if imm_byte and sz not in [1] + dynsizes[:1]:
                     raise Exception(f"imm_byte with opsize {sz} in {self}")
                 extraflags[f"imm_size"] = sz == 1 if imm_byte else sizes.index(sz)
             else: