diff --git a/src/asm_pdp8.cpp b/src/asm_pdp8.cpp
index 4be6b574..46d64b57 100644
--- a/src/asm_pdp8.cpp
+++ b/src/asm_pdp8.cpp
@@ -280,7 +280,7 @@ Error AsmPdp8::defineDec6String(StrScanner &scan, Insn &insn) {
             if (c == 0)
                 return insn.setErrorIf(p, MISSING_CLOSING_DELIMITER);
             uint8_t dec6 = (c < 0x40) ? c : c - 0x40;
-            if (c < 0x20 || c >= 0x60) {
+            if (c <= 0x20 || c >= 0x60) {
                 error.setErrorIf(p, ILLEGAL_CONSTANT);
                 dec6 = 0;
             }
diff --git a/src/dis_pdp8.cpp b/src/dis_pdp8.cpp
index 3b2a4e0e..8120c269 100644
--- a/src/dis_pdp8.cpp
+++ b/src/dis_pdp8.cpp
@@ -42,7 +42,7 @@ void DisPdp8::reset() {
 
 Error DisPdp8::decodeImpl(DisMemory &memory, Insn &_insn, StrBuffer &out) const {
     DisInsn insn(_insn, memory, out);
-    const auto opc = insn.readUint16();
+    const auto opc = insn.readUint12();
     insn.setOpCode(opc);
     if (TABLE.searchOpCode(cpuType(), insn, out))
         return _insn.setError(insn);
diff --git a/src/insn_pdp8.h b/src/insn_pdp8.h
index 7d2ab35f..5dbc9f5c 100644
--- a/src/insn_pdp8.h
+++ b/src/insn_pdp8.h
@@ -36,11 +36,12 @@ struct EntryInsn : EntryInsnBase<Config, Entry> {
 struct AsmInsn final : AsmInsnImpl<Config>, EntryInsn {
     AsmInsn(Insn &insn) : AsmInsnImpl(insn) {}
 
-    void emitInsn() { emitUint16(opCode()); }
+    void emitInsn() { emitUint16(opCode() & 07777); }
 };
 
 struct DisInsn final : DisInsnImpl<Config>, EntryInsn {
     DisInsn(Insn &insn, DisMemory &memory, const StrBuffer &out) : DisInsnImpl(insn, memory, out) {}
+    uint16_t readUint12() { return readUint16() & 07777; }
 };
 
 }  // namespace pdp8
diff --git a/test/test_asm_pdp8.cpp b/test/test_asm_pdp8.cpp
index 7c954515..d6941716 100644
--- a/test/test_asm_pdp8.cpp
+++ b/test/test_asm_pdp8.cpp
@@ -52,22 +52,23 @@ void test_cpu() {
 }
 
 void test_memory_reference() {
+    constexpr uint16_t ZP = 0000;
     constexpr uint16_t IA = 0400;
-    ATEST(04000, "AND 0022",   00000|0022);
-    ATEST(04000, "AND Z 0022", 00000|0022);
-    ATEST(04000, "AND 0022+7", 00000|0031);
-    ATEST(04000, "AND I 0043", 00000|0043|IA);
-    ATEST(04000, "AND I Z 0043", 00000|0043|IA);
-    ATEST(04000, "TAD 0064",   01000|0064);
-    ATEST(04000, "TAD I 0105", 01000|0105|IA);
-    ATEST(04000, "ISZ 0170",   02000|0170);
-    ATEST(04000, "ISZ I 0147", 02000|0147|IA);
-    ATEST(04000, "DCA 0170",   03000|0170);
-    ATEST(04000, "DCA I 0011", 03000|0011|IA);
-    ATEST(04000, "JMS 0032",   04000|0032);
-    ATEST(04000, "JMS I 0053", 04000|0053|IA);
-    ATEST(04000, "JMP 0074",   05000|0074);
-    ATEST(04000, "JMP I 0115", 05000|0115|IA);
+    ATEST(04000, "AND 0022",   00000|0022|ZP);
+    ATEST(04000, "AND Z 0022", 00000|0022|ZP);
+    ATEST(04000, "AND 0022+7", 00000|0031|ZP);
+    ATEST(04000, "AND I 0043", 00000|0043|ZP|IA);
+    ATEST(04000, "AND I Z 0043", 00000|0043|ZP|IA);
+    ATEST(04000, "TAD 0064",   01000|0064|ZP);
+    ATEST(04000, "TAD I 0105", 01000|0105|ZP|IA);
+    ATEST(04000, "ISZ 0170",   02000|0170|ZP);
+    ATEST(04000, "ISZ I 0147", 02000|0147|ZP|IA);
+    ATEST(04000, "DCA 0170",   03000|0170|ZP);
+    ATEST(04000, "DCA I 0011", 03000|0011|ZP|IA);
+    ATEST(04000, "JMS 0032",   04000|0032|ZP);
+    ATEST(04000, "JMS I 0053", 04000|0053|ZP|IA);
+    ATEST(04000, "JMP 0074",   05000|0074|ZP);
+    ATEST(04000, "JMP I 0115", 05000|0115|ZP|IA);
 
     constexpr uint16_t MP = 0200;
     ATEST(04000, "AND 4022",   00000|0022|MP);
@@ -95,12 +96,12 @@ void test_memory_reference() {
     symtab.intern(04017, "label4017");
     symtab.intern(04020, "label4020");
 
-    ATEST(04000, "AND zero020",     00000|0020);
-    ATEST(04000, "AND z zero020",   00000|0020);
-    ATEST(04000, "AND i auto017",   00000|0017|IA);
-    ATEST(04000, "AND i z auto017", 00000|0017|IA);
-    ATEST(04000, "AND i label4017", 00000|0017|MP|IA);
+    ATEST(04000, "AND zero020",     00000|0020|ZP);
+    ATEST(04000, "AND z zero020",   00000|0020|ZP);
+    ATEST(04000, "AND i auto017",   00000|0017|ZP|IA);
+    ATEST(04000, "AND i z auto017", 00000|0017|ZP|IA);
     ATEST(04000, "AND label4020",   00000|0020|MP);
+    ATEST(04000, "AND i label4017", 00000|0017|MP|IA);
 }
 
 void test_group1() {
@@ -720,17 +721,24 @@ void test_data_constant() {
     TEST("           10", 00010);
     ERUS("DUBL UNDEF", "UNDEF", 0, 0);
 
-    TEST(R"("A)", 'A'|0200);
-    TEST(R"("Z)", 'Z'|0200);
-    TEST(R"("0)", '0'|0200);
-    TEST(R"("9)", '9'|0200);
-    TEST(R"("()", '('|0200);
-    TEST(R"("[)", '['|0200);
+    TEST(R"("A)", 0301);
+    TEST(R"("Z)", 0332);
+    TEST(R"("0)", 0260);
+    TEST(R"("9)", 0271);
+    TEST(R"("@)", 0300);
+    TEST(R"(" )", 0240);
+    TEST(R"("()", 0250);
+    TEST(R"("[)", 0333);
 
     TEST("TEXT //",     00000);
     TEST("TEXT ATEXTA", 02405, 03024, 0000);
     TEST("TEXT /BOB/",  00217, 00200);
     TEST("TEXT ;BOB;",  00217, 00200);
+    TEST("TEXT /0123456789:;<=>?/", 06061, 06263, 06465, 06667, 07071, 07273, 07475, 07677, 00000);
+    TEST(R"(TEXT ;!"#$%&'()*+,-./;)", 04142, 04344, 04546, 04750, 05152, 05354, 05556, 05700);
+    TEST("TEXT /@ABCDEFGHIJKLMNO/", 00001, 00203, 00405, 00607, 01011, 01213, 01415, 01617, 00000);
+    TEST(R"(TEXT /PQRSTUVWXYZ[\]^_/)", 02021, 02223, 02425, 02627, 03031, 03233, 03435, 03637, 00000);
+    ERRT("TEXT /B B/",  ILLEGAL_CONSTANT, " B/", 00200, 00200);
     ERRT("TEXT /BOb/",  ILLEGAL_CONSTANT, "b/",  00217, 00000);
     ERRT("TEXT /(){3/", ILLEGAL_CONSTANT, "{3/", 05051, 00063, 00000);