diff --git a/unittests/32Bit_ASM/Primary/Primary_60_2.asm b/unittests/32Bit_ASM/Primary/Primary_60_2.asm
index c8fc84163..941f91f38 100644
--- a/unittests/32Bit_ASM/Primary/Primary_60_2.asm
+++ b/unittests/32Bit_ASM/Primary/Primary_60_2.asm
@@ -24,7 +24,11 @@ mov ebp, 4
 mov esi, 5
 mov edi, 6
 
-o16 pusha
+; operand-size override prefix
+; Nasm complains if o16 is used
+; `warning: invalid operand size prefix o16, must be o32`
+db 0x66
+pusha
 
 ; Invert the order
 mov ax, [esp + 2 * 0]
diff --git a/unittests/32Bit_ASM/Primary/Primary_61_2.asm b/unittests/32Bit_ASM/Primary/Primary_61_2.asm
index cbf41ea39..a7c155aea 100644
--- a/unittests/32Bit_ASM/Primary/Primary_61_2.asm
+++ b/unittests/32Bit_ASM/Primary/Primary_61_2.asm
@@ -33,6 +33,10 @@ push word 0x2
 push word 0x1
 push word 0x0
 
-o16 popa
+; operand-size override prefix
+; Nasm complains if o16 is used
+; `warning: invalid operand size prefix o16, must be o32`
+db 0x66
+popa
 
 hlt
diff --git a/unittests/ASM/Displacement_Encoding.asm b/unittests/ASM/Displacement_Encoding.asm
deleted file mode 100644
index a7a945abc..000000000
--- a/unittests/ASM/Displacement_Encoding.asm
+++ /dev/null
@@ -1,41 +0,0 @@
-%ifdef CONFIG
-{
-  "RegData": {
-    "RAX": "0x4142434445464748",
-    "RBX": "0x5152535455565758",
-    "RCX": "0x5152535455565758"
-  },
-
-  "MemoryRegions": {
-    "0x7FFFF000": "4096"
-  },
-
-  "MemoryData": {
-    "0x7FFFF000": "48 47 46 45 44 43 42 41"
-  }
-}
-%endif
-
-; Tests to ensure that 64-bit displacement encoding works correctly without being RIP relative.
-; x86-64 has two displacement encodings, one is RIP relative, one is 32-bit (signed) displacement only.
-; modrm.mod = 0b00 && modrm.rm = 0b101: Means no SIB, but address mode is RIP + disp32.
-; modrm.mod = 0b00 && modrm.rm = 0b100: Means SIB, but address mode is disp32.
-;  - if SIB.base = 0b101 && SIB.index = 0b100. Which means no registers for base and index.
-; Test disp32 by mapping a page at the limit of 2GB and read data from it. Also store and load.
-; If we were accidentally using RIP relative, then it would be 2GB + <low test base address>, which won't be mapped.
-
-; Test disp32 load.
-mov rax, [0x7FFF_F000]
-
-mov rbx, 0x5152535455565758
-
-; LEA with disp32.
-lea rcx, [0x7FFF_FFF8]
-
-; Test store with disp32 store.
-mov [0x7FFF_FFF8], rbx
-
-; Load back with the LEA to ensure it's correct.
-mov rcx, [rcx]
-
-hlt
diff --git a/unittests/ASM/FEX_bugs/BitConditionCheck.asm b/unittests/ASM/FEX_bugs/BitConditionCheck.asm
index 282bd92de..2b9805ea0 100644
--- a/unittests/ASM/FEX_bugs/BitConditionCheck.asm
+++ b/unittests/ASM/FEX_bugs/BitConditionCheck.asm
@@ -19,7 +19,7 @@
   %rep 2000
     add qword [rel data], rbx
   %endrep
-  lea rax, [%%top]
+  lea rax, [rel %%top]
   jpe %%top
 %endmacro
 
diff --git a/unittests/ASM/FEX_bugs/BranchConditionCheck.asm b/unittests/ASM/FEX_bugs/BranchConditionCheck.asm
index c22758402..238374f1f 100644
--- a/unittests/ASM/FEX_bugs/BranchConditionCheck.asm
+++ b/unittests/ASM/FEX_bugs/BranchConditionCheck.asm
@@ -20,7 +20,7 @@
   %rep 37500
     add qword [rel data], rbx
   %endrep
-  lea rax, [%%top]
+  lea rax, [rel %%top]
   jnz %%top
 %endmacro
 
diff --git a/unittests/ASM/MemoryData.asm b/unittests/ASM/MemoryData.asm
index c41b74944..66f34b767 100644
--- a/unittests/ASM/MemoryData.asm
+++ b/unittests/ASM/MemoryData.asm
@@ -14,5 +14,5 @@
 
 ; Simple test to prove that config loader's MemoryData is working
 
-mov rax, [0x10000000]
+mov rax, [abs 0x10000000]
 hlt
diff --git a/unittests/ASM/Primary/Primary_63_2.asm b/unittests/ASM/Primary/Primary_63_2.asm
index ac4ef67b4..cd1bfa929 100644
--- a/unittests/ASM/Primary/Primary_63_2.asm
+++ b/unittests/ASM/Primary/Primary_63_2.asm
@@ -18,7 +18,11 @@ mov rsp, 0x6666666681828384
 ; Default: 0x48, 0x63, 0xc4
 movsxd rax, esp
 ; Default with o16 prefix: 0x66, 0x48, 0x63, 0xc4
-o16 movsxd rbx, esp
+; operand-size override prefix
+; Nasm complains if o16 is used
+; `warning: invalid operand size prefix o16, must be o64`
+db 0x66
+movsxd rbx, esp
 ; No-rex widening prefix
 db 0x63, 0xcc ; movsxd ecx, esp
 ; o16 prefix with no-rex widening
diff --git a/unittests/ASM/Primary/Primary_87_Atomic16.asm b/unittests/ASM/Primary/Primary_87_Atomic16.asm
index df6c376b7..1e75f9aa5 100644
--- a/unittests/ASM/Primary/Primary_87_Atomic16.asm
+++ b/unittests/ASM/Primary/Primary_87_Atomic16.asm
@@ -32,27 +32,27 @@ mov [r15 + 8 * 9], rax
 
 ; Test 1 byte offset within 4byte boundary
 mov rax, 1
-lock xchg word [r15 + 8 * 0 + 1], ax
+xchg word [r15 + 8 * 0 + 1], ax
 mov r14, rax
 
 ; Test 3 byte offset across 4byte boundary
 mov rax, 1
-lock xchg word [r15 + 8 * 0 + 3], ax
+xchg word [r15 + 8 * 0 + 3], ax
 mov r13, rax
 
 ; Test 7 byte offset across 8byte boundary
 mov rax, 1
-lock xchg word [r15 + 8 * 0 + 7], ax
+xchg word [r15 + 8 * 0 + 7], ax
 mov r12, rax
 
 ; Test 15 byte offset across 16byte boundary
 mov rax, 1
-lock xchg word [r15 + 8 * 0 + 15], ax
+xchg word [r15 + 8 * 0 + 15], ax
 mov r11, rax
 
 ; Test 63 byte offset across cacheline boundary
 mov rax, 1
-lock xchg word [r15 + 8 * 0 + 63], ax
+xchg word [r15 + 8 * 0 + 63], ax
 mov r10, rax
 
 mov rax, qword [r15 + 8 * 0]
diff --git a/unittests/ASM/Primary/Primary_87_Atomic32.asm b/unittests/ASM/Primary/Primary_87_Atomic32.asm
index 186e925a4..ff2944459 100644
--- a/unittests/ASM/Primary/Primary_87_Atomic32.asm
+++ b/unittests/ASM/Primary/Primary_87_Atomic32.asm
@@ -31,22 +31,22 @@ mov [r15 + 8 * 9], rax
 
 ; Test 3 byte offset across 4byte boundary
 mov rax, 1
-lock xchg dword [r15 + 8 * 0 + 3], eax
+xchg dword [r15 + 8 * 0 + 3], eax
 mov r13, rax
 
 ; Test 7 byte offset across 8byte boundary
 mov rax, 1
-lock xchg dword [r15 + 8 * 0 + 7], eax
+xchg dword [r15 + 8 * 0 + 7], eax
 mov r12, rax
 
 ; Test 15 byte offset across 16byte boundary
 mov rax, 1
-lock xchg dword [r15 + 8 * 0 + 15], eax
+xchg dword [r15 + 8 * 0 + 15], eax
 mov r11, rax
 
 ; Test 63 byte offset across cacheline boundary
 mov rax, 1
-lock xchg dword [r15 + 8 * 0 + 63], eax
+xchg dword [r15 + 8 * 0 + 63], eax
 mov r10, rax
 
 mov rax, qword [r15 + 8 * 0]
diff --git a/unittests/ASM/Primary/Primary_87_Atomic64.asm b/unittests/ASM/Primary/Primary_87_Atomic64.asm
index e887bd305..ebb94f6b9 100644
--- a/unittests/ASM/Primary/Primary_87_Atomic64.asm
+++ b/unittests/ASM/Primary/Primary_87_Atomic64.asm
@@ -30,17 +30,17 @@ mov [r15 + 8 * 9], rax
 
 ; Test 7 byte offset across 8byte boundary
 mov rax, 1
-lock xchg qword [r15 + 8 * 0 + 7], rax
+xchg qword [r15 + 8 * 0 + 7], rax
 mov r13, rax
 
 ; Test 15 byte offset across 16byte boundary
 mov rax, 1
-lock xchg qword [r15 + 8 * 0 + 15], rax
+xchg qword [r15 + 8 * 0 + 15], rax
 mov r12, rax
 
 ; Test 63 byte offset across cacheline boundary
 mov rax, 1
-lock xchg qword [r15 + 8 * 0 + 63], rax
+xchg qword [r15 + 8 * 0 + 63], rax
 mov r11, rax
 
 mov rax, qword [r15 + 8 * 0]
diff --git a/unittests/ASM/Primary/Primary_C8_o16.asm b/unittests/ASM/Primary/Primary_C8_o16.asm
index 9f487ccea..57454c939 100644
--- a/unittests/ASM/Primary/Primary_C8_o16.asm
+++ b/unittests/ASM/Primary/Primary_C8_o16.asm
@@ -10,6 +10,10 @@
 mov rsp, 0xe0001000
 mov rbp, 0xe0001000
 
-o16 enter 0x10, 0
+; operand-size override prefix
+; Nasm complains if o16 is used
+; `warning: invalid operand size prefix o16, must be o64`
+db 0x66
+enter 0x10, 0
 hlt
 
diff --git a/unittests/ASM/Primary/Primary_C9_o16.asm b/unittests/ASM/Primary/Primary_C9_o16.asm
index 149adc6d2..b9ca868c3 100644
--- a/unittests/ASM/Primary/Primary_C9_o16.asm
+++ b/unittests/ASM/Primary/Primary_C9_o16.asm
@@ -20,7 +20,12 @@ jmp .end
 
 .target:
 mov rax, 1
-o16 leave
+
+; operand-size override prefix
+; Nasm complains if o16 is used
+; `warning: invalid operand size prefix o16, must be o64`
+db 0x66
+leave
 
 .end:
 hlt
diff --git a/unittests/ASM/Secondary/07_XX_04.asm b/unittests/ASM/Secondary/07_XX_04.asm
index 6b961539b..cf0262931 100644
--- a/unittests/ASM/Secondary/07_XX_04.asm
+++ b/unittests/ASM/Secondary/07_XX_04.asm
@@ -36,11 +36,16 @@ smsw cx
 smsw [rsi]
 mov rdx, [rsi]
 
-o16 smsw rdi
+; operand-size override prefix
+; Nasm complains if o16 is used
+; `warning: invalid operand size prefix o16, must be o64`
+db 0x66
+smsw rdi
 repe smsw rsp
 repne smsw rbp
 
-o16 smsw r8w
+db 0x66
+smsw r8w
 repe smsw r9w
 repne smsw r10w