diff --git a/sim/aviron/src/lib/Cpu.zig b/sim/aviron/src/lib/Cpu.zig
index 9525f46eb..36ff7175a 100644
--- a/sim/aviron/src/lib/Cpu.zig
+++ b/sim/aviron/src/lib/Cpu.zig
@@ -755,7 +755,7 @@ const instructions = struct {
         cpu.sreg.z = (res == 0);
         cpu.sreg.n = ((res & 0x8000) != 0);
         cpu.sreg.c = ((src & 0x8000) == 0) and ((res & 0x8000) != 0);
-        cpu.sreg.v = cpu.sreg.c;
+        cpu.sreg.v = ((src & 0x8000) != 0) and ((res & 0x8000) == 0);
         cpu.sreg.s = (cpu.sreg.n != cpu.sreg.v);
     }
 
diff --git a/sim/aviron/src/testrunner.zig b/sim/aviron/src/testrunner.zig
index a39bc2899..d38adcb77 100644
--- a/sim/aviron/src/testrunner.zig
+++ b/sim/aviron/src/testrunner.zig
@@ -172,8 +172,8 @@ fn run_test(
     };
 
     // Check if it was a system exit (via IO)
-    const exit_mode: ExitMode = if (io.exit_requested)
-        .{ .system_exit = io.exit_code.? }
+    const exit_mode: ExitMode = if (io.exit_code) |code|
+        .{ .system_exit = code }
     else switch (result) {
         .breakpoint => .breakpoint,
         .enter_sleep_mode => .enter_sleep_mode,
@@ -310,7 +310,6 @@ const IO = struct {
 
     // Exit status tracking
     exit_code: ?u8 = null,
-    exit_requested: bool = false,
 
     const DataBusType = aviron.Bus(.{ .address_type = u24 });
     const IOBusType = aviron.IOBus;
@@ -433,7 +432,6 @@ const IO = struct {
         switch (reg) {
             .exit => {
                 io.exit_code = value & mask;
-                io.exit_requested = true;
             },
 
             .stdio => io.stdout.append(value & mask) catch @panic("out of memory"),
@@ -497,7 +495,9 @@ const IO = struct {
 
     fn dev_check_exit(ctx: *anyopaque) ?u8 {
         const io: *IO = @ptrCast(@alignCast(ctx));
-        if (io.exit_requested) return io.exit_code;
+        if (io.exit_code) |code| {
+            return code;
+        }
         return null;
     }
 };
diff --git a/sim/aviron/testsuite.avr-gcc/instructions/sbiw.S b/sim/aviron/testsuite.avr-gcc/instructions/sbiw.S
new file mode 100644
index 000000000..fd8ea790a
--- /dev/null
+++ b/sim/aviron/testsuite.avr-gcc/instructions/sbiw.S
@@ -0,0 +1,56 @@
+//! {
+//!   "exit": "breakpoint",
+//!   "cpus": ["atmega2560", "atmega328p"],
+//!   "precondition": {
+//!     "r24": 0,
+//!     "r25": 0,
+//!     "r26": 0,
+//!     "r27": 128,
+//!     "r28": 8,
+//!     "r29": 0,
+//!     "r30": 255,
+//!     "r31": 255 },
+//!   "postcondition": {
+//!     "r16": 21,
+//!     "r17": 24,
+//!     "r18": 2,
+//!     "r19": 20,
+//!     "r24": 248,
+//!     "r25": 255,
+//!     "r26": 255,
+//!     "r27": 127,
+//!     "r28": 0,
+//!     "r29": 0,
+//!     "r30": 192,
+//!     "r31": 255 }
+//! }
+
+    ; Test case 1: 0x0000 - 0x08 = 0xFFF8
+    ; Precondition: r25:r24 = 0x0000
+    ; Expected: r25:r24 = 0xFFF8, C=1, N=1, V=0, S=1, Z=0
+    ; SREG should be 0x15 (C=1, Z=0, N=1, V=0, S=1)
+    sbiw r24, 0x8
+    in r16, 0x3F    ; Save SREG to r16
+
+    ; Test case 2: 0x8000 - 0x01 = 0x7FFF (overflow)
+    ; Precondition: r27:r26 = 0x8000
+    ; Expected: r27:r26 = 0x7FFF, C=0, N=0, V=1, S=1, Z=0
+    ; SREG should be 0x18 (C=0, Z=0, N=0, V=1, S=1)
+    sbiw r26, 0x1
+    in r17, 0x3F    ; Save SREG to r17
+
+    ; Test case 3: 0x0008 - 0x08 = 0x0000 (zero result)
+    ; Precondition: r29:r28 = 0x0008
+    ; Expected: r29:r28 = 0x0000, C=0, N=0, V=0, S=0, Z=1
+    ; SREG should be 0x02 (C=0, Z=1, N=0, V=0, S=0)
+    sbiw r28, 0x8
+    in r18, 0x3F    ; Save SREG to r18
+
+    ; Test case 4: 0xFFFF - 0x3F = 0xFFC0
+    ; Precondition: r31:r30 = 0xFFFF
+    ; Expected: r31:r30 = 0xFFC0, C=0, N=1, V=0, S=1, Z=0
+    ; SREG should be 0x14 (C=0, Z=0, N=1, V=0, S=1)
+    sbiw r30, 0x3F
+    in r19, 0x3F    ; Save SREG to r19
+
+    break
diff --git a/sim/aviron/testsuite/instructions/sbiw-atmega2560.elf b/sim/aviron/testsuite/instructions/sbiw-atmega2560.elf
new file mode 100755
index 000000000..3b8fd2a0f
Binary files /dev/null and b/sim/aviron/testsuite/instructions/sbiw-atmega2560.elf differ
diff --git a/sim/aviron/testsuite/instructions/sbiw-atmega2560.elf.json b/sim/aviron/testsuite/instructions/sbiw-atmega2560.elf.json
new file mode 100644
index 000000000..6086d1019
--- /dev/null
+++ b/sim/aviron/testsuite/instructions/sbiw-atmega2560.elf.json
@@ -0,0 +1,106 @@
+{
+  "exit": "breakpoint",
+  "exit_code": 0,
+  "stdout": "",
+  "stderr": "",
+  "stdin": "",
+  "precondition": {
+    "sreg": {
+      "c": null,
+      "z": null,
+      "n": null,
+      "v": null,
+      "s": null,
+      "h": null,
+      "t": null,
+      "i": null
+    },
+    "r0": null,
+    "r1": null,
+    "r2": null,
+    "r3": null,
+    "r4": null,
+    "r5": null,
+    "r6": null,
+    "r7": null,
+    "r8": null,
+    "r9": null,
+    "r10": null,
+    "r11": null,
+    "r12": null,
+    "r13": null,
+    "r14": null,
+    "r15": null,
+    "r16": null,
+    "r17": null,
+    "r18": null,
+    "r19": null,
+    "r20": null,
+    "r21": null,
+    "r22": null,
+    "r23": null,
+    "r24": 0,
+    "r25": 0,
+    "r26": 0,
+    "r27": 128,
+    "r28": 8,
+    "r29": 0,
+    "r30": 255,
+    "r31": 255
+  },
+  "postcondition": {
+    "sreg": {
+      "c": null,
+      "z": null,
+      "n": null,
+      "v": null,
+      "s": null,
+      "h": null,
+      "t": null,
+      "i": null
+    },
+    "r0": null,
+    "r1": null,
+    "r2": null,
+    "r3": null,
+    "r4": null,
+    "r5": null,
+    "r6": null,
+    "r7": null,
+    "r8": null,
+    "r9": null,
+    "r10": null,
+    "r11": null,
+    "r12": null,
+    "r13": null,
+    "r14": null,
+    "r15": null,
+    "r16": 21,
+    "r17": 24,
+    "r18": 2,
+    "r19": 20,
+    "r20": null,
+    "r21": null,
+    "r22": null,
+    "r23": null,
+    "r24": 248,
+    "r25": 255,
+    "r26": 255,
+    "r27": 127,
+    "r28": 0,
+    "r29": 0,
+    "r30": 192,
+    "r31": 255
+  },
+  "mileage": 0,
+  "cpu": "atmega2560",
+  "cpus": null,
+  "optimize": "ReleaseSmall",
+  "gcc_flags": [
+    "-g",
+    "-O2",
+    "-nostdlib",
+    "-nostdinc",
+    "-ffreestanding"
+  ]
+}
\ No newline at end of file
diff --git a/sim/aviron/testsuite/instructions/sbiw-atmega328p.elf b/sim/aviron/testsuite/instructions/sbiw-atmega328p.elf
new file mode 100755
index 000000000..1b87a09f2
Binary files /dev/null and b/sim/aviron/testsuite/instructions/sbiw-atmega328p.elf differ
diff --git a/sim/aviron/testsuite/instructions/sbiw-atmega328p.elf.json b/sim/aviron/testsuite/instructions/sbiw-atmega328p.elf.json
new file mode 100644
index 000000000..2a3da12aa
--- /dev/null
+++ b/sim/aviron/testsuite/instructions/sbiw-atmega328p.elf.json
@@ -0,0 +1,106 @@
+{
+  "exit": "breakpoint",
+  "exit_code": 0,
+  "stdout": "",
+  "stderr": "",
+  "stdin": "",
+  "precondition": {
+    "sreg": {
+      "c": null,
+      "z": null,
+      "n": null,
+      "v": null,
+      "s": null,
+      "h": null,
+      "t": null,
+      "i": null
+    },
+    "r0": null,
+    "r1": null,
+    "r2": null,
+    "r3": null,
+    "r4": null,
+    "r5": null,
+    "r6": null,
+    "r7": null,
+    "r8": null,
+    "r9": null,
+    "r10": null,
+    "r11": null,
+    "r12": null,
+    "r13": null,
+    "r14": null,
+    "r15": null,
+    "r16": null,
+    "r17": null,
+    "r18": null,
+    "r19": null,
+    "r20": null,
+    "r21": null,
+    "r22": null,
+    "r23": null,
+    "r24": 0,
+    "r25": 0,
+    "r26": 0,
+    "r27": 128,
+    "r28": 8,
+    "r29": 0,
+    "r30": 255,
+    "r31": 255
+  },
+  "postcondition": {
+    "sreg": {
+      "c": null,
+      "z": null,
+      "n": null,
+      "v": null,
+      "s": null,
+      "h": null,
+      "t": null,
+      "i": null
+    },
+    "r0": null,
+    "r1": null,
+    "r2": null,
+    "r3": null,
+    "r4": null,
+    "r5": null,
+    "r6": null,
+    "r7": null,
+    "r8": null,
+    "r9": null,
+    "r10": null,
+    "r11": null,
+    "r12": null,
+    "r13": null,
+    "r14": null,
+    "r15": null,
+    "r16": 21,
+    "r17": 24,
+    "r18": 2,
+    "r19": 20,
+    "r20": null,
+    "r21": null,
+    "r22": null,
+    "r23": null,
+    "r24": 248,
+    "r25": 255,
+    "r26": 255,
+    "r27": 127,
+    "r28": 0,
+    "r29": 0,
+    "r30": 192,
+    "r31": 255
+  },
+  "mileage": 0,
+  "cpu": "atmega328p",
+  "cpus": null,
+  "optimize": "ReleaseSmall",
+  "gcc_flags": [
+    "-g",
+    "-O2",
+    "-nostdlib",
+    "-nostdinc",
+    "-ffreestanding"
+  ]
+}
\ No newline at end of file