fix: select default drive

Author: maximilien-noal

src/Spice86.Core/Emulator/InterruptHandlers/Dos/DosInt21Handler.cs

@@ -1721,16 +1721,19 @@ public override void Run() {
     /// The number of potentially valid drive letters in AL.
     /// </returns>
     public void SelectDefaultDrive() {
-        if (_dosDriveManager.TryGetValue(DosDriveManager.DriveLetters.ElementAtOrDefault(State.DL).Key, out VirtualDrive? mountedDrive)) {
-            _dosDriveManager.CurrentDrive = mountedDrive;
-        }
-        if (State.DL > DosDriveManager.MaxDriveCount && LoggerService.IsEnabled(LogEventLevel.Error)) {
-            LoggerService.Error("DOS INT21H: Could not set default drive! Unrecognized index in State.DL: {DriveIndex}", State.DL);
+        byte driveIndex = State.DL;
+        if (driveIndex < DosDriveManager.MaxDriveCount) {
+            char driveLetter = (char)('A' + driveIndex);
+            if (_dosDriveManager.TryGetValue(driveLetter, out VirtualDrive? mountedDrive)) {
+                _dosDriveManager.CurrentDrive = mountedDrive;
+            }
+        } else if (LoggerService.IsEnabled(LogEventLevel.Error)) {
+            LoggerService.Error("DOS INT21H: Could not set default drive! Unrecognized index in State.DL: {DriveIndex}", driveIndex);
         }
         if (LoggerService.IsEnabled(LogEventLevel.Verbose)) {
             LoggerService.Verbose("SELECT DEFAULT DRIVE {@DefaultDrive}", _dosDriveManager.CurrentDrive);
         }
-        State.AL = _dosDriveManager.NumberOfPotentiallyValidDriveLetters;
+        State.AL = DosDriveManager.MaxDriveCount;
     }
 
     /// <summary>

tests/Spice86.Tests/Dos/DosInt21IntegrationTests.cs

@@ -925,6 +925,82 @@ public void StandardFileHandles_AreInheritedFromParentPsp() {
         testHandler.Results.Should().NotContain((byte)TestResult.Failure);
     }
 
+    /// <summary>
+    /// Tests INT 21h AH=0Eh (Select Default Drive).
+    /// Bug 1: DL=2 (C:) should return AL=26 (total drive count), not the mounted count (3).
+    /// Bug 2: DL >= 26 is invalid but the comparison used > instead of >=, so DL=26 was accepted.
+    /// Bug 3: ElementAtOrDefault on DriveLetters with an out-of-range index could silently pass
+    ///         a default '\0' key to TryGetValue instead of rejecting the drive index.
+    /// </summary>
+    [Fact]
+    public void SelectDefaultDrive_WithValidDrive_ReturnsMaxDriveCountAndSwitchesDrive() {
+        // Test plan:
+        //   1. Select C: (DL=2) via AH=0Eh → AL must be 26
+        //   2. Read current drive via AH=19h → AL must be 2 (C:)
+        //   3. Select invalid drive (DL=26) via AH=0Eh → drive must remain C:
+        //   4. Read current drive again via AH=19h → AL must still be 2
+        byte[] program = new byte[] {
+            // --- Part 1: Select C: drive (DL=2) ---
+            0xB4, 0x0E,             // mov ah, 0Eh  - Select Default Drive
+            0xB2, 0x02,             // mov dl, 02h  - C: drive index
+            0xCD, 0x21,             // int 21h      - AL = number of drives
+
+            // Write AL to details port so we can inspect the value
+            0xBA, 0x98, 0x09,       // mov dx, DetailsPort (0x998)
+            0xEE,                   // out dx, al
+
+            // Check AL == 26 (0x1A)
+            0x3C, 0x1A,             // cmp al, 1Ah
+            0x75, 0x22,             // jne failed   (offset 14 -> 48 = +34)
+
+            // --- Part 2: Verify current drive is C: ---
+            0xB4, 0x19,             // mov ah, 19h  - Get Current Drive
+            0xCD, 0x21,             // int 21h      - AL = current drive (0=A, 1=B, 2=C)
+
+            // Write AL to details port
+            0xBA, 0x98, 0x09,       // mov dx, DetailsPort
+            0xEE,                   // out dx, al
+
+            // Check AL == 2 (C:)
+            0x3C, 0x02,             // cmp al, 02h
+            0x75, 0x16,             // jne failed   (offset 26 -> 48 = +22)
+
+            // --- Part 3: Select invalid drive (DL=26, out of range) ---
+            0xB4, 0x0E,             // mov ah, 0Eh
+            0xB2, 0x1A,             // mov dl, 1Ah  - index 26 (invalid)
+            0xCD, 0x21,             // int 21h
+
+            // --- Part 4: Verify current drive is still C: ---
+            0xB4, 0x19,             // mov ah, 19h  - Get Current Drive
+            0xCD, 0x21,             // int 21h
+
+            // Write AL to details port
+            0xBA, 0x98, 0x09,       // mov dx, DetailsPort
+            0xEE,                   // out dx, al
+
+            // Check AL == 2 (C: unchanged)
+            0x3C, 0x02,             // cmp al, 02h
+            0x75, 0x04,             // jne failed
+
+            // Success
+            0xB0, 0x00,             // mov al, TestResult.Success
+            0xEB, 0x02,             // jmp writeResult
+
+            // failed:
+            0xB0, 0xFF,             // mov al, TestResult.Failure
+
+            // writeResult:
+            0xBA, 0x99, 0x09,       // mov dx, ResultPort
+            0xEE,                   // out dx, al
+            0xF4                    // hlt
+        };
+
+        DosTestHandler testHandler = RunDosTest(program);
+
+        testHandler.Results.Should().Contain((byte)TestResult.Success);
+        testHandler.Results.Should().NotContain((byte)TestResult.Failure);
+    }
+
     /// <summary>
     /// Runs the DOS test program and returns a test handler with results
     /// </summary>