Fix DCI string edge case

If a DCI string ended with a string delimiter or non-ASCII character
(e.g. a PETSCII char with no ASCII equivalent), the code generator
output the last byte as a hex value.  This caused an error because it
was outputting the raw hex value, with the high bit already set, which
the assembler did not expect.

This change corrects the behavior for code generation and on-screen
display, and adds a few samples to the regression test suite.

(see issue #102)

Asm65/StringOpFormatter.cs

@@ -31,8 +31,17 @@ namespace Asm65 {
         /// </summary>
         public enum ReverseMode { Forward, LineReverse, FullReverse };
 
+        /// <summary>
+        /// Character encoding conversion delegate.  This function converts a raw byte value
+        /// to a printable value, or CharEncoding.UNPRINTABLE_CHAR.
+        /// </summary>
         public CharEncoding.Convert CharConv { get; set; }
 
+        /// <summary>
+        /// True if the input bytes are a DCI string. Only compatible with ReverseMode==Forward.
+        /// </summary>
+        public bool IsDciString { get; set; } = false;
+
         // Output format for raw (non-printable) characters.  Most assemblers use comma-separated
         // hex values, some allow dense hex strings.
         public enum RawOutputStyle { DenseHex, CommaSep };
@@ -252,6 +261,8 @@ namespace Asm65 {
         /// </summary>
         public void FeedBytes(byte[] data, int offset, int length, int leadingBytes,
                 ReverseMode revMode) {
+            Debug.Assert(!IsDciString || revMode == ReverseMode.Forward);
+
             int startOffset = offset;
             int strEndOffset = offset + length;
 
@@ -288,7 +299,11 @@ namespace Asm65 {
             } else {
                 Debug.Assert(revMode == ReverseMode.Forward);
                 for (; offset < strEndOffset; offset++) {
-                    WriteChar(data[offset]);
+                    byte val = data[offset];
+                    if (IsDciString && offset == strEndOffset - 1) {
+                        val ^= 0x80;
+                    }
+                    WriteChar(val);
                 }
             }