Interrupt system implemented, some new programs.

Source/Assembler/assembly.h

@@ -23,18 +23,62 @@
 //   ..      3     "DAT"
 //   ..      2     Data Segment length
 //   ..      M     Data Segment
+//   ..      3     "VEC", optional
+//   ..      2     Vector Segment length, in bytes
+//   ..      K     Vector Segment, four bytes per entry
+//
+// The Vector Segment is optional and comes last, so a binary written before it existed
+// simply ends after its Data Segment and still loads. Each entry is two bytes saying
+// where in Program Memory the vector sits, then two bytes saying where its handler is,
+// most significant byte first. It is a list rather than an image of the table, so a
+// program with three handlers costs twelve bytes instead of a padded kilobyte.
 //
 // The feature flags are how a binary says it needs something the base machine does
 // not provide, so that an emulator which cannot provide it refuses to run the binary
 // rather than quietly doing the wrong thing. No features are defined yet; the field
 // is here so that adding one later does not need another format version.
 
+// ---- The vector table ----
+//
+// The top kilobyte of Program Memory is reserved for vectors. Both tools have to
+// agree on where it begins: the CPU starts execution through it, and the assembler
+// has to refuse program text that would run into it.
+//
+// Entries are two bytes each, most significant byte first, the same order the branch
+// instructions and this file format already use.
+//
+//   0xFC00  Software vectors 0 to 255
+//   0xFE00  Hardware vectors 0 to 255, one for each I/O port
+//
+// Software vectors 0 and 1 are start addresses rather than handlers. Vector 0 is
+// where the machine begins at power on and vector 1 is a warm restart, so a zero in
+// either of them is not "nothing installed" but the address 0x0000, which is where a
+// program carrying no vector table of its own begins. A zero in any other entry does
+// mean no handler is installed, and dispatching through one is a fault.
+
+#define SOFTWARE_VECTOR_BASE 0xFC00
+#define HARDWARE_VECTOR_BASE 0xFE00
+#define VECTOR_ENTRY_BYTES   2
+#define VECTOR_BOOT          0
+#define VECTOR_SOFT_RESET    1
+#define VECTOR_INVALID_OPCODE 2
+// Vectors 3 to 15 are held back for faults that do not exist yet, so that each cause
+// can have an entry of its own rather than sharing one and needing a cause register to
+// tell them apart. Everything from 16 up belongs to programs.
+#define VECTOR_FIRST_FREE    16
+
+// The first address the vector table occupies, and so the first address that program
+// text may not use.
+#define PROGRAM_TEXT_LIMIT SOFTWARE_VECTOR_BASE
+
 #define SPLITBIT_MAGIC "SPBT"
 #define SPLITBIT_MAGIC_LENGTH 4
 #define SPLITBIT_FORMAT_VERSION 1
 #define SPLITBIT_FLAGS_LENGTH 4
 #define SEGMENT_MARKER_LENGTH 3
 #define SEGMENT_LENGTH_BYTES 2
+// Where a vector sits, and where its handler is.
+#define VECTOR_ENTRY_FILE_BYTES 4
 
 // Everything the format costs a file, on top of the two segments themselves.
 #define SPLITBIT_HEADER_BYTES (SPLITBIT_MAGIC_LENGTH + 1 + SPLITBIT_FLAGS_LENGTH \