Interrupt system implemented, some new programs.

Programs/testPrograms/interruptExample.asm

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+; The worked example from the SplitBit Assembler Manual, kept here so that the manual
+; cannot quietly stop being true. If this test changes, the manual changes with it.
+;
+; Correct output is:
+;   ready
+;   trap
+;   device
+
+; Interrupt handling from all three directions.
+
+#Program
+
+start:
+  CIF               ; Hold devices off while we set up.
+  SETD.0 Greeting
+  CALL printString
+
+  SWI announce      ; A trap of our own, reached by name.
+
+  INIA 0d1
+  OUTA 0x10         ; Ask the test device for attention. Its line goes up.
+  SIF               ; Let it through. It is answered before the next instruction.
+
+  HALT
+
+; A trap. It is entered with a full frame, so it may use any register it likes
+; without agreeing anything with the code it interrupted.
+announce:
+  SETD.0 Trapped
+  CALL printString
+  RETI
+
+; The device handler. Reached because the device sits on port 0x10.
+deviceReady:
+  SETD.0 Device
+  CALL printString
+  RETI
+
+; The fault handler. It reports and stops, rather than trying to carry on.
+reportFault:
+  SETD.0 Broken
+  CALL printString
+  HALT
+
+printString:        ; Expects DP0 to be set to the beginning of the string.
+  LDA.0
+  BRA printDone
+  OUTA 0x00
+  INCD.0
+  BRI printString
+ printDone:
+  INIA 0x0A
+  OUTA 0x00
+  RET
+
+#Data
+
+Greeting:
+"ready"
+Trapped:
+"trap"
+Device:
+"device"
+Broken:
+"bad opcode"
+
+#Vectors
+
+  Boot          start         ; Begin here rather than at the first byte.
+  BadOpcode     reportFault
+  announce      announce      ; A name of our own. The assembler numbers it.
+  Device 0x10   deviceReady   ; Named by the port, because that is what decides it.