Seventy becomes seventy one: a machine that can wait

HALT is terminal - stepCPU returns at once when the Halt Flag is up, so a
halted machine does not execute, service devices, or take an interrupt -
and that has to stay true, because every test ends with a halt and "halted"
is how a program says it has finished. The consequence was that SplitBit
had no way to wait at all. Every wait was a spin, and a spin is bus
traffic: 11.5% of Type over a 14K file on a disk of ten thousand cycles,
after read-ahead had already hidden three quarters of the latency.

WAIT is 0xFE, one byte, no operands, sitting under HALT where the
instruction that almost stops the machine belongs. Three decisions in it:

- A line already standing means there is nothing to wait for, so WAIT does
  nothing. That is what makes test-then-wait race-free.
- Any line ends the wait, masked or not, so a program can sleep on a device
  it has no handler for and read its status afterwards. Masking says who
  answers a request, not whether it happened.
- A line that wakes the CPU without being dispatched is taken down by the
  WAIT. Left standing it would be found by the next WAIT, which would
  return at once - the program would spin exactly as before while looking
  as though it slept.

Waiting is NOT a Status bit, and that is the trap avoided rather than a
gap: Status rides into the interrupt frame and comes back out, so a machine
interrupted mid-wait would return from its handler still waiting, and wait
again for what it had already been given. An internal field instead.

Idle cycles are counted apart from bus cycles and the halt line says so
when there are any, which is what makes the difference observable at all -
with the line-clearing removed the total moves by ONE cycle, 20,100 against
20,099, and only the idle half changes, halving to 9,976. A test on
totals could never have seen it. Tests/terminal.sh asks that question,
being the file for things a recorded output cannot see, and fails with the
clear removed while "both reads finished" still passes.

Three collisions, all found by building it:

- 0xFE was the assembler's "not an instruction" sentinel. getOpcode now
  answers a negative NOT_AN_OPCODE, which is outside the range of every
  possible answer instead of inside the unused part of it.
- 0xFE was also what faultTest and faultResumeTest executed to provoke a
  fault. They now use 0xFD and say why, because they did not fail when it
  became an instruction - they HUNG, having started sleeping instead.
- Keys.asm has had a label called "wait" for a year, and mnemonics are
  matched uppercased. What that reported was "Branch without label" at the
  BRQ thirty lines away. The assembler now refuses a label that is already
  an instruction, at the label, by name; every instruction added takes a
  word out of the space of label names, so this will happen again.

Programs/testPrograms/faultResumeTest.asm

@@ -5,7 +5,12 @@
 ; the same byte again, which is why this handler moves the saved address on by one
 ; first. MVSD is what lets it reach the frame at all.
 ;
-; 0xFE is not an instruction. Writing it as a literal is the only way past the
+; CHOSEN AWAY FROM HALT, because the bytes next to it get used. This said 0xFE for
+; a long time, and then 0xFE became WAIT - so the test stopped faulting and started
+; SLEEPING. It hung instead of failing, which is the worst way for a test to notice
+; that the thing it was testing had moved.
+;
+; 0xFD is not an instruction. Writing it as a literal is the only way past the
 ; assembler, which is what makes a program containing one buildable.
 ;
 ; Correct output is:
@@ -20,7 +25,7 @@ start:
   INIA 0x0A
   OUTA 0x00
 
-  0xFE              ; Not an instruction. The handler steps over this.
+  0xFD              ; Not an instruction. The handler steps over this.
 
   INIA 0d75         ; 'K'. Reached only because the handler moved the address on.
   OUTA 0x00