Take a device's line down when its status port is read

A device raises a line and something has to take it down. Two things did:
being interrupted, and being woken from WAIT with the Interrupt Flag down -
the second because a masked program has nowhere to dispatch to, so nobody
else would.

There was a third way to learn a device had finished and nothing answered
it. The documented idiom reads the status, branches out if the device is
already done, and only WAITs otherwise; on a disk quick enough to finish
before the first look, which is every disk here, the WAIT is unreachable.
The line then stood for the rest of the machine's life.

The program that leaves it standing never pays for it - it was masked
throughout. The bill arrives at whoever next sets the Interrupt Flag. The
boot chain reads the disk to load a program, leaves the line up, and hands
over; the loaded program is then interrupted on behalf of a read that
finished before it existed, through a vector table with no entry for a
device it never touched, and faults on the instruction after its SIF.

Found by running Examples/tune.asm through Once. It set up its whole sound
and died four bytes before its first note, which is why it was silent
rather than wrong - and why it looked like a sound bug for a while.

So reading the port that answers a device takes its line down, the same way
taking the byte already took the console's down. Disk and screen do it on
their status port. And a reset now clears every line, which is the sentence
the manual already makes about the vector table: a handler left behind aims
an interrupt into a program that is no longer running, and so does a line.

testPrograms/diskLineTest.asm pins it - the racy idiom, then SIF with no
handler installed anywhere. It faults without the fix.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E2JrLzFvuFX9fgi1LDRjrW

Source/Emulator/io.c

@@ -709,6 +709,10 @@ void clearInterrupt(uint8_t port) {
     linesClear(&machineLines, port);
 }
 
+void clearAllInterrupts(void) {
+    memset(&machineLines, 0, sizeof(machineLines));
+}
+
 int linesNext(const InterruptLines *lines) {
     // Lowest numbered port wins. This is a scan rather than a priority encoder, which
     // means there is no arbitration to explain and a programmer can work out what
@@ -1156,7 +1160,24 @@ uint8_t InputHandler(uint8_t Address) {
         break;
         case DISK_BLOCK_HIGH: return (uint8_t)(diskBlock >> 8);
         case DISK_BLOCK_LOW:  return (uint8_t)(diskBlock & 0xFF);
-        case DISK_STATUS:     return diskStatus;
+        case DISK_STATUS:
+            // ---- Looking is what answers it ----
+            //
+            // The console takes its line down when the byte is read, because taking the byte
+            // is what answers the console. The disk's answer is this port: the operation
+            // finished, and whether it worked is what Status is for. So reading it takes the
+            // line down, the same way.
+            //
+            // WITHOUT THIS THE ORDINARY IDIOM LEAVES A LINE STANDING. The documented shape of
+            // waiting for a device reads the status, branches out if the device is already
+            // done, and only WAITs otherwise - so on a disk fast enough to finish before the
+            // first look, which is every disk here, the WAIT that would have taken the line
+            // down is never reached. Nothing else was going to answer it either: the program
+            // is masked and has no handler. The line then stands for the rest of the
+            // machine's life, and the next program to set the Interrupt Flag is interrupted
+            // on behalf of a read that finished before it was loaded.
+            clearInterrupt(PORT_DISK);
+            return diskStatus;
         case PORT_REFUSE:
             // Refuses reads as well, so both directions are covered.
             refuseAccess(VECTOR_GUARD_VIOLATION);