"Hack hack" test.

The 340 interrupt handling uses a special hack.  PDP-10 emulators may
not support this.  See src/maint/hhtest.1 for longer explanation.

src/maint/hhtest.1

@@ -0,0 +1,47 @@
+	TITLE "HACK HACK" TEST
+
+;This tests the ITS "hack hack" for handling Type 340 display
+;interrupts.  Data is output to the display through a BLKO from a low
+;priority data channel, whereas other conditions are handled through a
+;higher priority special channel.  The "hack hack" is that when the
+;BLKO overflows, ITS triggers an interrupt on the special channel.
+;This interrupt is handled still in the overflow state, so the second
+;interrupt location should be used.  This hack goes back to at least
+;ITS 672.
+
+NOSYMS
+RIM10
+
+APR==0
+PI==4
+TTY==120
+
+SCHN==4
+DCHN==6
+
+LOC 40+2*DCHN
+	BLKO TTY,PTR			;2. BLKO overflows.
+	CONO PI,4000+200_<-SCHN>	;3. Trigger special interrupt.
+
+LOC 40+2*SCHN
+	JRST 4,.
+	JSR OFLOW			;4. Execute second instruction.
+
+LOC 100
+
+GO:	CONO APR,675550			;Reset APR.
+	CONO PI,711377			;Reset PI.
+	CONO PI,2000+<200_<-SCHN>>+<200_<-DCHN>> ;Enable channels.
+	CONO TTY,DCHN
+	CONO PI,4000+200_<-DCHN>	;1. Trigger data interrupt.
+REPEAT 10,JFCL
+	JRST 4,.			;Unsuccessful test ends here.
+
+LOC 200
+
+OFLOW:	0
+	JRST 4,.			;5. Successful test ends here.
+
+PTR:	-1,,["A]-1
+
+END GO