I've created an area so that we can test kernel mode code in SimH.

src/minikern/README.md

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+#Readme
+
+This area is a place to write and try out kernel-mode test code. We can
+try out drivers, test the disk routines etc. The idea is to bring pieces
+of the real kernel in, test them, and keep incorporating stuff until we
+reach a working kernel.
+
+List filenames and comments here.

src/minikern/ttyout_test.s

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+" Code to test the teleprinter output. Some code borrowed from s7.s
+
+iof = 0700002                   " PIC: interrupts off
+ion = 0700042                   " PIC: interrupts on
+tsf = 0700401                   " TTY: skip if flag set
+tcf = 0700402                   " TTY: clear flag
+tls = 0700406                   " TTY: load buffer, select
+
+. = 020
+
+   iof			" Interrupts off, do we need this?
+
+1: tsf			" Is the teleprinter ready to print?
+   jmp 1b		" No, loop back
+
+   tcf			" Clear the ready flag
+   jms ttyrestart	" Print out a character
+   hlt			" and halt for now
+
+
+ttyrestart: 0
+   lac ttydelay		" Is the ttydelay positive?
+   spa
+     jmp ttyrestart i	" Yes, can't print the character yet
+   lac nttychar
+   dzm nttychar
+   sza
+     jmp 3f
+   isz ttydelay		" Increment the tty delay towards zero
+   lac d2		" Why 2?
+   jms getchar		" Get a character to print
+     jmp 2f		" Why the jump, does getchar skip? Yes if no char
+3:
+   tls			" Send the character to the teleprinter
+   sad o12
+     jms putcr		" It's a newline, also send a CR?
+   sad o15
+     skp		" It was CR, so now insert a delay
+   jmp ttyrestart i	" Not CR, so return from the routine
+
+   lac ttydelay		" Add 020 to the ttydelay
+   tad o20
+   rcr			" Rotate right once
+   cma			" Invert the AC
+   dac ttydelay		" and save back in ttydelay
+   jmp ttyrestart i	" Now return from the routine
+2:
+   " lac sfiles+1
+   " jms wakeup
+   " dac sfiles+1
+   jmp ttyrestart i	" Now return from the routine
+
+getchar: 0
+   lac d65		" For now, return ASCII A
+   isz getchar
+   jmp i getchar
+
+putcr: 0
+   lac o15
+   dac nttychar
+   cla
+   jmp putcr i
+
+d2: 2
+d65: 64
+o12: 012
+o15: 015
+o20: 020
+
+ttydelay: 0
+nttychar: .=.+1