To:	Users
From:	Bob Supnik
Subj:	PDP-8 Simulator Usage
Date:	15-Feb-2004

			COPYRIGHT NOTICE

The following copyright notice applies to both the SIMH source and binary:

   Original code published in 1993-2004, written by Robert M Supnik
   Copyright (c) 1993-2004, Robert M Supnik

   Permission is hereby granted, free of charge, to any person obtaining a
   copy of this software and associated documentation files (the "Software"),
   to deal in the Software without restriction, including without limitation
   the rights to use, copy, modify, merge, publish, distribute, sublicense,
   and/or sell copies of the Software, and to permit persons to whom the
   Software is furnished to do so, subject to the following conditions:

   The above copyright notice and this permission notice shall be included in
   all copies or substantial portions of the Software.

   THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
   IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
   FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
   ROBERT M SUPNIK BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
   IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
   CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

   Except as contained in this notice, the name of Robert M Supnik shall not
   be used in advertising or otherwise to promote the sale, use or other dealings
   in this Software without prior written authorization from Robert M Supnik.

This memorandum documents the PDP-8 simulator.


1. Simulator Files

sim/		scp.h
		sim_console.h
		sim_defs.h
		sim_fio.h
		sim_rev.h
		sim_sock.h
		sim_tape.h
		sim_timer.h
		sim_tmxr.h
		scp.c
		sim_console.c
		sim_fio.c
		sim_sock.c
		sim_tape.c
		sim_timer.c
		sim_tmxr.c

sim/pdp8/	pdp8_defs.h
		pdp8_cpu.c
		pdp8_df.c
		pdp8_dt.c
		pdp8_lp.c
		pdp8_mt.c
		pdp8_pt.c
		pdp8_rf.c
		pdp8_rk.c
		pdp8_rl.c
		pdp8_rx.c
		pdp8_sys.c
		pdp8_td.c
		pdp8_tsc.c
		pdp8_tt.c
		pdp8_ttx.c

2. PDP-8 Features

The PDP-8 simulator is configured as follows:

device		simulates
name(s)

CPU		PDP-8/E CPU with 4KW-32KW of memory
-		KE8E extended arithmetic element (EAE)
-		KM8E memory management and timeshare control
TSC		TSC8-75 ETOS operating system timeshare control
PTR,PTP		PC8E paper tape reader/punch
TTI,TTO		KL8E console terminal
TTIX,TTOX	KL8JA additional terminals
LPT		LE8E line printer
CLK		DK8E line frequency clock (also PDP-8/A compatible)
RK		RK8E/RK05 cartridge disk controller with four drives
RF		RF08/RS08 fixed head disk controller with 1-4 platters, or
DF		DF32/DS32 fixed head disk controller with 1-4 platters
RL		RL8A/RL01 cartridge disk controller with four drives
RX		RX8E/RX01, RX28/RX02 floppy disk controller with two drives
DT		TC08/TU56 DECtape controller with eight drives
TD		TD8E/TU56 DECtape controller with two drives
MT		TM8E/TU10 magnetic tape controller with eight drives

Most devices can be disabled or enabled, by the commands:

	SET <dev> DISABLED
	SET <dev> ENABLED

The simulator allows most device numbers to be changed, by the command:

	SET <dev> DEV=<number>

The PDP-8 can support only one of the set {DF32, RF08, RL8A} using the
default device numbers, since they all use device numbers 60-61.  The
default is the RF08.  To change the disk at device numbers 60-61:

	sim> SET RF DISABLED		disable RF08
	sim> SET DF ENABLED, or		enable DF32
	sim> SET RL ENABLED		enable RL8A

The PDP-8 can only support one of the set {TC08, TD8E} using the default
device numbers, since both use device number 77.  The default is the
TC08.  To change the DECtape controller to the TD8E:

	sim> SET DT DISABLED		disable TC08
	sim> SET TD ENABLED		enable TD8E

Alternately, the device conflict can be eliminated by changing device
numbers:

	sim> SET RL DEV=50
	sim> SET RL ENA
	sim> SET TD DEV=74
	sim> SET TD ENA

However, devices can only be BOOTed with their default device numbers.

The PDP-8 simulator implements several unique stop conditions:

	- if an undefined instruction (unimplemented IOT or OPR) is
	  decoded, and register STOP_INST
	- if a simulated DECtape runs off the end of its reel

The PDP-8 loader supports both RIM format and BIN format tapes.  If the file
extension is .RIM, or the -r switch is specified with LOAD, the file is
assumed to be RIM format; if the file extension is not .RIM, or if the -b
switch is specified, the file is assumed to be BIN format.

2.1 CPU

The only CPU options are the presence of the EAE and the size of main
memory; the memory extension and time-share control is always included,
even if memory size is 4K.

	SET CPU EAE		enable EAE
	SET CPU NOEAE		disable EAE
	SET CPU 4K		set memory size = 4K
	SET CPU 8K		set memory size = 8K
	SET CPU 12K		set memory size = 12K
	SET CPU 16K		set memory size = 16K
	SET CPU 20K		set memory size = 20K
	SET CPU 24K		set memory size = 24K
	SET CPU 28K		set memory size = 28K
	SET CPU 32K		set memory size = 32K

If memory size is being reduced, and the memory being truncated contains
non-zero data, the simulator asks for confirmation.  Data in the truncated
portion of memory is lost.  Initial memory size is 32K.

CPU registers include the visible state of the processor as well as the
control registers for the interrupt system.

	name		size	comments

	PC		15	program counter, including IF as high 3 bits
	AC		12	accumulator
	MQ		12	multiplier-quotient
	L		1	link
	SR		12	front panel switches
	IF		3	instruction field
	DF		3	data field
	IB		3	instruction field buffer
	SF		7	save field
	UF		1	user mode flag
	UB		1	user mode buffer
	SC		5	EAE shift counter
	GTF		1	EAE greater than flag
	EMODE		1	EAE mode (0 = A, 1 = B)
	ION		1	interrupt enable
	ION_DELAY	1	interrupt enable delay for ION
	CIF_DELAY	1	interrupt enable delay for CIF
	PWR_INT		1	power fail interrupt
	UF_INT		1	user mode violation interrupt
	INT		15	interrupt pending flags
	DONE		15	device done flags
	ENABLE		15	device interrupt enable flags
	PCQ[0:63]	15	PC prior to last JMP, JMS, or interrupt;
				most recent PC change first
	STOP_INST	1	stop on undefined instruction
	WRU		8	interrupt character

The CPU can maintain a history of the most recently executed instructions.
This is controlled by the SET CPU HISTORY and SHOW CPU HISTORY commands:

	SET CPU HISTORY		clear history buffer
	SET CPU HISTORY=0	disable history
	SET CPU HISTORY=n	enable history, display length = n
	SHOW CPU HISTORY	print CPU history

The maximum length for the history is 65536 entries.

2.2 TSC8-75 ETOS Timeshare Control (TSC)

ETOS is a timeshared operating system for the PDP-8, providing multiple
virtual OS/8 environments for up to 32 users.  It requires a special
timeshare control option, the TSC8-75.  The TSC8-75 is normally disabled;
to run ETOS, it must be enabled with the command:

	SET TSC ENABLED

The TSC8-75 implements these registers:

	IR			most recently trapped instruction
	PC			PC of most recently trapped instruction
	CDF			1 if trapped instruction is CDF, 0 otherwise
	ENB			interrupt enable flag
	INT			interrupt pending flag

Except for operation of ETOS, the TSC8-75 should be left disabled.

2.3 Programmed I/O Devices

2.3.1 PC8E Paper Tape Reader (PTR)

The paper tape reader (PTR) reads data from a disk file.  The POS
register specifies the number of the next data item to be read.  Thus, 
by changing POS, the user can backspace or advance the reader.

The paper tape reader supports the BOOT command.  BOOT PTR copies the
RIM loader into memory and starts it running.

The paper tape reader implements these registers:

	name		size	comments

	BUF		8	last data item processed
	DONE		1	device done flag
	ENABLE		1	interrupt enable flag
	INT		1	interrupt pending flag
	POS		32	position in the input file
	TIME		24	time from I/O initiation to interrupt
	STOP_IOE	1	stop on I/O error

Error handling is as follows:

	error	     STOP_IOE	processed as

	not attached	1	report error and stop
			0	out of tape

	end of file	1	report error and stop
			0	out of tape

	OS I/O error	x	report error and stop

2.3.2 PC8E Paper Tape Punch (PTP)

The paper tape punch (PTP) writes data to a disk file.  The POS register
specifies the number of the next data item to bewritten.  Thus, by
changing POS, the user can backspace or advance the punch.

The paper tape punch implements these registers:

	name		size	comments

	BUF		8	last data item processed
	DONE		1	device done flag
	ENABLE		1	interrupt enable flag
	INT		1	interrupt pending flag
	POS		32	position in the output file
	TIME		24	time from I/O initiation to interrupt
	STOP_IOE	1	stop on I/O error

Error handling is as follows:

	error	     STOP_IOE	processed as

	not attached	1	report error and stop
			0	out of tape

	OS I/O error	x	report error and stop

2.3.3 KL8E Terminal Input (TTI)

The terminal interfaces (TTI, TTO) can be set to one of three modes:
KSR, 7B, or 8B.  In KSR mode, lower case input and output characters
are automatically converted to upper case, and the high order bit is
forced to one on input.  In 7B mode, input and output characters are
masked to 7 bits.  In 8B mode, characters are not modified.  Changing
the mode of either interface changes both.  The default mode is KSR.

The terminal input (TTI) polls the console keyboard for input.  It
implements these registers:

	name		size	comments

	BUF		8	last data item processed
	DONE		1	device done flag
	ENABLE		1	interrupt enable flag
	INT		1	interrupt pending flag
	POS		32	number of characters input
	TIME		24	keyboard polling interval

If the simulator is compiled under Windows Visual C++, typing ^C to the
terminal input causes a fatal run-time error.  Use the following command
to simulate typing ^C:

	SET TTI CTRL-C

2.3.4 KL8E Terminal Output (TTO)

The terminal output (TTO) writes to the simulator console window.  It
implements these registers:

	name		size	comments

	BUF		8	last data item processed
	DONE		1	device done flag
	ENABLE		1	interrupt enable flag
	INT		1	interrupt pending flag
	POS		32	number of characters output
	TIME		24	time from I/O initiation to interrupt

2.3.5 LE8E Line Printer (LPT)

The line printer (LPT) writes data to a disk file.  The POS register
specifies the number of the next data item to be read or written.  Thus,
by changing POS, the user can backspace or advance the printer.

The line printer implements these registers:

	name		size	comments

	BUF		8	last data item processed
	ERR		1	error status flag
	DONE		1	device done flag
	ENABLE		1	interrupt enable flag
	INT		1	interrupt pending flag
	POS		32	position in the output file
	TIME		24	time from I/O initiation to interrupt

Error handling is as follows:

	error	     STOP_IOE	processed as

	not attached	1	report error and stop
			0	set error flag

	OS I/O error	x	report error and stop

2.3.6 DK8E Line-Frequency Clock (CLK)

The real-time clock (CLK) frequency can be adjusted as follows:

	SET CLK 60HZ		set frequency to 60Hz
	SET CLK 50HZ		set frequency to 50Hz

The default is 60Hz.

The clock implements these registers:

	name		size	comments

	DONE		1	device done flag
	ENABLE		1	interrupt enable flag
	INT		1	interrupt pending flag
	TIME		24	clock interval

The real-time clock autocalibrates; the clock interval is adjusted up or
down so that the clock tracks actual elapsed time.

2.3.7 KL8JA Additional Terminals (TTIX, TTOX)

The additional terminals consist of two independent devices, TTIX and
TTOX.  The entire set is modelled as a terminal multiplexor, with TTIX
as the master unit.  The additional terminals perform input and output
through Telnet sessions connected to a user-specified port.  The ATTACH
command specifies the port to be used:

	ATTACH TTIX <port>	set up listening port

where port is a decimal number between 1 and 65535 that is not being used
for other TCP/IP activities.

The additional terminals can be set to one of three modes: UC, 7B, or
8B.  In UC mode, lower case input and output characters are converted
automatically to upper case.  In 7B mode, input and output characters
are masked to 7 bits.  In 8B mode, characters are not modified.  The
default mode is UC.  Finally, each line supports output logging.
The SET TTOXn LOG command enables logging on a line:

	SET TTOXn LOG=filename	log output of line n to filename

The SET TTOXLn NOLOG command disables logging and closes the open log
file, if any.

Once TTIX is attached and the simulator is running, the terminals listen
for connections on the specified port.  They assume that the incoming
connections are Telnet connections.  The connections remain open until
disconnected either by the Telnet client, a SET TTIX DISCONNECT command,
or a DETACH TTIX command.

The SHOW TTIX CONNECTIONS command displays the current connections to the
extra terminals.  The SHOW TTIX STATISTICS command displays statistics for
active connections.  The SET TTIX DISCONNECT=linenumber disconnects the
specified line.

The input device (TTIX) implements these registers:

	name		size	comments

	BUF[0:3]	8	input buffer, lines 0-3
	DONE		4	device done flags (line 0 rightmost)
	ENABLE		4	interrupt enable flag
	INT		4	interrupt pending flag
	TIME		24	initial polling interval
	TPS		10	polls per second after calibration

The output device (TTOX) implements these registers:

	name		size	comments

	BUF[0:3]	8	last data item processed, lines 0-3
	DONE		4	device done flag (line 0 rightmost)
	ENABLE		4	interrupt enable flag
	INT		4	interrupt pending flag
	TIME[0:3]	24	time from I/O initiation to interrupt,
				lines 0-3

The additional terminals do not support save and restore.  All open
connections are lost when the simulator shuts down or TTIX is detached.

2.3.8 TD8E/TU56 DECtape (TD)

The TD8E is a programmed I/O, non-interrupt controller, supporting two
DECtape drives (0 and 1).  The TD8E simulator puts a high burden on the
host processor, because tape activity is simulated a line (3b) at a time.
Unless the PDP-8 software requires the TD8E, the TC08 should be used
to simulate DECtapes.  The TD8E is disabled by default.

TD8E options include the ability to make units write enabled or write
locked.

	SET DTn LOCKED		set unit n write locked
	SET DTn WRITEENABLED	set unit n write enabled

Units can also be set ONLINE or OFFLINE.  The TD8E supports the BOOT command.

The TD8E supports supports PDP-8 format, PDP-11 format, and 18b format
DECtape images.  ATTACH tries to determine the tape format from the DECtape
image; the user can force a particular format with switches:

	-r			PDP-8 format
	-s			PDP-11 format
	-t			18b format

The TD8E controller is a data-only simulator; the timing and mark
track, and block header and trailer, are not stored.  Thus, read always
produces standard values for header and trailer words, and write throws
header and trailer words into the bit bucket.

The TD8E controller implements these registers:

	name		size	comments

	TDCMD		4	command register
	TDDAT		12	data register
	TDMTK		6	mark track register
	TDSLF		1	single line flag
	TDQLF		1	quad line flag
	TDTME		1	timing error flag
	TDQL		2	quad line counter
	LTIME		31	time between lines
	DCTIME		31	time to decelerate to a full stop
	POS[0:7]	32	position, in lines, units 0-7
	STATT[0:7]	18	unit state, units 0-7
	STOP_OFFR	1	stop on off-reel error

The LTIME parameter should not be changed, or OS/8 may fail to run
correctly.  The DCTIME parameter should always be at least 100 times
greater than LTIME.  Acceleration time is 75% of deceleration time.

2.4 Moving Head Disks

2.4.1 RK8E Cartridge Disk (RK)

RK8E options include the ability to make units write enabled or write locked:

	SET RKn LOCKED		set unit n write locked
	SET RKn WRITEENABLED	set unit n write enabled

Units can also be set ONLINE or OFFLINE.  The RK8E supports the BOOT command.

The RK8E implements these registers:

	name		size	comments

	RKSTA		12	status
	RKCMD		12	disk command
	RKDA		12	disk address
	RKMA		12	current memory address
	BUSY		1	control busy flag
	INT		1	interrupt pending flag
	STIME		24	seek time, per cylinder
	RTIME		24	rotational delay
	STOP_IOE	1	stop on I/O error

Error handling is as follows:

	error	     STOP_IOE	processed as

	not attached	1	report error and stop
			0	disk not ready

	end of file	x	assume rest of disk is zero

	OS I/O error	x	report error and stop

2.4.2 RL8A Cartridge Disk (RL)

RL8A options include the ability to make units write enabled or write locked:

	SET RLn LOCKED		set unit n write locked
	SET RLn WRITEENABLED	set unit n write enabled

Units can also be set ONLINE or OFFILE.  The RL8A supports the BOOT command.

The RL8A implements these registers:

	name		size	comments

	RLCSA		12	control/status A
	RLCSB		12	control/status B
	RLMA		12	memory address
	RLWC		12	word count
	RLSA		6	sector address
	RLER		12	error flags
	RLSI		16	silo top word
	RLSI1		16	silo second word
	RLSI2		16	silo third word
	RLSIL		1	silo read left/right flag
	INT		1	interrupt request
	DONE		1	done flag
	ERR		1	composite error flag
	STIME		1	seek time, per cylinder
	RTIME		1	rotational delay
	STOP_IOE	1	stop on I/O error

Error handling is as follows:

	error	     STOP_IOE	processed as

	not attached	1	report error and stop
			0	disk not ready

	end of file	x	assume rest of disk is zero

	OS I/O error	x	report error and stop

2.5 RX8E/RX01, RX28/RX02 Floppy Disk (RX)

The RX can be configured as an RX8E with two RX01 drives, or an RX28 with
two RX02 drives:

	SET RX RX8E		set controller to RX8E/RX01
	SET RX RX28		set controller to RX28/RX02

The controller is set to the RX8E by default.  The RX28 is not backwards-
compatible with the RX8E and will not work with the standard OS/8 V3D floppy
disk driver.

RX8E options include the ability to set units write enabled or write locked:

	SET RXn LOCKED		set unit n write locked
	SET RXn WRITEENABLED	set unit n write enabled

RX28 options include, in addition, the ability to set the unit density to
single density, double density, or autosized; autosizing is the default:

	SET RXn SINGLE		set unit n single density
	SET RXn DOUBLE		set unit n double density
	SET RXn AUTOSIZE	set unit n autosize

The RX8E and RX28 support the BOOT command.

The RX8E and RX28 implement these registers:

	name		size	comments

	RXCS		12	status
	RXDB		12	data buffer
	RXES		12	error status
	RXTA		8	current track
	RXSA		8	current sector
	STAPTR		4	controller state
	BUFPTR		8	buffer pointer
	INT		1	interrupt pending flag
	DONE		1	device done flag
	ENABLE		1	interrupt enable flag
	TR		1	transfer ready flag
	ERR		1	error flag
	CTIME		24	command completion time
	STIME		24	seek time, per track
	XTIME		24	transfer ready delay
	STOP_IOE	1	stop on I/O error
	SBUF[0:255]	8	sector buffer array

Error handling is as follows:

	error	     STOP_IOE	processed as

	not attached	1	report error and stop
			0	disk not ready

RX01 and RX02 data files are buffered in memory; therefore, end of file
and OS I/O errors cannot occur.

2.6 Fixed Head Disks

Either the RF08 or the DF32 can be present in a configuration, but
not both, with default device addressing.

2.6.1 RF08/RS08 Fixed Head Disk (RF)

RF08 options include the ability to set the number of platters to a
fixed value between 1 and 4, or to autosize the number of platters
from the attached file:

	SET RF 1P		one platter (256K)
	SET RF 2P		two platters (512K)
	SET RF 3P		three platters (768K)
	SET RF 4P		four platters (1024K)
	SET RF AUTOSIZE		autosized on attach

The default is one platter.

The RF08 implements these registers:

	name		size	comments

	STA		12	status
	DA		20	current disk address
	MA		12	memory address (in memory)
	WC		12	word count (in memory)
	WLK		32	write lock switches
	INT		1	interrupt pending flag
	DONE		1	device done flag
	TIME		24	rotational delay, per word
	BURST		1	burst flag
	STOP_IOE	1	stop on I/O error

The RF08 supports the BOOT command.  The default bootstrap is for OS/8.  To
bootstrap the 4K Disk Monitor, use the BOOT -D RF command.

The RF08 is a three-cycle data break device.  If BURST = 0, word transfers
are scheduled individually; if BURST = 1, the entire transfer occurs in
a single data break.

Error handling is as follows:

	error	     STOP_IOE	processed as

	not attached	1	report error and stop
			0	disk not ready

RF08 data files are buffered in memory; therefore, end of file and OS
I/O errors cannot occur.

2.6.2 DF32/DS32 Fixed Head Disk (RF)

DF32 options include the ability to set the number of platters to a
fixed value between 1 and 4, or to autosize the number of platters
from the attached file:

	SET DF 1P		one platter (32K)
	SET DF 2P		two platters (64K)
	SET DF 3P		three platters (98K)
	SET DF 4P		four platters (128K)
	SET DF AUTOSIZE		autosized on attach

The default is one platter.

The DF32 implements these registers:

	name		size	comments

	STA		12	status, disk and memory address extension
	DA		12	low order disk address
	MA		12	memory address (in memory)
	WC		12	word count (in memory)
	WLK		16	write lock switches
	INT		1	interrupt pending flag
	DONE		1	device done flag
	TIME		24	rotational delay, per word
	BURST		1	burst flag
	STOP_IOE	1	stop on I/O error

The DF32 supports the BOOT command.  The default bootstrap is for OS/8.  To
bootstrap the 4K Disk Monitor, use the BOOT -D DF command.

The DF32 is a three-cycle data break device.  If BURST = 0, word transfers
are scheduled individually; if BURST = 1, the entire transfer occurs in
a single data break.

Error handling is as follows:

	error	     STOP_IOE	processed as

	not attached	1	report error and stop
			0	disk not ready

DF32 data files are buffered in memory; therefore, end of file and OS
I/O errors cannot occur.

2.7 TC08/TU56 DECtape (DT)

DECtapes drives are numbered 1-8; in the simulator, drive 8 is unit 0.
TC08 options include the ability to make units write enabled or write
locked.

	SET DTn LOCKED		set unit n write locked
	SET DTn WRITEENABLED	set unit n write enabled

Units can also be set ONLINE or OFFLINE.  The TC08 supports the BOOT command.

The TC08 supports supports PDP-8 format, PDP-11 format, and 18b format
DECtape images.  ATTACH tries to determine the tape format from the DECtape
image; the user can force a particular format with switches:

	-r			PDP-8 format
	-s			PDP-11 format
	-t			18b format

The TC08 controller is a data-only simulator; the timing and mark
track, and block header and trailer, are not stored.  Thus, the WRITE
TIMING AND MARK TRACK function is not supported; the READ ALL function
always returns the hardware standard block header and trailer; and the
WRITE ALL function dumps non-data words into the bit bucket.

The DECtape controller implements these registers:

	name		size	comments

	DTSA		12	status register A
	DTSB		12	status register B
	INT		1	interrupt pending flag
	ENB		1	interrupt enable flag
	DTF		1	DECtape flag
	ERF		1	error flag
	CA		12	current address (memory location 7754)
	WC		12	word count (memory location 7755)
	LTIME		31	time between lines
	DCTIME		31	time to decelerate to a full stop
	SUBSTATE	2	read/write command substate
	POS[0:7]	32	position, in lines, units 0-7
	STATT[0:7]	31	unit state, units 0-7
	STOP_OFFR	1	stop on off-reel error

It is critically important to maintain certain timing relationships
among the DECtape parameters, or the DECtape simulator will fail to
operate correctly.

	- LTIME must be at least 6
	- DCTIME needs to be at least 100 times LTIME

Acceleration time is set to 75% of deceleration time.

2.8 TM8E Magnetic Tape (MT)

Magnetic tape options include the ability to make units write enabled or
or write locked.

	SET MTn LOCKED		set unit n write locked
	SET MTn WRITEENABLED	set unit n write enabled

Units can also be set ONLINE or OFFLINE.

The magnetic tape controller implements these registers:

	name		size	comments

	CMD		12	command
	FNC		12	function
	CA		12	memory address
	WC		12	word count
	DB		12	data buffer
	STA		12	main status
	STA2		6	secondary status
	DONE		1	device done flag
	INT		1	interrupt pending flag
	STOP_IOE	1	stop on I/O error
	TIME		24	record delay
	UST[0:7]	24	unit status, units 0-7
	POS[0:7]	32	position, units 0-7

Error handling is as follows:

	error			processed as

	not attached		tape not ready; if STOP_IOE, stop

	end of file		bad tape

	OS I/O error		parity error; if STOP_IOE, stop

2.9 Symbolic Display and Input

The PDP-8 simulator implements symbolic display and input.  Display is
controlled by command line switches:

	-a			display as ASCII character
	-c			display as (sixbit) character string
	-t			display as (TSS/8 sixbit) character string
	-m			display instruction mnemonics

Input parsing is controlled by the first character typed in or by command
line switches:

	' or -a			ASCII character
	" or -c			two character sixbit string
	# or -t			two character TSS/8 sixbit string
	alphabetic		instruction mnemonic
	numeric			octal number

Instruction input uses standard PDP-8 assembler syntax.  There are four
instruction classes: memory reference, IOT, field change, and operate.

Memory reference instructions have the format

	memref {I} {C/Z} address

where I signifies indirect, C a current page reference, and Z a zero page
reference.  The address is an octal number in the range 0 - 07777; if C or
Z is specified, the address is a page offset in the range 0 - 177.  Normally,
C is not needed; the simulator figures out from the address what mode to use.
However, when referencing memory outside the CPU (eg, disks), there is no
valid PC, and C must be used to specify current page addressing.

IOT instructions consist of single mnemonics, eg, KRB, TLS.  IOT instructions
may be or'd together

	iot iot iot...

The simulator does not check the legality of the proposed combination.  IOT's
for which there is no opcode may be specified as IOT n, where n is an octal
number in the range 0 - 0777.

Field change instructions (CIF, CDF) have the format

	fldchg field

where field is an octal number in the range 0 - 7.  Field change instructions
may be or'd together.

Operate instructions have the format

	opr opr opr...

The simulator does not check the legality of the proposed combination.  EAE
mode A and B mnemonics may be specified regardless of the EAE mode.  The
operands for MUY and DVI must be deposited explicitly.
