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To:	Users
From:	Bob Supnik
Subj:	IBM 1401 Simulator Usage
Date:	15-Nov-2004


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.


   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 IBM 1401 simulator.

1. Simulator Files

[ list of files used in building simulator 
   deleted as we users don't care ]

2. IBM 1401 Features

The IBM 1401 simulator is configured as follows:

device		simulates

CPU		IBM 1401 CPU with 16K of memory
CDR,CDP		IBM 1402 card reader/punch
LPT		IBM 1403 line printer
INQ		IBM 1407 inquiry terminal
DP		IBM 1311 disk pack with five drives
MT		IBM 729 7-track magnetic tape controller with six drives

The IBM 1401 simulator implements many unique stop conditions.  On almost
any kind of error the simulator stops:

	unimplemented opcode
	reference to non-existent memory
	reference to non-existent device
	no word mark under opcode
	invalid A address
	invalid B address
	invalid instruction length
	invalid modifier character
	invalid branch address
	invalid magtape unit number
	invalid magtape record length
	write to locked magtape drive
	skip to unpunched carriage control tape channel
	card reader hopper empty
	address register wrap-around
	single character A field in MCE
	single character B field in MCE
	hanging $ in MCE with EPE enabled
	I/O check with I/O stop switch set
	invalid disk drive
	invalid disk sector address
	invalid disk sector count
	invalid disk address compare

The LOAD command is used to load a line printer carriage-control tape.
The DUMP command is not implemented.

2.1 CPU

The CPU options include a number of special features and the size of main
memory.  Note that the Modify Address special feature is always included
when memory size is greater than 4K.

	SET CPU XSA		enable advanced programming special feature
	SET CPU NOXSA		disable advanced programming
	SET CPU HLE		enable high/low/equal special feature
	SET CPU NOHLE		disable high/low/equal
	SET CPU BBE		enable branch on bit equal special feature
	SET CPU NOBBE		disable branch on bit equal
	SET CPU MR		enable move record special feature
	SET CPU NOMR		disable move record
	SET CPU EPE		enable extended print edit special feature
	SET CPU NOEPE		disable extended print edit
	SET CPU MDV		enable multiply/divide special feature
	SET CPU NOMDV		disable multiply/divide
	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

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.  Initially, memory size is 16K, and all special
features are enabled.

Memory is implemented as 7 bit BCD characters, as follows:

	6	5 	4	3	2	1	0

	word	B bit	A bit	8	4	2	1
	mark	<-- zone -->	<-------- digit -------->

In BCD, the decimal digits 0-9 are (octal) values 012, 001, 002, 003, 004,
005, 006, 007, 010, 011, respectively.  Signs are encoded in the zone bits,
with 00, 01, and 11 being positive, and 10 being negative.

CPU registers include the visible state of the processor.  The 1401 has no
interrupt system.

	name		size	comments

	IS		14	instruction storage address register (PC)
	AS		14	A storage address register
	BS		14	B storage address register
	ASERR		1	AS invalid flag
	BSERR		1	BS invalid flag
	SSA		1	sense switch A
	SSB		1	sense switch B
	SSC		1	sense switch C
	SSD		1	sense switch D
	SSE		1	sense switch E
	SSF		1	sense switch F
	SSG		1	sense switch G
	EQU		1	equal compare indicator
	UNEQ		1	unequal compare indicator
	HIGH		1	high compare indicator
	LOW		1	low compare indicator
	OVF		1	overflow indicator
	IOCHK		1	I/O check switch
	PRCHK		1	process check switch
	ISQ[0:63]	14	IS prior to last branch;
				most recent IS change first
	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, length = n
	SHOW CPU HISTORY	print CPU history
	SHOW CPU HISTORY=n	print first n entries of CPU history

The maximum length for the history is 65536 entries.

2.2 1402 Card Reader/Punch (CDR, CDP, STKR)

The IBM 1402 card/reader punch is simulated as three independent devices:
the card reader (CDR), the card punch (CDP), and the reader and punch
stackers (STKR).  STRK units 0, 1, 2, and 4 correspond to the reader
normal stacker, reader stacker 1, shared stacker 2/8, and punch stacker
4, respectively.

The card reader supports the BOOT command.  BOOT CDR reads a card image
into locations 1-80, sets a word mark under location 1, clears storage,
and then transfers control to location 1.

The card reader reads data from disk files, while the punch and stackers
write data to disk files.  Cards are simulated as ASCII text lines with
terminating newlines; column binary is not supported.  For each unit,
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 these

The card reader registers are:

	name		size	comments

	LAST		1	last card indicator
	ERR		1	error indicator
	S1		1	stacker 1 select flag
	S2		1	stacker 2 select flag
	POS		32	position
	TIME		24	delay window for stacker select
	BUF[0:79]	8	reader buffer

The card punch registers are:

	ERR		1	error indicator
	S4		1	stacker 4 select flag
	S8		1	stacker 8 select flag

The stacker registers are:

	POS0		32	position, normal reader stack
	POS1		32	position, reader stacker 1
	POS2		32	position, shared stacker 2/8
	POS4		32	position, punch stacker 4

Error handling is as follows:

	device		error		processed as

	reader		end of file	if SSA set, set LAST indicator
					on next Read, report error and stop

	reader,punch	not attached	report error and stop
			OS I/O error	print error message
					if IOCHK set, report error and stop
					otherwise, set ERR indicator

	stacker		not attached	ignored
			OS I/O error	print error message
					if IOCHK set, report error and stop

2.3 1403 Line Printer (LPT)

The IBM 1403 line printer (LPT) writes its data, converted to ASCII, to
a disk file.  The line printer supports three different print character
sets or "chains":

	SET LPT PCF			full 64 character chain
	SET LPT PCA			48 character business chain
	SET LPT PCH			48 character FORTRAN chain

In addition, the line printer can be programmed with a carriage control
tape.  The LOAD command loads a new carriage control tape:

	LOAD 			load carriage control tape file

The format of a carriage control tape consists of multiple lines.  Each
line contains an optional repeat count, enclosed in parentheses, optionally
followed by a series of column numbers separated by commas.  Column numbers
must be between 1 and 12; a column number of zero denotes top of form.  The
following are all legal carriage control specifications:

				no punch
	(5)				5 lines with no punches
	1,5,7,8				columns 1, 5, 7, 8 punched
	(10)2				10 lines with column 2 punched
	1,0				column 1 punched; top of form

The default form is 66 lines long, with column 1 and the top of form mark
on line 1, and the rest blank.

The line printer registers are:

	name		size	comments

	LINES		8	number of newlines after next print
	LFLAG		1	carriage control flag (1 = skip, 0 = space)
	CCTP		8	carriage control tape pointer
	CCTL		8	carriage control tape length (read only)
	ERR		1	error indicator
	POS		32	position
	CCT[0:131]	32	carriage control tape array	

Error handling is as follows:

	error			processed as

	not attached		report error and stop

	OS I/O error		print error message
				if IOCHK set, report error and stop
				otherwise, set ERR indicator

2.4 1407 Inquiry Terminal (INQ)

The IBM 1407 inquiry terminal (INQ) is a half-duplex console.  It polls
the console keyboard periodically for inquiry requests.  The inquiry
terminal registers are:

	name		size	comments

	INQC		7	inquiry request character (initially ESC)
	INR		1	inquiry request indicator
	INC		1	inquiry cleared indicator
	TIME		24	polling interval

When the 1401 CPU requests input from the keyboard, the message [Enter]
is printed out, followed by a new line.  The CPU hangs waiting for input
until either the return/enter key is pressed, or the inquiry request
character is typed in.  The latter cancels the type-in and sets INC.

The inquiry terminal has no errors.

2.5 1311 Disk Pack (DP)

The disk pack controller supports 5 drives, numbered 0 through 4.  Disk
pack options include the ability to enable address writing (formatting).

	SET DPn ADDROFF		set unit n address enable off
	SET DPn ADDRON		set unit n address enable on

Units can also be set ENABLED or DISABLED.

Unlike most simulated disks, the 1311 includes explicit representation
for sector addresses.  This is to support non-standard formats, such as
the inclusion of the drive number in the sector address.  As a result,
1311 sectors are 106 characters long: 6 address characters and 100
data characters.  If the 1311 has not been formatted, the addresses
are blanks and are synthesized, if needed, based on the sector number. 

The 1311 also supports two modes of operation: move mode and load mode.
In move mode, word marks are ignored on writes and left untouched on reads,
and sectors hold 100 characters.  In load mode, word marks are included
on writes and stored on reads, and sectors hold 90 characters.  No attempt
is made to deal with sectors written in load mode and read in move mode,
or vice versa; on a real 1401, this causes a fatal parity error.

The disk pack controller implements these registers:

	name		size	comments

	ACC		1	access error indicator
	PWC		1	parity or write check error indicator
	WLR		1	wrong length record error indicator
	UNA		1	unequal address compare error indicator
	DSK		1	any disk error indicator
	BSY		1	disk access busy indicator
	LASTF		3	most recent function
	TIME		24	seek time

The 1311 has a primative overlapped seek capability.  If TIME is set
non-zero, the 1311 will report itself busy for the specified amount
of time following a seek.  This allows programs to utilize the seek
time for processing.

Error handling is as follows:

	error			processed as

	not attached		set DSK indicator
				if IOCHK set, report error and stop

1311 data files are buffered in memory; therefore, end of file and OS
I/O errors cannot occur.

2.6 729 Magnetic Tape (MT)

The magnetic tape controller supports six drives, numbered 1 through 6.
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 ENABLED or DISABLED.  The magnetic tape simulator
supports the BOOT command.  BOOT MT reads the first record off tape,
starting at location 1, and then branches to it. 

The magnetic tape controller implements these registers:

	name		size	comments

	END		1	end of file indicator
	ERR		1	error indicator
	PAR		1	parity error indicator
	POS1..6		32	position, drives 1..6

Error handling is as follows:

	error			processed as

	not attached		report error and stop

	end of file		set error indicator

	OS I/O error		print error message
				set error indicator
				if IOCHK set, report error and stop

2.7 Symbolic Display and Input

The IBM 1401 simulator implements symbolic display and input.  Display is
controlled by command line switches:

	-c			display as single character
				(BCD for CPU and MT, ASCII for others)
	-s			display as wordmark terminated BCD string
				(CPU only)
	-m			display instruction mnemonics
				(CPU only)
	-d			display 50 characters per line, with word
				marks denoted by "1" on the line below

In a CPU character display, word marks are denoted by ~.

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

	' or " or -c or -s	characters (BCD for CPU and MT, ASCII
				for others)
	alphabetic		instruction mnemonic
	numeric			octal number

Instruction input is free format, with spaces separating fields.  There
are six instruction formats: 1, 2, 4, 5, 7, and 8 characters:

	1 character		opcode
	2 character		opcode 'modifier
	4 character		opcode address
	5 character		opcode address 'modifier
	7 character		opcode address address
	8 character		opcode address address 'modifier

Addresses are always decimal, except for special I/O addresses in the A
field, which may be specified as %xy, where x denotes the device and y
the unit number.

For the CPU, string input may encompass multiple characters.  A word mark
is denoted by ~ and must precede the character to be marked.  All other
devices can only accept single character input, without word marks.

2.7 Character Sets

The IBM 1401 uses a 6b character code called BCD (binary coded decimal).
Some of the characters have no equivalent in ASCII and require different

BCD	    ASCII	IBM 1401		print
code	representation	character		chains

00	space
01	1
02	2
03	3
04	4
05	5
06	6
07	7
10	8
11	9
12	0
13	#					= in H chain
14	@					' in H chain
15	:					blank in A, H chains
16	>					blank in A, H chains
17	(		tape mark		blank in A, H chains
20	^		alternate blank		blank in A, H chains
21	/
22	S
23	T
24	U
25	V
26	W
27	X
30	Y
31	Z
32	'		record mark
33	,
34	%					( in H chain
35	=		word mark		blank in A, H chains
36	\					blank in A, H chains
37	+					blank in A, H chains
40	-
41	J
42	K
43	L
44	M
45	N
46	O
47	P
50	Q
51	R
52	!
53	$
54	*
55	]					blank in A, H chains
56	;					blank in A, H chains
57	_		delta			blank in A, H chains
60	&
61	A
62	B
63	C
64	D
65	E
66	F
67	G
70	H
71	I
72	?
73	.
74	)		lozenge
75	[					blank in A, H chains
76	<					blank in A, H chains
77	"		group mark		blank in A, H chains