IBM 1401

IBM 1401 Data Processing System. From the left: 1402 Card Read-Punch, 1401 Processing Unit, 1403 Printer.

The IBM 1401 was a variable wordlength decimal computer that was announced by IBM on October 5, 1959. The first member of the highly successful IBM 1400 series, it was aimed at replacing electromechanical unit record equipment for processing data stored on punched cards. Over 10,000 units were produced and many were leased or resold in less developed countries after they were replaced with newer technology. The 1401 was withdrawn on February 8, 1971.

History

A volunteer repairing an IBM 1401 at the Computer History Museum

Closeup of a swing-out card cage (or gate in IBM parlance) showing some of the 1401's SMS circuit cards

From the announcement:

The all-transistorized IBM 1401 Data Processing System places the features found in electronic data processing systems at the disposal of smaller businesses, previously limited to the use of conventional punched card equipment. These features include: high speed card punching and reading, magnetic tape input and output, high speed printing, stored program, and arithmetic and logical ability.'
The 1401 may be operated as an independent system, in conjunction with IBM punched card equipment, or as auxiliary equipment to IBM 700 or 7000 series systems.^[1]

Monthly rental for 1401 configurations started at US$2,500.^[2]

"IBM was pleasantly surprised (perhaps shocked) to receive 5,200 orders in just the first five weeks – more than predicted for the entire life of the machine!"^[3] By late 1961, the 2000 installed in the USA were about one quarter of all electronic stored-program computers by all manufacturers. The number of installed 1401s peaked above 10,000 in the mid-1960s. "In all, by the mid-1960s nearly half of all computer systems in the world were 1401-type systems."^[3] The system was marketed until February 1971.^[4]

Commonly used by small businesses, the IBM 1401 was also frequently used as an off-line peripheral controller for mainframe computers. In such installations, with an IBM 7090 for example, the mainframe computers used only magnetic tape for input-output. It was the 1401 that transferred input data from peripherals, such as the IBM 1402 Card Read-Punch, to tape, and transferred output data from tape to the card punch, the IBM 1403 Printer, or other peripherals.

Elements within IBM, notably John Haanstra, an executive in charge of 1401 deployment, supported its continuation in larger models for evolving needs (e.g., the IBM 1410) but the 1964 decision at the top to focus resources on the System/360 ended these efforts rather suddenly. Then, faced with the competitive threat of the Honeywell 200 and the 360's incompatibility with the 1401 design, IBM pioneered the use of microcode emulation, in the form of ROM, so that some System/360 models could run 1401 programs.^[5]

Two 1401 systems are being restored to operating order at the Computer History Museum in Mountain View, California, complete with a raised floor typical of the mainframe era (and modern data centers), used to hide cabling.^[6][7]

Architecture

IBM 1401 Console and, below, the Auxiliary Console.

Each alphanumeric character in the 1401 was encoded by six bits, called B,A,8,4,2,1. The B,A bits were called zone bits and the 8,4,2,1 bits were called numeric bits, terms taken from the IBM 80 column punched card.

• For digits 1 through 9, the bits B,A were zero, the digit BCD encoded in bits 8,4,2,1. Digit 0 was encoded 8,2.
• For alphabetic characters the setting of bits was derived from the zone and digit punches of the IBM 80 column punched card character code: B,A from 12, B from 11, and A from 0; the setting of bits 8,4,2,1 from BCD encoding of the 1 through 9 punches. Thus the letter A, 12,1 in the punched card character code, was encoded B,A,1.
• Encodings of punched card characters with two or more digit punches can be found in the Character and Op codes table.

IBM called the 1401's character code BCD, even though that term describes only the decimal digit encoding.^[8] The 1401's alphanumeric collating sequence was compatible with the punched card collating sequence.

Associated with each memory location were two other bits, called C for odd parity check and M for word mark.

Each memory location then, had the following bits:

C B A 8 4 2 1 M

The 1401 was available in six memory configurations: 1.4K,^[9] 2K, 4K, 8K, 12K, or 16K locations. A very small number of 1401s were expanded to 32K by special request. Each memory location was addressable, addresses were 0 through 15999.

Some operations used specific memory locations (those locations were not reserved and could be used for other purposes). Read a card stored the 80 columns of data from a card into memory locations 001-080. Index registers 1, 2 and 3 were in memory locations 087-089, 092-094 and 097-099 respectively. Punch a card punched the contents of memory locations 101-180 into a card. Write a line printed the contents of memory locations 201-332.

The 1401's instruction format was

  Opcode   with   [A-or-I-or-unit-address [B-address]] [modifier] word mark

Opcodes were one character. Memory addresses ("I" a branch target, "A" and "B" data) and unit address were three characters. The opcode modifier was one character. Instruction length was then 1, 2, 4, 5, 7, or 8 characters. Most instructions had to be followed by a word mark (a requirement commonly met by the word mark with the opcode of the next instruction).

See Character and Op codes for a list of operations.

A three character memory address in an instruction was an encoding of a five character memory address. The three low order digits of the five character address, 000 to 999, were specified by the numeric bits of the three characters. The zone bits of the high-order character specified an increment as follows: A 1000, B 2000, B and A together 3000, giving an addressability of 4,000 memory locations. The zone bits of the low-order character specified increments of 4000, 8000, or 12000, to address 16,000 memory locations (with an IBM 1406 Storage Unit). For example, the three character address "I99" was a reference to memory location 3000 + 999, or 3999.

The zone bits of the middle character of a three character memory address specified index registers, one of many optional features.

Operands referenced by the A-address and B-address were: a single memory location, a variable length field, or a variable length record. Variable length fields were addressed at their loworder position, their length defined by a word mark set at their highorder position. The only limit on the length of such fields was the available memory. Instructions applicable to variable length fields included: Add, Subtract, Multiply, Divide, Compare, Move Characters to A or B Word Mark, Move Characters and Edit. One or more adjacent variable length fields could make up a variable length record. A variable length record was addressed at its highorder position, its length defined by a group-mark character with a word mark or a record-mark character in its loworder position. The instruction Move Characters to Record or Group Mark could be used to assemble a block of records. A variable length record, or block of records, to be written to magnetic tape was addressed at its highorder position, its length defined by a group-mark character with a word mark immediately following its loworder position.

A sequence of operations on adjacent fields could be "chained", using the addresses left in the address registers by the previous operation. For example, addition of adjacent data fields might be coded as "A 700,850", "A 695,845", "A 690,840". With chaining, this could be coded as "A 700,850", "A", "A" - omitting data address from the 2nd and 3rd instructions.^[10]

Booting and Sample Program

When the LOAD button on the IBM 1402 Card Read-Punch was pressed, a card was read into memory locations 001-080, a word mark was set in location 001, the word marks in locations 002-080 were cleared, and execution started with the instruction at location 001. That was always the dyadic Set Word Mark (it was the only instruction not requiring a following word mark) to set word marks for the two following instructions. Execution of instructions in the card deck continued, loading the program into memory, setting word marks, and then branching to the program's start address.

One-card programs could be written for various tasks. Commonly available were a one-card program to print the deck of cards following it, and another to duplicate a deck to the card punch. From Tom Van Vleck's web site^[11] here is a one-card program which will print "HELLO WORLD". Pressing LOAD (above) begins execution at location 001 (the first ","). The program is: set word marks (opcode "," operands 008 015), set more word marks, clear storage - part of the print area 333-300 (opcode "/" operand 333), clear storage - the rest of the print area 299-200 (opcode "/" using a chained address), move "HELLO WORLD" to the print area (opcode "M", operands 079 and 250), print a line (opcode "2"), eject the page in the printer (opcode "F" modifier "1"), and halt (opcode "." operand 065 - a branch address, to this same halt command, if start is pressed).

, 008015,022029,036043,050054,055062,06 3065,069080/333/M0792502F1.065HELLO WORLD

Hardware implementation

IBM 1403 high speed line printer in the "Haus zur Geschichte der IBM Datenverarbeitung" museum, Sindelfingen, Germany

Most of the logic circuitry of the 1401 was a type of diode-transistor logic (DTL), that IBM referred to as CTDL. Other IBM circuit types used were referred to as: Alloy (some logic, but mostly various non-logic functions, named for the kind of transistors used), CTRL (a type of resistor-transistor logic (RTL)). Later upgrades (e.g., the TAU-9 tape interface) used a faster type of DTL using "drift" transistors (a type of transistor invented by Herbert Kroemer in 1953) for their speed, that IBM referred to as SDTDL. Typical logic levels of these circuits were (S & U Level) high: 0 V to -0.5V, low: -6 V to -12 V; (T Level) high: 6 V to 1 V, low: -5.5 V to -6 V.

A single width SMS card of the type used in the 1401

These circuits were constructed of discrete components (resistors, capacitors, transistors) mounted on single sided paper-epoxy printed circuit boards either 2.5 by 4.5 inches (64 by 110 mm) with a 16-pin gold plated edge connector (single wide) or 5.375 by 4.5 inches (136.5 by 110 mm) with two 16-pin gold plated edge connectors (double wide), that IBM referred to as SMS cards (Standard Modular System). The amount of logic on one card was similar to that in one 7400 series SSI or simpler MSI package (e.g., 3 to 5 logic gates or a couple of flip-flops on a single wide card up to about 20 logic gates or 4 flip-flops on a double wide card).

The SMS cards were inserted in sockets on hinged swing out racks, that IBM referred to as gates.

Like most machines of the day, the 1401 used magnetic-core memory. The cores were about 1 mm in diameter and used a four-wire arrangement (x, y, sense, and inhibit). The memory was arranged in planes of 4000 cores each, each core storing one bit. A stack of eight such planes stored the six data bits, word mark bit, and parity bit for 4000 memory locations. Together with eight additional planes with fewer cores on them for additional storage functions, this made up a 4K memory module.^[12] One such module could be housed within the 1401's primary enclosure. Systems were commonly available with 8K, 12K, or 16K of memory, the additional modules being contained in an add-on box, the 1406 Core Memory Unit, which was about two feet square and three high. Operands in memory were accessed serially, one memory location at a time, and the 1401 could read or write one memory location within its basic cycle time of 11.5 microseconds.^[13] All instruction timings were cited in multiples of this cycle time.^[14]

Software

1403 line printer opened, with 729 tape drives in the background.

Software for the 1401 included:

• IBM 1401 Symbolic Programming System assembler.
• Autocoder on Tape, a more advanced assembler, required at least 4K memory and four tape drives.
• Autocoder on Disk, similar to, but not compatible with, Autocoder on Tape, required one or more 1311 disk drives.
• FARGO (Fourteen-o-one Automatic Report Generation Operation), a predecessor of RPG, required 4K memory.
• FORTRAN II required at least 8K memory; the 1401 Fortran compiler is described in Haines, L.H. (1965), below. The Fortran compiler, to generate code for small memories, used a pioneering form of interpreted "p-code" although, of course, its programmers had no name for what it was that they did.
• FORTRAN IV required at least 12K memory and either 4 tape drives or 1 IBM 1311 disk drive.
• COBOL required at least 4k memory and 4 tape drives.
• IBM RPG (Report Program Generator) Basic RPG required at least 4K memory.

For the IBM Catalog of 1401 software, see IBM 1400 series.

Art inspired by the IBM 1401

In October 2006, Icelandic avante-garde musician Jóhann Jóhannsson released the album 'IBM 1401, A User's Manual through music publisher 4AD.^[15] The concept is based upon work done in 1964 by his father, Jóhann Gunnarsson, chief maintenance engineer of one of the country’s first computers, and Elias Davidsson, one of the first programmers in the country. The album was originally written for a string quartet, organ and electronics and to accompany a dance piece by long-standing collaborator friend, Erna Ómarsdóttir. For the album recording, Johann has rewritten it for a sixty-piece string orchestra, adding a new final movement and incorporating electronics and vintage reel-to-reel recordings of a singing IBM 1401 found in his father’s attic.^[16]

1401 culture

A reel of half-inch, 7-track tape being loaded onto an IBM 729 tape drive. An IBM 1403 line printer is in the foreground.

More well-known were various demo programs to play music on transistor radios placed on the CPU^[17] and computer "art", mostly kitschy pictures printed using Xs and 0s on chain printers.^[18]

The 1401 was made before FCC rules came into effect limiting unintentional radiation from electronic devices. As a result the 1401 would radiate signals covering the entire AM radio band. This had some utility as an AM radio placed on top of the computer would emit a stream of audible notes until the program ended, at which time the sounds would stop. This allowed a solo operator of a 1401 to do other work until they were needed to start another program.^{[citation needed]} Each operation transmitted a specific note, thus allowing someone to put together music by executing the right op codes in a specific order. As an operator this also had the effect that specific jobs would play the same musical notes when then run. For example, an operator familiar with the sounds could easily identify an Autocoder compiler execution from a payroll job based on the sequence of notes.^{[citation needed]}

Another feature of the 1401 was small muffin fans that blew cool air into the bottom of each gate, up around the electronics and out the top. It was common to take two standard IBM punch cards, stapled together into a circle, and with the top cut and folded over, and place them over the air stream from a fan. The cards would stay centered on the air stream and spin round and round as the computer operated. This allowed operators to check at a glance from outside the room whether the computer was operating or not based on the motion of the cards.^{[citation needed]}

The simplicity of the 1401 made it, if not a modern ruggedized machine, fairly reliable in stressful conditions. Thus IBM 1401 systems were more extensively used for US military logistics applications, in Vietnam and elsewhere, than were other, more advanced, systems.

During the 1970s, IBM installed many 1401s in India and Pakistan where they were in use well into the 1980s. Some of today's Indian and Pakistani software entrepreneurs started on these 1401s. The first computer in Pakistan, for example, was an IBM 1401 installed at Pakistan International Airlines.^[19]

Character and Op codes

The table is in Character Collating Sequence.

Note: If Word mark bit is set, then the C bit will be opposite of shown. Of course, the C bit was determined and checked automatically by the machine - normally it was of no concern to the programmers. The only way the C bit could be entered was by manually using the switches on the Auxiliary Console. A programmer might use these switches to make quick patches while debugging.

BCD Character	Print-A	Print-H	Card	BCD w/o M	Operation	Definition & Notes
Blank				C
.	.	.	12-3-8	BA8 21	Halt
¤	¤	)	12-4-8	CBA84	Clear Word Mark	Lozenge
[			12-5-8	BA84 1
<			12-6-8	BA842		Less Than
			12-7-8	CBA8421		Group Mark
&	&	+	12	CBA
$	$	$	11-3-8	CB 8 21
*	*	*	11-4-8	B 84
]			11-5-8	CB 84 1
			11-6-8	CB 842
Δ			11-7-8	B 8421		Delta (Mode Change)
-	-	-	11	B
/	/	/	0-1	C A   1	Clear Storage
,	,	,	0-3-8	C A8 21	Set Word Mark
%	%	(	0-4-8	A84	Divide	Optional special feature.
ˠ			0-5-8	C A84 1		Word Separator
\			0-6-8	C A842		Left Oblique
⧻			0-7-8	A8421		Tape Segment Mark
ƀ	‡	‡	N/A 0	A		Cannot be read from card without a no-cost RPQ, in which case it is read as 8-2. Punches as zero. Blank with "even-parity" on tape.
#	#	=	3-8	8 21	Modify Address	Optional (requires more than 4000 characters of memory)
@	@	'	4-8	C  84	Multiply	Optional special feature.
:			5-8	84 1
>			6-8	842		Greater Than
√			7-8	C  8421		Tape Mark
?	&	&	12-0	CBA8 2	Zero and Add	Plus Zero
A	A	A	12-1	BA   1	Add
B	B	B	12-2	BA  2	Branch
C	C	C	12-3	CBA  21	Compare
D	D	D	12-4	BA 4	Move Numerical	(Bits)
E	E	E	12-5	CBA 4 1	Move Characters and Edit
F	F	F	12-6	CBA 42	Control Carriage	(Printer)
G	G	G	12-7	BA 421
H	H	H	12-8	BA8	Store B-Address Register	Optional special feature.
I	I	I	12-9	CBA8  1
!	-	-	11-0	B 8 2	Zero and Subtract	Minus Zero
J	J	J	11-1	CB    1
K	K	K	11-2	CB   2	Select Stacker and other device controls	(Card and some others)
L	L	L	11-3	B   21	Load Characters to Word Mark
M	M	M	11-4	CB  4	Move Characters to Word Mark
N	N	N	11-5	B  4 1	No Operation
O	O	O	11-6	B  42
P	P	P	11-7	CB  421	Move Characters to Record or Group Mark	Optional special feature.
Q	Q	Q	11-8	CB 8	Store A-Address Register	Optional special feature.
R	R	R	11-9	B 8  1
‡	‡	‡	0-2-8	A8 2		Record Mark
S	S	S	0-2	C A  2	Subtract
T	T	T	0-3	A  21	Translate	(1460 only)
U	U	U	0-4	C A 4	Control Unit	(Tape)
V	V	V	0-5	A 4 1	Branch if Word Mark and/or Zone
W	W	W	0-6	A 42	Branch if Bit Equal	Optional special feature.
X	X	X	0-7	C A 421	Move and Insert Zeros	Optional special feature.
Y	Y	Y	0-8	C A8	Move Zone	(Bits)
Z	Z	Z	0-9	A8  1	Move Characters and Suppress Zeros
0	0	0	0	C  8 2
1	1	1	1	1	Read a Card
2	2	2	2	2	Write a Line
3	3	3	3	C    21	Write and Read
4	4	4	4	4	Punch a Card
5	5	5	5	C   4 1	Read and Punch
6	6	6	6	C   42	Write and Punch
7	7	7	7	421	Write, Read, and Punch
8	8	8	8	8	Start Read Feed	Optional special feature.
9	9	9	9	C  8  1	Start Punch Feed	Optional special feature.

Modifiers for Select Stacker (K) instruction

Modifiers for five-character Branch on Indicator (B) instruction

References

Further reading

• Bashe, Charles J.; Johnson, Lyle R; Palmer, John H.; Pugh, Emerson W. (1986). IBM's Early Computers. MIT. pp. 717. ISBN 0-262-02225-7. Chapter 12 Broadening the Base pages 465-494, a history of IBM's 1401 and 1403 development
• IBM (April 1966) (PDF). IBM 1401 System Summary. A24-1401-1. http://bitsavers.org/pdf/ibm/140x/A24 -1401-1_1401_System_Summary_Sep64.pdf. Brief descriptions of the machine features, components, configurations, and special features

• IBM (April 1962) (PDF). IBM 1401 Data Processing System: Reference Manual. A24-1403-5. http://bitsavers.org/pdf/ibm/140x/A24 -1403-5_1401_Reference_Apr62.pdf.

External links

• IBM 1401 documents on bitsavers.org
• A Century of Smart: The IBM 1401 (1959). IBM Social Media. 2009-11-16. http://www.youtube.com/watch?v=hQcaYv bwLPo. Retrieved 2009-11-17. Video captures thoughts and reflections of some of the original 1401 team members from a reunion held in Endicott, NY in 2009. Includes footage from IBM 1401 marketing films.
• IBM 1401 videos and sounds
• 1401s I have Known, Tom Van Vleck
• Haines, L. H. (1965). "Serial compilation and the 1401 FORTRAN compiler". IBM Systems Journal 4 (1): 73–80. doi:10.1147/sj.41.0073. http://domino.research.IBM.com/tchjr/ journalindex.nsf/495f80c9d0f539778525 681e00724804/cde711e5ad6786e485256bfa 00685a03?OpenDocument.. This article was reprinted, edited, in both editions of Lee, John A. N. (1967(1st), 1974(2nd)). Anatomy of a Compiler. Van Nostrand Reinhold.
• Music inspired by the IBM 1401: Johann Johannsson's "IBM 1401: A User's Manual"