IBM 1401

IBM 1401
Designer IBM
Bits 6-bits plus word mark and parity
Introduced 1959
Design CISC
Type Memory-Memory
Encoding Variable
Branching Branch instruction with modifier character
Endianness Big
3 index, in memory, optional
IBM 1401 Data Processing System. From the left: 1402 Card Read-Punch, 1401 Processing Unit, 1403 Printer.
1403 high speed line printer (At House for the History of IBM Data Processing (closed), Sindelfingen)

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 unit record equipment for processing data stored on punched cards and at providing peripheral services for larger computers.[1] Over 12,000 units were produced and many were leased or resold after they were replaced with newer technology. The 1401 was withdrawn on February 8, 1971.


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.[2]

Monthly rental for 1401 configurations started at US$2,500 (worth about $20,328 today).[3]

"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!"[4] 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."[4] The system was marketed until February 1971.[5]

Commonly used by small businesses as their primary data processing machines, the 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 slow 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. This allowed the mainframe's throughput to not be limited by the speed of a card reader or printer. (For more information, see Spooling.)

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.[6]

A volunteer repairing a 1401 at the Computer History Museum

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 a 1401 installed at Pakistan International Airlines.[7]

Two 1401 systems have been 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 and distribute cooled air.[8][9]


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.

IBM called the 1401's character code BCD, even though that term describes only the decimal digit encoding.[10] 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: 1400, 2000, 4000, 8000, 12000, or 16000 characters.[lower-alpha 1] Each character was addressable, addresses ranging from 0 through 15999. A very small number of 1401s were expanded to 32,000 characters by special request.

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

   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 digit memory address. The three low order digits of the five digit 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).[lower-alpha 2] 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 could specify one of three 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 low-order (highest-addressed) position, their length defined by a word mark set at their high-order (lowest-addressed) position. When an operation such as addition was performed, the processor began at the low-order position of the two fields and worked its way to the high-order, just as a person would when adding with pencil and paper.

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 high-order position, its length defined by a group-mark character with a word mark or a record-mark character in its low-order position. The instruction Move Characters 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 high-order position, its length defined by a group-mark character with a word mark immediately following its low-order 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.[11]

Booting and sample program

When the LOAD button on the 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[12] here is a one-card program which will print "HELLO WORLD". Pressing LOAD (above) begins execution at location 001 (the first ",").

,008015,022029,036043,050054,055062,063065,069080/333/M0792502F1.065HELLO WORLD

The program is:

Hardware implementation

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

Most of the logic circuitry of the 1401 was a type of diode-transistor logic (DTL), that IBM referred to as CTDL (Complemented Transistor Diode Logic). Other IBM circuit types used were referred to as: Alloy (some logic, but mostly various non-logic functions, named for the germanium-alloy transistors used), CTRL (Complemented Transistor Resistor Logic, 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 (Saturated Drift Transistor Diode Logic). 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.

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 114 mm) with a 16-pin gold plated edge connector (single wide) or 5.375 by 4.5 inches (136.5 by 114.3 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).

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

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 4000-character memory module.[13] One such module could be housed within the 1401's primary enclosure. Systems were commonly available with two, three, or four such modules. The additional modules were 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.[14] All instruction timings were cited in multiples of this cycle time.[15]


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

IBM software for the 1401 included:

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

Character and op codes

The 1401's operation codes were single characters. In many cases, particularly for the more common instructions, the character chosen was mnemonic for the operation: A for add, B for branch, S for subtract, etc.

The table is in Character Collating Sequence.

Note: If Word mark bit is set, then the C bit will be opposite of shown. 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.
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.
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


  A       Cannot be read from card without a no-cost RPQ, in which case it is read as 8-2.[lower-alpha 3]

Punches as zero (or 8-2 with the RPQ). 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 or Branch on Indicator See "Modifiers for five-character Branch on Indicator (B) instruction" section
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 See "Modifiers for Select Stacker (K) instruction" section
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.

Two of the instructions, Branch on Indicator (B) and Select Stacker (K), use a "modifier" operand.

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

The B opcode, if followed by a three-character operand, was a simple unconditional branch to the indicated address. If a fourth operand character was present (making five characters total including the opcode), this made it a conditional branch. This "modifier" character specified the condition to be tested.

BCD Character Unit Meaning
Blank 1401 Unconditional—no word mark needed in next character
1 1009 in run condition
  1011 punch in backspace operation
  1011 reader parity indicator on
  1231 auto select
  1285 error
  1412 control-check indicator on
  1418 late read
  1419 document to be read
  1428 late read or late reading mode change
  1445 printer error
  7340 hypertape unusual end
  7740 transmission complete with abnormal status
  DDC transmission error
2 1009 buffer available
  1011 punch ready
  1011 reader ready
  1231 full buffer
  1285 end of line
  1412 reader-not-ready signal on
  1418 ready to engage
  1419 document under read head (PDS 4)
  1428 ready to engage
  7340 hypertape normal end
  7740 transmission complete successfully
  DDC transmission ended by GMWM
3 1009 good transmission occurred
  1011 punch not ready to receive data
  1231 ready to read
  1285 reader transporting
  1412 read-check indicator on
  1418 document under selected read station
  1419 valid amount field
  1428 document under selected read station
  7340 hypertape control unit 7641 busy
  7740 receive request
  DDC read request
4 1009 reply-bad indicator on
  1011 punch not ready to read
  1231 empty hopper
  1285 marked line
  1412 amount-field indicator on
  1419 valid process-control field
  7340 hypertape attention
  7740 7740 attention
  DDC write request
5 1009 error reply acknowledgement
  1011 punch overextended
  1231 read error or overrun
  1285 header information
  1412 process-control indicator on
  1418 document end
  1419 valid account-number field
  1428 document end
  1445 printer busy
6 1009 program attention required
  1011 punch supply reel low or chad box full
  1231 timing mark check
  1285 ready to read a line
  1412 account-number indicator on
  1418 character on line
  1419 valid transit-number field
  1428 character on line
  1445 carriage busy
  DDC write in progress
7 1009 end of message
  1285 reader ready
  1412 transit-number indicator on
  1418 empty hopper and transport (end of file)
  1419 valid serial-number field
  1428 empty hopper and transport (end of file)
  1445 carriage channel 9
  DDC read in progress
8 1009 end of file
  1285 end of file
  1412 document-spacing-check indicator on
  1418 ready to read
  1419 auto-select
  1428 ready to read
  1448 carriage channel 12
  DDC system A stopped
9 1403 carriage tape channel 9
0 1404 validity error
' or @ 1403 carriage tape channel 12
> 1448 end of block
/ 1401 unequal compare
S 1401 equal compare
T 1401 low compare
U 1401 high compare
V 1301 disk error
  1311 disk error
  1405 read or write parity check or read back check error
W 1301 wrong-length record
  1311 wrong-length record
  1405 wrong-length record
X 1301 unequal address compare
  1311 unequal address compare
  1405 unequal address compare
Y 1301 any disk-unit error condition
  1311 any disk-unit error condition
  1405 any disk-unit error condition
Z 1401 arithmetic overflow
1403 printer error (record mark)
( or % 1401 process check with process check switch off
  1440 I/O check stop switch off
\ 1301 access busy
  1311 access busy
J 1419 I/O channel-busy indicator
  TAU tape busy
  SIO serial input-output busy
K TAU end of reel (writing) or tape mark (reading)
L TAU tape error
N 1301 access inoperable
  1311 access inoperable
  1405 access inoperable
P 1403 printer busy
Q 1407 inquiry request
R 1403 carriage busy
! 1402 punch error
* 1407 inquiry clear
+ or & 1442 last card (Reader unit 2)
A 1402 last card if SS A is on
B 1401 SS B
  7340 hypertape attention response
C 1401 SS C
D 1401 SS D
E 1401 SS E
  7340 hypertape end response
F 1401 SS F
G 1401 SS G
H 1402 reader busy
I 1402 punch busy
? 1402 reader error
< 1448 early warning

Modifiers for Select Stacker (K) instruction

The Select Stacker (K) instruction sent commands to various devices. It was named for its relevance to the 1402 reader/punch.

BCD Character Unit Meaning
Blank 1401 set program activity recording at BBB only if A is, dd
1 1402 select last card read to stacker 1 if within 10 ms
2 1402 select last card read to stacker 8/2 if within 10 ms
4 1402 select previous card punched to stacker 4
8 1402 select previous card punched to stacker 8/2
= or # 1444 select previous card punched to stacker 2
L 1009 operate in load mode
$ 1402 overlap on
A 1009 suppress 3-second alarm
C DDC read request
D 1009 set direction to receive
  DDC write request
E 1009 set direction to transmit
  DDC reset
F 1009 send EOF
. 1402 overlap off
) 1402 reset overlap

1401 culture

In October 2006, Icelandic avant-garde musician Jóhann Jóhannsson released the album IBM 1401, A User's Manual through music publisher 4AD.[17] 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 Elías Daviðsson,[18] 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, Jóhann 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 1401 found in his father's attic.[18]

More well-known were various demo programs to play music on transistor radios placed on the CPU[19] and computer "art", mostly kitschy pictures printed using Xs and 0s on chain printers.[20]


  1. The 1401 was a decimal computer, so e.g. "8000" characters is not 8,192 characters.
  2. More simply stated: the four zone bits encoded the decimal values 0 to 15 as the binary numbers 0000 to 1111. The encoded value was used as the thousands part of a 1401 address. Thus making the 1401 a curious machine with memory addresses stored as a combination of 3 decimal digits and 4 binary digits. IBM did not use the word "binary" in describing the 1401's addressing (see the 1401 Reference Manual).
  3. The RPQ "Reader/Punch Card Code 8-2 and A-Bit Compatibility" (898148) causes the 1401 to read/punch the A bit as 8-2 for compatibility with the IBM 1410.[16]


  1. Boshe, Charles J.; Johnson, Lyle R.; Palmer, John H.; Pugh, Emerson W. (1986). IBM's Early Computers. MIT Press. p. 473. ... configured for stand-alone use as well as peripheral service for larger computers ... A small configuration, without tapes and with the minimum memory capacity, was available for just under $2500 per month, a much lower rental for much higher performance than three 407 accounting machines plus a 604 calculator.
  2. "1401 Data Processing System". IBM Archives. October 5, 1959. Retrieved June 2, 2010.
  3. Columbia University, Computing History Project
  4. 1 2 Spicer, Dag. "Back to Life: The story behind CHM's IBM 1401 Restoration" (PDF). Computer History Museum. Retrieved May 27, 2011.
  5. "FAQ's for Products and Services". IBM Archives. Retrieved June 2, 2010.
  6. Pugh, Emerson W. (1995). Building IBM: Shaping an Industry and Its Technology. MIT. p. 273. ISBN 0-262-16147-8.
  7. "History of PIA".
  8. "1401 Restoration Project".
  9. "1401 "Rebuilding the IBM," by Philip E. Ross, IEEE Spectrum, November 2009".
  10. IBM and BCD
  11. IBM (April 1962). IBM 1401 Data Processing System: Reference Manual (PDF). p. 20. A24-1403-5.
  12. "Tom Van Vleck: 1401s I have known".
  13. Rob Storey. "Core memory frame from a 1401". Retrieved July 15, 2012.
  14. "The IBM 1401". IBM 1401 Restoration Project. Computer History Museum. Retrieved July 15, 2012. The 1401’s clock frequency is 86,957 cycles per second, or about 87 kiloHertz! This corresponds to an 11.5 micro-second system clock cycle time. ... The 1401 CPU does everything in a character-serial manner. In order to add say two N-digit numbers, the CPU takes several cycles to fetch the instruction itself and then one cycle for every character of the instruction’s two operands or arguments, or 2N cycles total.
  15. IBM Corporation (1961). "IBM 1401 Principles of Programming, Section 7" (PDF). IBM Personal Study Program. IBM Corporation. p. 19. Retrieved July 15, 2012. The timing of the IBM 1401 is described in terms of the time required for one complete core storage cycle, which is 11.5 microseconds ... The time required for any internal processing instruction is always a multiple of this interval of time.
  16. "Custom Features for IBM 1401, 1440, and 1460 Data Processing Systems" (PDF). Retrieved September 23, 2015.
  17. "IBM 1401, A User's Manual-Live in Italy". Retrieved January 29, 2015.
  18. 1 2 "Jóhann Jóhannsson: IBM 1401, A User's Manual". work's web site. Retrieved January 29, 2015.
  19. "1401-music-Movie".
  20. Gansing, Kristoffer (2007). "Working Paper version - Humans Thinking Like Machines - Incidental Media Art in the Swedish Welfare State" (PDF). University of Malmö, School of Arts & Communication. Retrieved May 2011. Check date values in: |access-date= (help) A full version to be published in Place Studies in Art, Media, Science and Technology, VDG Weimar 2009


Further reading

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