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IBM 1401

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The IBM 1401 was a variable wordlength decimal computer that was announced by IBM on October 5, 1959 and marketed as an inexpensive "Business Computer". It was withdrawn on February 8, 1971.

Although described as a (BCD) computer, each byte (or alphameric character) in the 1401 was represented by six bits, called A, B, 8, 4, 2 and 1. The A and B bits were called zone bits and the 8, 4, 2 and 1 bits were called numeral or BCD bits. Associated with each six-bit byte were two other bits, called C for odd parity check and M for wordmark, in the following format:

  C B A 8 4 2 1 M

An IBM 1401 core memory address consisted of three six-bit bytes. The decimal address within a 1000-byte page was specified by the BCD bits of the address. Addresses that did not contain valid BCD codes in these bits caused a hard halt. Early machines used the A and B bits of the high-order byte to specify which of four pages was referred to, giving an addressability of 4,000 bytes in all. Several storage sizes were available up to this maximum. Later machines used the zone bits of the low-order byte to increase this maximum to 16,000 bytes, with an IBM 1406 memory expansion unit. The zone bits of the middle byte were used to specify index registers, one of many optional features.

The 1401 was the first member of the IBM 1400 series. The IBM 1410 was a similar design, but with a larger address space. The last member was the IBM 1460, logically but not physically identical to a fully optioned 1401 with 16,000 bytes of memory.

Instructions were of four lengths. Four-byte instructions consisted of an opcode followed by an address, five byte instructions an opcode, address and modifier byte, seven byte instructions an opcode followed by two addresses, and eight byte instructions an opcode, two addresses and a modifier byte.

Instructions were only valid if the M bit was set on the low-order (opcode) byte and nowhere else in the instruction. There was one exception to this rule: The dyadic (seven byte) SET WORDMARK instruction, which set two wordmarks, was valid provided the wordmark was set by the completion of the instruction. This was necessary as at power-on and following some reset conditions, all wordmarks were cleared. Thus, the first instruction of any bootstrap program was a dyadic set wordmark, which validated itself and one other instruction. In practice, the first few cards of a card-deck bootstrap program would consist entirely of dyadic set wordmark instructions, no-op instructions and a "read card", which would set up a pattern of wordmarks in the card read buffer. By use of no-op instructions of various lengths, the next few cards would conform to this pattern of wordmarks.

The IBM 1401 was also commonly used as an off-line peripheral controller in many installations of both large "Scientific Computer"s and large "Business Computer"s. In these installations the big computer (e.g., an IBM 7090) did all of its input-output on magnetic tapes and the 1401 was used to format input data from other peripherials (e.g., punch card readers) on the tapes and transfer output data from the tapes to other peripherals (e.g., punch card punches or the IBM 1403 lineprinter).

At peak, there were over 10,000 installed systems running in the mid-1960s. The IBM 1401 was withdrawn in February 1971. During its lifetime about 20,000 total systems were manufactured, making the IBM 1401 one of IBM's most successful products.

Major software on the 1401 included a simple assembler called the Symbolic Programming System (SPS) and a more advanced form of assembler, Autocoder. The only high-level language in common use was the RPG (Report Program Generator) language, a declarative language primarily for specifying accounting reports still in use on IBM's midrange AS/400. Fortran was available for systems containing at least 8000 memory locations and the optional hardware unit for multiplication and division and is described in an appendix of John A. N. Lee's 1968 book The Anatomy of a Compiler; 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 is that they did.

Elements within IBM, notably John Haanstra, an executive in charge of 1401 deployment, supported its continuation in larger models for evolving needs but the 1964 decision at the top to focus resources on the System/360 ended these efforts rather suddenly. Nonetheless, to preserve customer investment in 1401 software, IBM pioneered a use of microcode in the form of ROM which caused 360 models to emulate 1401 instructions well into the modern era... in some cases, perhaps, until Y2K efforts caused the still-running 1401 code to be rewritten.

Notable installations included a high end 1440 at the Chicago police department installed by reformist superintendant Orlando Wilson in the early 1960s. During the 1970s, many installations in India used the 1401 and some of today's Indian software entrepreneurs started on this machine.

An extant 1401 is being restored to operation at this writing at the Computer History Museum in Mountain View, California, complete with the old "false floor" of the mainframe era, used to hide cabling.

Announced as marginal, and for the simple accounting of midrange companies and as a sort of slave print server to the IBM 7090, the 1401 (like many other platforms before it and since) attracted its own hard core of devoted followers, who could see that although marketed as limited in function, the 1401 was in logical terms a real computer, whose secrets could be unlocked by people sufficiently patient to understand its arcane addressing scheme, and to devise ways around its limitations.

Character and Op codes

The table below is listed in Character Collating Sequence.

BCD Character Print-A Print-H  Card  BCD 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.
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
ˉ     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 (Card)
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    
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.


Hardware implementation

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)).

These circuits were constructed of individual discrete components mounted on single sided paper-epoxy printed circuit boards either 2.5 by 4.5 inches (38 by 114 mm) with a 16 pin gold plated edge connector (single wide) or 5.375 by 4.5 inches (82 by 114 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).

These boards were inserted in sockets on racks, that IBM referred to as gates.

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