About the Computer History Museum’s IBM 1401 Machines

 Guy Fedorkow   February 19, 2015  Guest Blog  0

We’ve all heard it: “Today, your phone has a zillion times more processing power than giant computers that filled an entire room in days past!” Rather than review bits and bytes, let’s look at CHM’s 1401 computer from the point of view of what it can do, and its long-lasting impact on the development of business computing.

IBM 1401 Promotional Photo

IBM 1401 Promotional Photo

At its introduction in 1959, the 1401 was not the largest computer on the block; for IBM, that would have been the massive 7090 scientific mainframe, introduced a year earlier, followed by the even-larger STRETCH supercomputer in 1961. But the 1401 would have been the cheapest, priced to appeal to ordinary businesses that simply wanted to run their finance and accounting systems more efficiently, and in particular, to replace single-purpose electromechanical machines that used levers, knobs and plugboards to program their operations.

The World According to Punched Cards

IBM's 80-column punched cards

IBM’s 80-column punched cards

By the time the 1401 was introduced, electromechanical systems based on punched cards were widely used to manage business operations. These large and unwieldy machines — sorters, collators, punched card calculators and tabulators – each had unique functions and were used together to solve large-scale business problems. For example, inventory management, general ledger or payroll were all performed by passing punched cards through predetermined sequences of operations on different machines, with operators to carry boxes of cards from one operation (and machine) to the next.

The IBM 402 - punched card-based tabulating machine

The IBM 402 – punched card tabulating machine

Although the punched card methodology was clearly superior to paper, pencils and ledger books, it was still very labor-intensive. A task as routine as producing a monthly sales report, for example, might involve the following machines and steps:

  • Punched cards would be created for each individual sale, containing the part number of the item that had been sold, the numeric ID of the salesperson who sold it, plus other information such as dates and quantities. These cards would be compiled into a so-called detail deck.1
  • A master deck would also be maintained, with one or more cards for each salesperson. The master deck might be updated monthly, and could record annual sales to date, as well as unchanging information relating to the salesperson, such as name and address.

To run the job at the end of the month, operators in the Tab Room (the place where the tabulating machines were) would perform a series of steps with these decks, which could run to many thousands of cards.

  • Before the first punched card is stacked, machines in the Tab Room had to be ‘programmed’ by making mechanical settings and wiring plugboards, setting up the electrical connections that would result in the desired behavior.
  • The detail deck would be sorted in order of salespersons’ ID numbers using a special-purpose machine called a sorter. Sorting stacks of punched cards is no easy task: the machine could sort on only one digit at a time, so to sort based on a simple six-digit employee ID number, these thousands of cards would have to be fed through the sorter six times.
  • The master deck might also need sorting, although with care it could be kept in the right order from the previous month’s run.
  • Here’s the first reason sorting matters: in the next step, the two decks are then “collated,” that is, merged together into one deck with a master card followed by as many detail cards as there are for that person. The collating machine reads a card from the master deck, then reads from the detail deck until the next employee ID number comes up. Then it’s time for the next master card, and so on until the two decks are perfectly intermingled, all sorted by employee ID number.
  • Here’s the second reason sorting matters. The next step is to push the cards through a tabulating machine, an adding machine with punched card input and a printer for output. The tab machine prints the actual report by reading the master card to print the name and previous totals, then the machine reads all the detail cards, printing a line for each one and accumulating new totals. When a new master card turns up, it can print the totals it has accumulated, and then move on to the next salesperson. The tab machine might also punch a new master card for next month, with the new sales-to-date figures.
  • And if the report requires arithmetic more complicated than adding and subtracting, expect to have to push the card deck through another machine a few times, a punched card-controlled calculator like the IBM 604.
  • Once the reports were done, another pass through a sorting machine would separate the master deck from the detail cards, so the master deck would be ready for next month, and the detail cards could be archived.
IBM Plugboard

IBM Plugboard

For each one of these steps, an operator has to supervise the machine, load cards into the input hopper, remove cards from the output hopper and fix jams. Each pass introduced new perils; as tab Veteran W. Van Snyder said: “My first full-time job was in a tab room, sorting decks for input to the 1401.  We had model 84 sorters that could accordion-fold dozens of cards before you could say Jack Robinson.  I rapidly learned how to flatten them out, dupe them on a 026 [keypunch -- see below], and ‘sight check’ them.”  So when IBM brought out the 1401, designed specifically to fit the existing punched card environment, they could offer several radical improvements, all of which cut labor costs (this is in the post-war boom years, when labor costs were getting to be a big deal):

  • A bare-bones 1401 system could replace several of the tabulating and calculating machines, and some of the sorting steps, without disrupting the process flow, while consuming much less operator time.
  • The 1401 also eliminated the accounting machines’ plugboards and mechanical programming. The 1401 was a “stored program computer,” allowing programmers to write (and share) applications loaded into the machine from punched cards or magnetic tape, all without the need to physically reconfigure the machine for each task.
  • With the addition of one magnetic tape drive to store the master deck on tape instead of cards, a 1401 could eliminate the collator machine.
  • With several tape drives, the same machine could do the many sorting jobs required in preparing monthly business reports. The transition from punched cards to magnetic tape was so attractive that ultimately few of the bare-bones systems without tape drives were sold.
  • Finally, unlike the tab machines, a 1401 could do much more than just add and subtract totals, allowing for reports that could offer greater insight into business operations. Not exactly Big Data yet, but the capabilities gave managers a better view into how their business was functioning.

And of course with all this, the 1401 had to be inexpensive enough to displace the punched card machines that were all so deeply entrenched. CHM’s 1401 Demo Lab actually contains two independent machines. Both machines started out under lease from IBM to insurance companies, and both escaped the crusher by being sold second-hand into home businesses. One was acquired from a small business in Germany in 2004, the other retrieved from its owner in Connecticut in 2008, who had remarkably kept the machine in operation running a billing application until as recently as 1995. Let’s look at what’s inside.

So What Is a Computer Anyway?

Just about every recent electronic computer has a similar block diagram if you stand far enough back: There are only three major sections:


  • The Central Processing Unit (CPU) is the part that actually computes things… CPUs can add, subtract, sometimes multiply and occasionally divide, plus perform a host of other operations like copying and comparing data elements. A key component of the CPU is the Arithmetic & Logic Unit (ALU), which actually does the adding and subtracting.
  • The CPU communicates with a Memory system, the place where instructions to tell the CPU what to do are stored as well the data on which the system is operating.  In current computers, even laptops, the memory systems are measured in gigabytes (1,000,000,000s of characters), while in the 1401, memory was measured in 1,000s of characters. Machines could be configured with as little as 1,400 characters of memory, or with as many as 16,000 characters.2 Not a lot, but enough that careful programmers could compute very useful results.
  • The third block contains all the Input/Output (I/O) devices, the elements that communicate with the outside world.

Here’s one of the CHM 1401’s pictured in an earlier configuration during restoration:

The 1401Restoration Lab

The 1401Restoration Lab

What, No Touch Screen??

Of course any computer’s I/O devices are critical, as that’s where programs and data go in, and answers come out. While many I/O devices were added during the 1401 family’s lifetime, the machine started out with only three:

  • As noted, punched cards were critical to data processing using tabulating machines long before what we’d recognize as programmable computers came along. The 1401 relied on a combination card reader (the Input part of I/O) and card punch (the Output part) to read programs and data from stacks of punched cards, and to produce new card decks with results.
  • The function of the Printer is obvious enough – at some point, The Boss needs the Report on Business Operations, and he’s not going to read it on punched cards. The 1401 printer (called a 1403) was a breakthrough in its own right, being able to print over 600 lines of text per minute3 with consistently good print quality. The 1403 printer lived on even longer than the computer for which it was first designed, being sold well into the S/370 era (1970s).
  • The 1401 also could be connected to magnetic tape units. Magnetic tape provided a format much more convenient than punched cards for storing larger amounts of data… a single two-pound magnetic tape could hold 12 million characters of data,4 while the same information on punched cards might take up 160,000 cards, requiring 80 boxes and weighing in at 800 pounds. And once a 1401 had several tape drives, mechanical punched card sorting machines were no longer needed.
  • Later in the 1401 life history, the machine was equipped with random access disk drives (the 1405, based on IBM’s RAMAC technology), which put an end to the era of computing when sorting data before it could be processed was mandatory. Disk drives allowed for instantaneous access to data and real-time monitoring of business operations—a first.

But Where did Punched Cards Come From?

The 1401 doesn’t have a keyboard or display, so how would all that important business data get into the computer in the first place? The 1401 could read the data from punched cards, but where did they come from? For that, there’s another machine in the CHM lab, called a keypunch, where data entry operators could type in the data, one card at a time. The keypunch operates as a typewriter, punching everything the operator types into the body of the card as patterns of holes. Some keypunches also printed the typed characters along the top edge of the card, but many operators became adept at reading the patterns of holes directly.

Keypunch Demo in the CHM 1401 Lab

Keypunch demonstration in the CHM 1401 Demo Lab

Keypunches were a mainstay of the data processing industry long before the 1401 came along, as operators would transcribe data to punched cards for existing card sorters and tabulating gear. CHM’s keypunch is an IBM Model 026, first sold in 1949.

So What’s Interesting About the 1401 Computer?

For those of us interested in how computers work, the 1401 fits into a transitional phase in the development of modern computers. Many characteristics are still very familiar, but some are quite different because of the focus on the “unit record”1 one-punched-card-at-a-time paradigm. As shown above, the machine’s internal architecture lines up well with modern conventional computers, instructions and data in the same memory bank, and an arithmetic unit for doing calculations. In fact, the internal data path for the machine is on display on the 1401 operator’s panel.

IBM 1401 Operator Control Panel

IBM 1401 Operator Control Panel

But compared to modern computing, the very small memory capacity and the initial lack of mass storage like disk drives (on which data could be accessed non-sequentially), the programming paradigm would look unfamiliar. Remember sorting? It’s easy to sort records if you can read all the data into memory, sort it and write it out, but not so easy when only a few records will fit in memory at one time.1 Not impossible, just different, and much more time-consuming.

In Conclusion

The 1401 was certainly not the biggest computer on the block when it was introduced, but the focus on modest cost and compatibility with existing business processes made it a very successful machine. Thousands were sold, and by 1965, half of all computers in the world were of the 1401 family, spawning an entire generation of programmers who first learned computing on these machines. For the businesses that used these machines, the 1401 marked the end of rooms full of single-purpose business machines programmed with plugboards and mechanical controls, to be replaced by universal machines programmed via the powerful stored-program paradigm. The result was a revolution in efficiency and visibility into business operations that continues to the current day.

For Further Reading

  • 1 A “card deck” is a term-of-art for what an ordinary person would call a stack of punched cards.
  • 2 The 1401 used decimal (not binary) addressing, so 16K really means 16,000, not 16,384. The unit of storage in a 1401 was eight bits, large enough to hold a single character plus parity and “wordmark” bits.  The term “byte” wasn’t universally used until later.
  • 3 That’s a full page every 6 seconds. Another popular kind of printer of the era, a Teletype machine, could barely print a single line of text in 6 seconds.
  • 4 The amount of data that can fit on a tape depends on a lot of factors: see http://ibm-1401.info/TapeBlocking.html
  • 5 A unit record would be a fixed-format unit of data representing a single operation, e.g. a weekly time-card for an hourly employee, or a single line item on a bill of sale for a manufacturing company.
  • 6 Of course in this era of Big Data, programmers still struggle with data sets that are far too large to fit in one computer’s memory, but now use tools like Hadoop and MapReduce to distribute the work across many computers and disks.


Read all posts by Guy Fedorkow

Guy C. Fedorkow serves as a System Architect for Juniper Networks, where he develops system-level designs for router and telecommunication products used in Internet Service Provider networks around the world. Guy has done similar work at Cisco Systems, and worked in high-performance computing at Bolt Beranek and Newman in Cambridge MA. Guy received ...