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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:

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 spaceIn computing, an address space defines a context in which an address makes sense. That is, two addresses may be the same but refer to different things if they belong to different address spaces. Some example address spaces include: Main memory (physical m. 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 opcodeA computer can perform operations, each of which is assigned a numeric code called an opcode . To assist in the use of these numeric codes, mnemonics are used as textual abbreviations. It's much easier to remember ADD than 05, for example. Opcodes operate 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-opThis article is about the NOP assembly language mnemonic. NOP also stands for National Opinion Polls, a public opinion polling company in the UK''. In most assembly languages, NOP (short for N o OP eration is an instruction that does nothing at all during instructions and a "read card", which would set up a pattern of wordmarks in the card read bufferBuffer can have various meanings: In chemistry, the term buffer refers to a buffer solution, usually used to stabilize the acidity ( pH) of a liquid. In computing a buffer is a portion of memory set aside to store data, often before it is sent to an exter. 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 7090The IBM 7090 a second-generation transistorized version of the earlier IBM 709 vacuum tube mainframe computers and was designed for "large-scale scientific and technological applications", was the third member of the IBM 700/7000 series scientific compute) did all of its input-output on magnetic tapeMagnetic tape is an information storage medium consisting of a magnetisable coating on a thin plastic strip. Nearly all recording tape is of this type, whether used for video with a video cassette recorder, audio storage ( reel-to-reel tape, compact audios and the 1401 was used to format input data from other peripherials (e.g., punch cardThe punch card (or Hollerith card) is a recording medium for holding information for use by automated data processing machines. Made of stiff cardboard, the punch card represents information by the presence or absence of holes in predefined positions on t 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.