Editing IBM 1401 (section)

==Hardware implementation==
{{main | transistor computer}}

[[File:IBM 1401 card cage.jpg|thumb|Closeup of a swing-out card cage (or ''gate'' in IBM parlance) showing some of the 1401's [[IBM Standard Modular System|SMS circuit cards]]]]
Most of the logic circuitry of the 1401 is a type of [[diode–transistor logic]] (DTL), that IBM referred to as ''CTDL (Complemented Transistor Diode Logic)''. Other IBM circuit types 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) use a faster type of DTL using [[drift-field transistor|"drift" transistor]]s (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&nbsp;V to -0.5V, low: -6&nbsp;V to -12&nbsp;V; (T Level) high: 6&nbsp;V to 1&nbsp;V, low: -5.5&nbsp;V to -6&nbsp;V.

These circuits are constructed of discrete components (resistors, capacitors, transistors) mounted on single-sided paper-epoxy [[printed circuit board]]s either {{convert|2.5|by|4.5|in|mm}} with a 16-pin [[gold-plated]] edge connector (single wide) or {{convert|5.375|by|4.5|in|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 is similar to that in one [[7400 series]] [[Integrated circuit#SSI|SSI]] or simpler [[Integrated circuit#MSI|MSI]] package (e.g., three to five logic gates or a couple of [[Flip-flop (electronics)|flip-flops]] on a single-wide card up to about twenty logic gates or four flip-flops on a double-wide card).

[[File:SMScard.jpg|thumb|left|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''.

The modules used were fairly delicate, compared to previous unit-record equipment, so IBM shipped them enclosed in a newly invented packing material, [[bubble wrap]].  This was one of the first widespread uses of this packing; it greatly impressed recipients, and brought great publicity to the material.

Like most machines of the day, the 1401 uses [[magnetic-core memory]]. The cores are about 1&nbsp;mm in diameter and use a four-wire arrangement (x, y, sense, and inhibit).
The memory is arranged in planes of {{not a typo|4000}} <!-- NOT 4096. This was a decimal-addressed machine. -->
cores each, each core storing one bit. A stack of eight such planes store 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.<ref>{{cite web
 |url=http://members.optushome.com.au/intaretro/1401Core.htm
 |title=Core memory frame from a 1401
 |author=Rob Storey
 |access-date=July 15, 2012
 |archive-date=July 16, 2012
 |archive-url=https://web.archive.org/web/20120716203935/http://members.optushome.com.au/intaretro/1401Core.htm
 |url-status=dead
 }}</ref> One such module is housed within the 1401's primary enclosure. Systems were commonly available with two, three, or four such modules. The additional modules are contained in an add-on box, the 1406 Core Memory Unit, which is about two feet square and three feet high.

[[File:IBM 1403 opened up2.jpg|thumb|120px|1403 line printer opened, with 729 tape drives in the background.]]

Operands in memory are accessed serially, one memory location at a time, and the 1401 can read or write one memory location within its basic cycle time of 11.5 microseconds.<ref>{{cite web
 |url=http://www.ibm-1401.info/1401GuidePosterV9.html
 |title=The IBM 1401
 |work=IBM 1401 Restoration Project   |publisher=Computer History Museum
 |access-date=July 15, 2012
 |quote=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.}}</ref>

All instruction timings are cited in multiples of this cycle time.<ref>{{cite web
 |url=http://ibm-1401.info/IBM-PrinProg-07.pdf |archive-url=https://ghostarchive.org/archive/20221010/http://ibm-1401.info/IBM-PrinProg-07.pdf |archive-date=2022-10-10 |url-status=live
 |title=IBM 1401 Principles of Programming, Section 7
 |year=1961   |work=IBM Personal Study Program   |publisher=IBM Corporation
 |page=19   |access-date=July 15, 2012
 |quote=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.}}</ref>