NCR Century 100

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NCR Century 100 Logo

The NCR Century 100 was NCR's first all integrated circuit computer.[1] All logic gates were created by wire-wrapping NAND gates together to form flip-flops and other complex circuits. The console of the system had only 18 lights and switches and allowed entry of a boot routine, or changes to loaded programs or data in memory. A typewriter console was also available.


The 615-100 Series integrated a complete data processing system had 16KB or 32KB of short rod memory, 80-column card reader or paper tape reader, two 5MB removable disc drives, 600-line per minute printer. The system could be provided with a punched paper tape reader, or an external card reader/punch, and also allowed for the attachment of multiple 9 track 1/2 inch reel to reel magnetic tape drives. Two more disk drives could be attached to the system. The Century series used an instruction set with two instruction lengths: 4 bytes (32 bits) and 8 bytes (64 bits).

Dancing rods[edit]

The memory of the Century Series computers used machine made, short (1/16 inch long and approximately the diameter of a human hair) iron-oxide coated, ceramic rods as their random access memories, instead of the hand-labor-intensive core memories that were used by other computers of the time. These rods were inserted into a plastic alignment sheet which was wound with read, write, and sense wire coils arranged in columns and rows. To get the rods to stand up straight on the sheet (so that they would drop into the coils for assembly) a large electro-magnet was turned on and made the rods stand up and "dance" into the individual holes. The economy of machine assembly was augmented by selling rod memory without paying patent royalties on core memory to (NCR's competitor) IBM.

Flying heads[edit]

The Model 655 removable disc drives were the first to employ floating or flying heads. Early marketing material made a big deal of this, but there were a number of problems that plagued all of the Century Series systems. Head crashes were common, because the head flew less than a human hair's width above the disc surface. (Unless a drive unit was repaired and carefully cleaned after a crash, the next disc pack loaded in that drive would also crash. And, if a crashed disc pack was loaded on an operational drive, it would destroy the head on that drive unit. Contributing to this was the fact that disc packs were hand-loaded: spun tight onto a spindle, then uncovered by hand, in open air, before the drive cover was closed. A head crash could be caused merely by repeatedly bumping the smoked polycarbonate pack cover against the edge of a disc platter while lifting it off. Although the typical head crash ruined heads and the pack, the worst kind of head crash was caused by an entire pack coming loose from its spindle and rising freely inside the machine while spinning at high speeds!)

A feature of the 655 drives was 12 read / write heads per surface. This reduced track to track movement and thus access times. However, this meant that there were 12 times more heads per drive, further increasing the likelihood of head crashes. These flying heads were moved using a 16 position magnetic actuator. The actuator used four different magnets to create the 16 positions. The magnetic actuators were later replaced with hydraulic actuators and later yet the hydraulic actuators were replaced with voice coil actuators. Later NCR used disk units from other manufacturers, who had significantly reduced head crashes.

Programming languages[edit]

The NCR Century 100 supported several programming languages. The first was NEAT/3 (National's Easy Auto-coding Technique) which was a later version of the NEAT/1 language that ran on the NCR 315 computer system. The second was COBOL. The third was FORTRAN, and the fourth was BASIC. If minor program logic changes were necessary, instead of recompiling the program, one could go to the control panel, stop the operation and with the dial switches, put in address/command changes with the dial switches.


The system had 39 hardware instructions. Early versions of the hardware did NOT employ hardware multiply or divide instructions. They were instead emulated using software.[citation needed]. The machine ran in ascii 8 bit per byte code. Files could contain packed fields with or without a "sign". That is, if you had the number 1234, you could store it in just two bytes, with each of the 8 bits of the character holding 2 numbers, such as 0001 0010 0011 0100 (for 1234). So if you knew you had a field that could only be positive, you could pack it into an unsigned packed field.

A typical hardware configuration consisted of a panel with toggle switches and lights to enter the boot loader, and a Teletype writer to input operating system commands, a punched card reader that gravity feed the cards (they dropped into the read station, and were ejected and turned 180 degrees and then placed in the exit hopper), 2 655 disc drives, and a printer that printed about 600 lines per minutes. The boot loader and peripherals were usually on punched cards, which notified the operating system which devices to use via a PAL (Peripheral Availability List) entry cards. The "go" command to the operating system was infamous: "EE" control-G (bell).[citation needed]

The Century 100 lacked hardware sense switches, which the Century 200 had. Programs that utilized sense switches on the Century 100 architecture would simply halt with the humour line: "You find the switches, and I'll test them!" displayed on the console typewriter.[citation needed]

A unique feature of the Century's hardware/software design allowed the normal 4K executive to be reduced to a mere 512 bytes, freeing up precious storage.[citation needed]

The Century 50 was slower than the Century 100 and only had 16K of the thin film rod memory.[citation needed]