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The IBM 305 RAMAC was the first commercial computer that used a moving head hard disk drive (magnetic disk storage) for secondary storage. IBM introduced the storage unit on September 4, 1956 before unveiling the entire computer nine days later on September 13.^[1][2][3] RAMAC stood for "Random Access Method of Accounting and Control". Its design was motivated by the need to replace the punch card tub file used by most businesses at the time. The first RAMAC to be used in the US auto industry was installed at Chrysler's MOPAR Division in 1957. It replaced a huge tub file which was part of MOPAR's parts inventory control and order processing system. The 305 was one of the last vacuum tube computers that IBM built. The IBM 350 disk system stored 5 million 8-bit (7-bits plus 1 odd parity bit) characters (about 4.4 MB). It had fifty 24-inch diameter disks. Two independent access arms moved up and down to select a disk and in and out to select a recording track, all under servo control. Average time to locate a single record was 600 milliseconds. Several improved models were added in the 1950s. The IBM RAMAC 305 system with 350 disk storage leased for $3,200 per month in 1957 dollars, equivalent to a purchase price of about $160,000. More than 1000 systems were built. Production ended in 1961, the RAMAC computer became obsolete in 1962 when the IBM 1405 Disk Storage Unit for the IBM 1401 was introduced, and the 305 was withdrawn in 1969.

During the 1960 Olympic Winter Games in Squaw Valley (USA), IBM provided the first electronic data processing systems for the Games. The system featured an IBM RAMAC 305 computer, punch card data collection, and a central printing facility.

Programming the 305 involved not only writing machine language instructions to be stored on the drum memory, but also almost every unit in the system (including the computer itself) was programmed by inserting wire jumpers into a plug-board.

The original 305 RAMAC computer system could be housed in a room of about 9 m (30') by 15 m (50'); the 350 disk storage unit measured around 1,5 m² (5' square). The first hard disk unit was shipped Sept. 13, 1956^[4]. The additional components of the computer were a card punch, a central processing unit, a power supply unit, an operator's console/card reader unit, and a printer.

In an interview^[5] published in the Wall Street Journal with Currie Munce, research vice president for Hitachi Global Storage Technologies, which acquired the IBM's storage business, said the entire RAMAC unit weighed over a ton and had to be moved around with forklifts and delivered via large cargo airplanes. According to Munce, while the storage capacity of the drive could have been increased above five megabytes, the marketing department at IBM was against a larger capacity drive because they didn't know how to sell a product with more storage.

Architecture

The 305 was a character-oriented variable "word" length decimal (BCD) computer with a drum memory rotating at 6000 RPM that held 3200 alphanumeric characters. A core memory buffer of 100 characters was used for temporary storage during data transfers.

Each character was 7 bits, composed of two zone bits ("X" and "O"), four BCD bits for the value of the digit, and an odd parity bit ("R") in the following format: X O 1 2 4 8 R

Instructions could only be stored on 20 tracks of the drum memory and were fixed length (10 characters), in the following format:

T₁ A₁ B₁ T₂ A₂ B₂ M N P Q

Field positions	Function
T1 A1 B1	Source operand address – Track, low order AB character
T2 A2 B2	Destination operand address – Track, low order AB character
M N	Length of operands (each operand must be entirely on its specified track)
P	Program exit code; used to select test conditions, perform jumps, and initiate input/output. The 305's plug-board programming determines the action(s) performed.
Q	Control code; modifies the operation (similar to an op code), the default operation being a copy from source to destination. Other operations were: "1" Compare, "2" Field compare, "3" Compare & Field compare, "5" Accumulator reset, "6" Blank transfer test, "7" Compress & Expand, "8" Expand, "9" Compress

Fixed-point data "words" could be any size from one decimal digit up to 100 decimal digits, with the X bit of the least significant digit storing the sign (signed magnitude).

Data records could be any size from one character up to 100 characters.

Drum memory

The drum memory was organized into 32 tracks of 100 characters each.

The color code of this table is:

• Yellow – Storage
• Blue – Arithmetic
• Green – Input/Output
• Red – Special function

Track Specifier	Source Function	Destination Function
W X Y Z	General Storage
0 1 2 3 4 5 6 7 8 9 & A B C D E F G H I	Instruction Storage, General Storage
L	Read accumulator	Add to accumulator
M	Read & clear accumulator	Subtract from accumulator
V	Multiplicand (1 to 9 characters) or Divisor (1 to 9 characters)
N	N/A	Multiply (1 to 11 characters) Stores 2 to 20 character product in accumulators 0 & 1
P	N/A	Divide (option)
K	380 Punch Card Input	N/A
S T	N/A	323 Punch Card Output, 370 Printer Output, 407 Printer Output
Q	380 Inquiry Input/Output
J	N/A	350 File Address
R	350 File Data Input/Output
-	Core buffer	Character selector
$	382 Paper Tape Input/Output (option)

L and M select the same track, containing ten 10-character "Accumulators". As a destination L specifies addition, M specifies subtraction. (Numbers in these accumulators were stored in ten's complement form, with the X bit of the most significant digit storing the sign. The sign of each accumulator was also held in a relay. However the 305 automatically converted between its standard signed magnitude format and this format without the need for special programming.)

J, R, and - do not select tracks on the drum, they specify other sources and destinations.

Jumps

The 305's instruction set does not include any jumps, instead these are programmed on the plug-board:

• Unconditional jump – the program exit code (P field) specifies a Program exit hub on the plug-board, which has a wire plugged into it and the Program entry hub specifying the instruction to jump to.
• Conditional jump – the program exit code (P field) specifies a Program exit hub on the plug-board, which has a wire plugged into it and the appropriate Condition selector common hub to be tested, the corresponding two Condition selector output hubs have wires plugged into them and the Program entry hubs specifying the instructions to jump to or the Program advance hub to continue in sequence. Complicated conditions involving many Condition selectors could be wired to execute in a single instruction (e.g., Testing the sign and zero states of multiple accumulators), with one of several Program entry hubs activated.
• Multi-way jump – the destination track (T₂ field) is set to "-" and the appropriate Character selector hubs on the plug-board have wires plugged into them and the Program entry hubs specifying the instructions to jump to or the Program advance hub to continue in sequence.

Timing

All timing signals for the 305 were derived from a factory recorded Clock Track on the drum. The clock track contained 816 pulses 12 µs apart with a 208 µs gap for sync.

Reading or writing a character took 96 µs.

The 305's typical instruction took three revolutions of the drum (30ms): one (I phase) to fetch the instruction, one (R phase) to read the source operand and copy it to the core buffer, and one (W phase) to write the destination operand from the core buffer. If the P field (Program exit code) was not blank, then two (D phase and P phase) additional revolutions of the drum (20ms) were added to the execution time to allow relays to be picked. The Improved Processing Speed option could be installed that allowed the three instruction phases (IRW) to immediately follow each other instead of waiting for the next revolution to start; with this option and well optimized code and operand placement a typical instruction could execute in as little as one revolution of the drum (10ms).

Certain instructions though took far longer than the typical 30ms to 50ms. For example, multiply took six to nineteen revolutions of the drum (60ms to 190ms) and divide (an option) took ten to thirty seven revolutions of the drum (100ms to 370ms). Input/Output instructions could interlock the processor for as many revolutions of the drum as needed by the hardware.

Hardware implementation

The logic circuitry of the 305 was built of one and two tube pluggable units and relays.

Related peripheral units

A basic system was composed of the following units:

IBM 305	–	Processing unit	The magnetic process drum, magnetic core register and electronic logical and arithmetic circuits
IBM 350	–	Disk Storage unit
IBM 370	–	Printer
IBM 323	–	Card Punch
IBM 380	–	Console	The Card Reader and IBM Electric typewriter model B1
IBM 340	–	Power Supply

External links

• IBM Archives on the 305
• IBM 305 RAMAC Data Processing System

Footnotes