English Electric KDF9 facts for kids

The KDF9 was an early British computer. It was designed and built by a company called English Electric. This company later joined with another to form International Computers Limited (ICL).

The first KDF9 computer started working in 1964. It was a powerful machine for its time. Only 29 of these computers were ever made. The last one stopped working in 1980. It was located at the National Physical Laboratory.

The KDF9 was mostly used for science and math problems. Many were used in universities and colleges in the UK. Another computer, the KDF8, was made at the same time. It was designed for business tasks.

The KDF9 was one of the first computers to handle multiprogramming. This means it could run several programs at the same time. It used special parts of its core memory to keep programs separate. Different operating systems were made for the KDF9. Some even allowed people to use the computer interactively. This was done through smaller computers called PDP-8s acting as smart terminals.

How the KDF9 Computer Was Built

The KDF9 used advanced technology for its time. Its circuits were made entirely from solid-state parts. This means it used transistors instead of older, less reliable vacuum tubes.

Inside the KDF9's Circuits

The KDF9 used a type of circuit called transformer-coupled diode–transistor logic. It had about 20,000 transistors. It also had about 2,000 special parts called toroid pulse transformers. The computer ran at a speed of 1 MHz. This means it could do 1 million operations per second.

Memory and Data Storage

The KDF9 could have up to 32,000 "words" of memory. Each word was 48 bits long. This is like 192,000 bytes of information. This memory could hold different types of data. It could store a single 48-bit number. It could also store two 24-bit numbers. Or it could hold six 8-bit instruction parts. It could even store eight 6-bit characters. The KDF9 could also handle very large 96-bit numbers.

However, the KDF9 was not good at handling single characters or bytes. This made it less useful for tasks that weren't about numbers. The computer used a special character set. It was based on a paper tape code. This code included unusual characters for math and programming.

Special Memory Areas: Registers

The KDF9's main processing unit (CPU) had three special sets of memory areas called registers.

• The Nest was like a stack of 16 arithmetic registers. It was used for calculations.
• The SJNS (Subroutine Jump Nesting Store) was another stack. It stored return addresses for programs.
• The Q Store had 16 index registers. Each was 48 bits long. These registers helped the computer quickly find and change memory addresses. This made it very efficient for repeating tasks, like counting loops.

Computer Instructions

The KDF9 used instructions that were 1, 2, or 3 "syllables" long. A syllable is like a small piece of an instruction. Most math operations used short, 1-syllable instructions. These instructions worked on the numbers at the top of the Nest.

The KDF9's instructions were very efficient. They didn't need to access the main memory very often. This helped make up for the memory being a bit slow. Because of this, the KDF9 was quite fast. It was about one-third as fast as the much more expensive Atlas Computer.

Running Multiple Programs (Multiprogramming)

The KDF9 was one of the first computers that could securely run many programs at once. This is called multiprogramming. Its operating system, called the Timesharing Director, could run up to four programs at once. Each program had its own protected area of memory. This kept them from interfering with each other.

Each program also had its own Nest and Q Store registers. When the computer switched between programs, these registers were quickly swapped. This made changing between tasks very fast.

Programs could use hardware devices like printers directly. But the Director operating system controlled which devices each program could use. If a program tried to use a device it wasn't allowed to, it would cause an error. The computer also had ways to prevent programs from using memory that was busy. If a program needed to wait for a device, the Director would switch to another program. This made sure the computer was always busy.

Later operating systems for the KDF9 were developed. These included Eldon 2 and COTAN. They allowed many users to interact with the computer at the same time. They used PDP-8 computers to handle the user terminals.

The KDF9 also had important compilers for a programming language called Algol 60. One compiler focused on making programs run very fast. Another compiler helped programmers find and fix errors in their code. The work done with these compilers led to important computer performance tests, like the Whetstone benchmark.

Interesting Facts and History

The KDF9 used a special way to write its machine code. It was called syllabic octal. This system used three octal digits to represent 8 bits of information. At the time, the word "byte" wasn't well known. So, English Electric used the word "syllable" for what we now call a byte.

Before the KDF9, English Electric had another computer called DEUCE. That computer used vacuum tubes, which were not very reliable. So, it had a way to check for errors in calculations. When the KDF9 came out, it used transistors. These were much more reliable. So, the error checking system was no longer needed and was eventually removed.

The name "KDF9" was chosen randomly by a marketing manager. The KDF8 was a similar computer developed at the same time for business use.

Some of the KDF9's operating systems had interesting names. The EGDON operating system was named after Egdon Heath. This is a place in a book by Thomas Hardy. The Eldon operating system was named because the computer at Leeds University was in a building that used to be an Eldon chapel.

Physical Size of the KDF9

The KDF9 was a very large computer. It weighed more than 5.2 short tons (about 4.7 metric tons).

• The control desk and typewriter weighed 300 pounds (136 kg).
• The main memory and input/output control unit weighed 3,500 pounds (1,587 kg).
• The arithmetic and main control unit weighed 3,500 pounds (1,587 kg).
• The power supply unit weighed 3,000 pounds (1,360 kg).

See also

• Reverse Polish notation (RPN)