How were 18-bit instructions encoded in paper tape?

Question

Early 18-bit computers like PDP-1 were based on 18-bit instruction sets, and used punched paper tape for storage etc. As far as I know paper tape was usually 8 holes, which works nicely with 16-bit etc. I haven't heard of 9-hole tape. How did they encode 18-bit instructions on tape?

Answer 1

The answer is invariably specific for a particular computer, and indeed may be specific depending on what is doing the reading (say, hardware bootstrap versus software loader, or what it's reading from (say, photoelectric reader versus teletype attachment).

On the PDP-1 reading 8-channel tape in binary mode from the photoelectric reader, an 18-bit word was assembled from 3 frames, taking 6 bits from each.

PDP-1 I/O manual, page 12

Assembly takes place in the I/O register, and it seems that this is accomplished by hardware obeying the "read papertape binary" instruction, rather than the reader software needing to read each frame, shift, and merge.

Read Punched Tape, Binary

For each In-Out Transfer instruction, three lines of punched tape are read and assembled in the In-Out Register to form a full computer word. For a line to be recognized in this mode, the eighth hole must be punched; i.e., lines with no eighth hole will be skipped over. The seventh hole is ignored. The pattern of holes in the binary tape is arranged so as to be easily interpreted visually in terms of machine instruction

1961 PDP-1 Handbook, page 22

The 1963 Handbook, page 23, has a concrete example:

The instruction sub 5137, for example, appears on tape and is assembled by rpb as follows:

Channel   8 7 6 5 4 : 3 2 1
Line 1    X   X     :   X
Line 2    X   X   X :     X
Line 3    X     X X : X X X

Reader Buffer 100 010 101 001 011 111

This answer, though about the PDP-1, shows the general nature of binary I/O (whether paper tape or otherwise) -- the medium supports units that are so-many bits wide, and there is some convention for representing machine words with multiple units. Magnetic tape needs to do the same thing.

Answer 2

In addition to anotehr-dave's in depth answer maybe some (not PDP-1 specific) background information to show the environment:

8 hole punch tape was only a thing rather late on. Paper tape was commonly used in 5 to 8 hole versions (special types up to 24 and more), depending on purpose/equipment. 5 hole was 'small' Baudot type tapes, while 6 to 8 hole shared the same 25.4mm tape.

Thanks to this physically compatible medium, any 6..8 hole reader could any of these tapes and produce the lower 6 bits correctly. Read characters were always framed as a byte, so any 6 bit portion was always put into the lower 6 bit of whatever size that byte was, independent of paper tape 'width' or transmission byte size. That's why splitting any binary data it up into 6 bit chunks not only made a fine threesome of 18 bit words, but created as well a mostly device independent storage format. A machine could thus load a tape from any 6..8 bit reader as long as the serial interface as configured to the right word format.

At the time of the PDP-1 the most common teletypes were still operating on 6 bit.

Answer 3

This is explained here starting at page 21. There are basically two modes in which a paper tape can be used:

• Alphanumeric: 8 bits per tape line.
• Binary: 3 tape lines of 6 effective bits each per 18 bit word. Positions 7 and 8 are unpunched and punched respectively when punching. Lines with position 8 unpunched are skipped while reading.

Instructions were encoded using the binary mode.