5 ms·

This had a 36-bit word, as did its predecessor, and as did the later DEC 10 and DEC 20 systems used famously at MIT and elsewhere, and I understand that people

This had a 36-bit word, as did its predecessor, and as did the later DEC 10 and DEC 20 systems used famously at MIT and elsewhere, and I understand that people found that word size to have various advantages once they'd used it extensively. But how was 36 bits originally decided upon back in the 1950s? What were the tradeoffs that made them say "two 18 bit half words are just right, but two 20 bit half words is clearly too big", and so on? These days everyone takes power-of-two character and integer and floating sizes for granted; I'm not talking about that, I'm just wondering how they looked at it back at the beginning.

082349872349872 4y ago
backwards compatibility: https://en.wikipedia.org/wiki/36-bit_computing#History https://en.wikipedia.org/wiki/36-bit_computing#History cf https://datatracker.ietf.org/doc/html/rfc4042 https://datatracker.ietf.org/doc/html/rfc4042 (note date)
- DougMerritt 4y ago
  I'm asking about the original thinking, not about later backward compatibility.
  - greenyoda 4y ago
    The Wikipedia article cited above has some practical reasons: > Early binary computers aimed at the same market therefore often used a 36-bit word length. This was long enough to represent positive and negative integers to an accuracy of ten decimal digits (35 bits would have been the minimum). It also allowed the storage of six alphanumeric characters encoded in a six-bit character code.
    - DougMerritt 4y ago
      Coincidentally I commented on this above at the same time you were posting -- thanks.
- DougMerritt 4y ago
  Your link does say: "Early binary computers aimed at the same market therefore often used a 36-bit word length. This was long enough to represent positive and negative integers to an accuracy of ten decimal digits (35 bits would have been the minimum). It also allowed the storage of six alphanumeric characters encoded in a six-bit character code." Which helps a little, but it still begs the question: why ten decimal digits? Why not nine or eleven or something? Are they implying that six characters of six bits was the critical issue? If so, why not seven characters? Or five? Etc.
  - tgv 4y ago
    Six character is (in)famously the maximum length of linker symbols on IBM systems, at least for FORTRAN. Perhaps that had something to do with it. And of course, there comes a time when you have to pick a number, so why no 6x6, which is also good for 10^10 integers?
    - vincent-manis 4y ago
      6 was the magic number on 700/7000 systems. With System/360, the magic number grew by 1/3, to the magnificent total of 8.
  - lapsed_lisper 4y ago
    If you're keen to go down the wikipedia hole, https://en.wikipedia.org/wiki/Six-bit_character_code https://en.wikipedia.org/wiki/Six-bit_character_code and then https://en.wikipedia.org/wiki/BCD_(character_encoding) https://en.wikipedia.org/wiki/BCD_(character_encoding) explain that IBM created a 6-bit card punch encoding for alphanumeric data in 1928, that this code was adopted by other manufacturers, and that IBM's early electronic computers' word sizes were based on that code. (Hazarding a guess, but perhaps to take advantage of existing manufacturing processes for card-handling hardware, or for compatibility with customers existing card handling equipment, teletypes, etc.) So backward compatibility is likely the most historically accurate answer. Fewer bits wouldn't have been compatible, more bits might not have been usable! (Why 6 bit codes for punch cards in 1928? Dunno. Perhaps merely the physical properties of paper cards and the hardware for reading them. This article talks about that stuff: https://web.archive.org/web/20120511034402/http://www.ieeeghn.org/wiki/index.php/STARS%3APunched_Card_Equipment https://web.archive.org/web/20120511034402/http://www.ieeegh...)
kps 4y ago
Early computers had many different word sizes. If you look at the 1955 Ballistic Research Laboratory document, A survey of domestic electronic digital computing systems, you find lengths from a low of 10 bits to a high of 42 decimal digits. (pp234-235; Text: http://ed-thelen.org/comp-hist/BRL.html http://ed-thelen.org/comp-hist/BRL.html Scan: https://babel.hathitrust.org/cgi/pt?id=wu.89037555299&view=1up&seq=234 https://babel.hathitrust.org/cgi/pt?id=wu.89037555299&view=1... ) The influential machine to look at here, then, would be the IBM 701. Edit: The IBM 701 patent <https://patents.google.com/patent/US3197624A https://patents.google.com/patent/US3197624A> says, “A binary number of a full word of 36 bits has a precision equal to that of about a 10 decimal digit number”. It doesn't mention characters, and the machine was designed for scientific (military) calculations, not text/data processing. The associated IBM 716 printer did not use a 6-bit code. However, this does not rule out a 6-bit character code as a design consideration, even if this machine didn't use one, since IBM did have a large business in pre-computer data processing, using punched-card character sets that would fit in no less than 6 bits. So that may have driven the design of shared peripherals like 7-track tape (6 bits plus parity) and led to a multiple-of-6 word size. Edit 2: The paper, The logical organization of the new IBM scientific calculator <https://doi.org/10.1145/609784.609791 https://doi.org/10.1145/609784.609791> again mentions 10 decimal digits. It also describes card and printer I/O, and it is explicit that these work on binary card images, row by row, not character by character. The machine handles only 72 columns of an 80-column card, reading a row at a time into two 36-bit words. A 40-bit word would have let it use the whole card!
- kps 4y ago
  From Stephen H. Kaisler, First Generation Mainframes: The IBM 700 Series: > IBM conducted a survey of existing machines and applications, ultimately determining that a word length of between 10 and 12 digits or about 35 to 41 bits was desirable. Additionally, single address machines required at least 18 bits for storage of operation codes and addresses given a specified memory size. Since the secondary memory was magnetic tape, a tape technology study showed that 6 parallel channels for data storage were optimal. Thus, the word size should be a multiple of 6 bits so that subparts of the word could be stored in successive locations on tape. As a result, IBM arrived at 36 bits for the IBM 701 word size. https://books.google.ca/books?id=B9l9DwAAQBAJ&lpg=PR4&pg=PA5#v=onepage&q&f=false https://books.google.ca/books?id=B9l9DwAAQBAJ&lpg=PR4&pg=PA5...
klelatti 4y ago
I've previously had a look at IBM Stretch and how they considered and rejected 60 bits. A '6-bit' byte was considered to be important - more so that 8-bit - so having a word a multiple of 6 was seen as an advantage. https://thechipletter.substack.com/p/60-bit-computing https://thechipletter.substack.com/p/60-bit-computing
- DougMerritt 4y ago
  Useful interesting historical information, thanks. Seymour Cray was amazing, inventing and proving CPU architectural techniques (like out of order execution) that continue to be used today. "Control Data Corporation 6600 was the fastest computer in the world from 1964 to 1969. It was, in many ways, a pioneer of modern computing using, for example, some of the techniques that would form the basis of RISC architectures... Wikipedia: Generally considered to be the first successful supercomputer, it outperformed the industry's prior recordholder, the IBM 7030 Stretch, by a factor of three." Wow, 3x. Mind boggling.

082349872349872 4y ago

backwards compatibility: https://en.wikipedia.org/wiki/36-bit_computing#History https://en.wikipedia.org/wiki/36-bit_computing#History cf https://datatracker.ietf.org/doc/html/rfc4042 https://datatracker.ietf.org/doc/html/rfc4042 (note date)

DougMerritt 4y ago
I'm asking about the original thinking, not about later backward compatibility.
- greenyoda 4y ago
  The Wikipedia article cited above has some practical reasons: > Early binary computers aimed at the same market therefore often used a 36-bit word length. This was long enough to represent positive and negative integers to an accuracy of ten decimal digits (35 bits would have been the minimum). It also allowed the storage of six alphanumeric characters encoded in a six-bit character code.
  - DougMerritt 4y ago
    Coincidentally I commented on this above at the same time you were posting -- thanks.
DougMerritt 4y ago
Your link does say: "Early binary computers aimed at the same market therefore often used a 36-bit word length. This was long enough to represent positive and negative integers to an accuracy of ten decimal digits (35 bits would have been the minimum). It also allowed the storage of six alphanumeric characters encoded in a six-bit character code." Which helps a little, but it still begs the question: why ten decimal digits? Why not nine or eleven or something? Are they implying that six characters of six bits was the critical issue? If so, why not seven characters? Or five? Etc.
- tgv 4y ago
  Six character is (in)famously the maximum length of linker symbols on IBM systems, at least for FORTRAN. Perhaps that had something to do with it. And of course, there comes a time when you have to pick a number, so why no 6x6, which is also good for 10^10 integers?
  - vincent-manis 4y ago
    6 was the magic number on 700/7000 systems. With System/360, the magic number grew by 1/3, to the magnificent total of 8.
- lapsed_lisper 4y ago
  If you're keen to go down the wikipedia hole, https://en.wikipedia.org/wiki/Six-bit_character_code https://en.wikipedia.org/wiki/Six-bit_character_code and then https://en.wikipedia.org/wiki/BCD_(character_encoding) https://en.wikipedia.org/wiki/BCD_(character_encoding) explain that IBM created a 6-bit card punch encoding for alphanumeric data in 1928, that this code was adopted by other manufacturers, and that IBM's early electronic computers' word sizes were based on that code. (Hazarding a guess, but perhaps to take advantage of existing manufacturing processes for card-handling hardware, or for compatibility with customers existing card handling equipment, teletypes, etc.) So backward compatibility is likely the most historically accurate answer. Fewer bits wouldn't have been compatible, more bits might not have been usable! (Why 6 bit codes for punch cards in 1928? Dunno. Perhaps merely the physical properties of paper cards and the hardware for reading them. This article talks about that stuff: https://web.archive.org/web/20120511034402/http://www.ieeeghn.org/wiki/index.php/STARS%3APunched_Card_Equipment https://web.archive.org/web/20120511034402/http://www.ieeegh...)

kps 4y ago

Early computers had many different word sizes. If you look at the 1955 Ballistic Research Laboratory document, A survey of domestic electronic digital computing systems, you find lengths from a low of 10 bits to a high of 42 decimal digits. (pp234-235; Text: http://ed-thelen.org/comp-hist/BRL.html http://ed-thelen.org/comp-hist/BRL.html Scan: https://babel.hathitrust.org/cgi/pt?id=wu.89037555299&view=1up&seq=234 https://babel.hathitrust.org/cgi/pt?id=wu.89037555299&view=1... ) The influential machine to look at here, then, would be the IBM 701. Edit: The IBM 701 patent <https://patents.google.com/patent/US3197624A https://patents.google.com/patent/US3197624A> says, “A binary number of a full word of 36 bits has a precision equal to that of about a 10 decimal digit number”. It doesn't mention characters, and the machine was designed for scientific (military) calculations, not text/data processing. The associated IBM 716 printer did not use a 6-bit code. However, this does not rule out a 6-bit character code as a design consideration, even if this machine didn't use one, since IBM did have a large business in pre-computer data processing, using punched-card character sets that would fit in no less than 6 bits. So that may have driven the design of shared peripherals like 7-track tape (6 bits plus parity) and led to a multiple-of-6 word size. Edit 2: The paper, The logical organization of the new IBM scientific calculator <https://doi.org/10.1145/609784.609791 https://doi.org/10.1145/609784.609791> again mentions 10 decimal digits. It also describes card and printer I/O, and it is explicit that these work on binary card images, row by row, not character by character. The machine handles only 72 columns of an 80-column card, reading a row at a time into two 36-bit words. A 40-bit word would have let it use the whole card!

kps 4y ago
From Stephen H. Kaisler, First Generation Mainframes: The IBM 700 Series: > IBM conducted a survey of existing machines and applications, ultimately determining that a word length of between 10 and 12 digits or about 35 to 41 bits was desirable. Additionally, single address machines required at least 18 bits for storage of operation codes and addresses given a specified memory size. Since the secondary memory was magnetic tape, a tape technology study showed that 6 parallel channels for data storage were optimal. Thus, the word size should be a multiple of 6 bits so that subparts of the word could be stored in successive locations on tape. As a result, IBM arrived at 36 bits for the IBM 701 word size. https://books.google.ca/books?id=B9l9DwAAQBAJ&lpg=PR4&pg=PA5#v=onepage&q&f=false https://books.google.ca/books?id=B9l9DwAAQBAJ&lpg=PR4&pg=PA5...

klelatti 4y ago

I've previously had a look at IBM Stretch and how they considered and rejected 60 bits. A '6-bit' byte was considered to be important - more so that 8-bit - so having a word a multiple of 6 was seen as an advantage. https://thechipletter.substack.com/p/60-bit-computing https://thechipletter.substack.com/p/60-bit-computing

DougMerritt 4y ago
Useful interesting historical information, thanks. Seymour Cray was amazing, inventing and proving CPU architectural techniques (like out of order execution) that continue to be used today. "Control Data Corporation 6600 was the fastest computer in the world from 1964 to 1969. It was, in many ways, a pioneer of modern computing using, for example, some of the techniques that would form the basis of RISC architectures... Wikipedia: Generally considered to be the first successful supercomputer, it outperformed the industry's prior recordholder, the IBM 7030 Stretch, by a factor of three." Wow, 3x. Mind boggling.