> In a recent note, McKown, John said: > > > Date: Wed, 6 Jul 2005 08:17:39 -0500 > > > > there were some systems in the past in which an "int" was > 36 bits. As > > you can see if you do the math, that 36 is not evenly > divisible by 8 > > (36/8==4.5). On these systems, a byte was usually 9 bits in > length. In > > this context, a "byte" is not "8 bits", but rather "the fundamental > > unit of memory addressing" (or some such thing). If you do any C > > programming > > > On most such machines, "the fundamental unit of memory > addressing" was 36 bits. Some of them had an extended > pointer format which the hardware used to fetch or store a > bitfield of arbitrary length at an arbitrary offset from the > beginning of a 36-bit word. The PDP-6 et. seq. stored 5 > 7-bit ASCII characters in a 36-bit word with one bit left > over. Some language processors used this bit as a flag to > indicate that the word contained a 5-digit line number, not > to be otherwise processed. > > The PDP-11 was interesting in that its fundamental unit of > memory addressing was an 8-bit byte, yet their early > marketing material described memory sizes in units of 16-bit > words; a bold step bucking the trend that has produced the > 13-ounce pound of coffee. > > > (or UNIX work), you will likely notice that octal is used > quite a bit > > instead of hexadecimal. The reason, IIRC, is that the PDP > systems upon > > which C and UNIX were originally developed were 36 bit > machines. A 9 > > bit "byte" could be displayed as 3 octal digits. But could not be > > displayed at all using hex. > > > Also, the octal digits were a subset of the decimal digits. > And nicely divided the 6-bit (not 8 or 9) characters that > prevailed at the time. The PDP-6 OS must have been designed > at the boundary between 6-bit characters and ASCII. Text was > generally represented in 7-bit characters, but most arguments > to system calls (filenames, > etc.) were represented in a 6-bit ASCII subset. > > PDP-11 struggled with representing 16-bit words and 8-bit > characters in octal for a while, then turned to hex, which I > perceived at the time as an IBM 360 innovation. > > PDP-11 is little-endian. Its dumps display the text > representation with low addresses at the left; hex with high > addresses at the left so copying to a hex calculator was simplified.
In the late seventies I was brought up on CDC 6000/7000 series machines, designed by Seymour Cray. These were word-addressable, with a 60 bit word length. Each word contained a nominal 10 six bit characters but these were only accessible by loading the word into a register and using shift/mask. And you only had 64 instructions to play with, RISC years before IBM thought of it! -Robin. --------------------------------------------------------------------------------------------------------------- This e-mail is intended only for the above addressee. It may contain privileged information. If you are not the addressee you must not copy, distribute, disclose or use any of the information in it. If you have received it in error please delete it and immediately notify the sender. evolvebank.com is a division of Lloyds TSB Bank plc. Lloyds TSB Bank plc, 25 Gresham Street, London, EC2V 7HN. Registered in England, number 2065. Telephone No: 020 7626 1500 Lloyds TSB Scotland plc, Henry Duncan House, 120 George Street, Edinburgh EH2 4LH. Registered in Scotland, number 95237. Telephone No: 0131 225 4555 Lloyds TSB Bank plc and Lloyds TSB Scotland plc are authorised and regulated by the Financial Services Authority and represent only the Scottish Widows and Lloyds TSB Marketing Group for life assurance, pensions and investment business. Signatories to the Banking Codes. --------------------------------------------------------------------------------------------------------------- ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [EMAIL PROTECTED] with the message: GET IBM-MAIN INFO Search the archives at http://bama.ua.edu/archives/ibm-main.html
