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. -- gil -- StorageTek INFORMATION made POWERFUL ---------------------------------------------------------------------- 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
