From: Chuck Guzis Sent: Monday, December 05, 2016 6:15 PM > On 12/05/2016 01:09 PM, Lars Brinkhoff wrote:
>> As Charles wrote, the PDP-10 commonly uses 7-bit bytes for ASCII >> text, but that's only part of the truth. The architecture is quite >> byte size agnostic. There are instructions to operate on any byte >> size from 1 to 36 bits, at any position inside a word. (Well, a >> later extension to the architecture restricted this a bit.) The restriction Lars mentions only applies to what are referred to as One-Word Global Byte Pointers (OWGBPs), which encode the divisions of a 36-bit word into 6, 7, 8, 9, or 18 bit bytes into a 6-bit value in the high-order bits of a word with a 30-bit address filling the rest. There are also Two-Word Global Byte Pointers (which I've never seen abbreviated) which carry the standard "any size byte at any position" in the first word, with a zero address in the right half, and the 30-bit extended address (with 0's in the 6 high-order bits) in the second. > I've seen PDP-10 9-track tapes done two ways--one character per frame > and then 4 frames (36 bits) with 5 7-bit characters and the sign bit > left over. Neither of those is entirely accurate. 9-track tapes on the PDP-10 used one of the following encodings: 1. Core-Dump: 4 frames of 8 bits, 5th frame with 4 leading 0's (or 0100 on one type of controller) and the last 4 bits. 2. Industry-Compatible: 4 frames of 8 bits, and ignore the low order 4. 3. ANSI-ASCII: 4 frames of 7 bits padded with a leading 0, 5th frame with low order bit (B35) followed by the remaining 7 bits. In this case, B35 is usually 0, but in the case of line-numbered files B35 = 1 is the indicator that the 5 ASCII digits are a line number (and the parity bit is set incorrectly on the tape). 4. High-Density: 4 frames of 8 bits, 5th frame has low order 4 bits of the 1st word in its high order bits + high order 4 bits of the 2nd word in its low order bits, then 4 frames of 8 bits finishing up the 2nd word. I've been dealing with PDP-10 tapes for 40 years now. Rich Rich Alderson Vintage Computing Sr. Systems Engineer Living Computer Museum 2245 1st Avenue S Seattle, WA 98134 mailto:ri...@livingcomputermuseum.org http://www.LivingComputerMuseum.org/