PDP-10 hello world live coding on ITS.

[@noland65]

Thanks for sharing! I learned quite a bit…

[@larsbrinkhoff]

Good! You're always welcome to 36-bit land. :-)

[@n7275]

I usually lurk on your irc channel as "n7275". Nice video. Sorry I wasn't around to watch it live.

[@larsbrinkhoff]

Thanks!

[@alicewyan]

Great video, I'm learning quite a bit! But one thing I can't understand: what do you mean at 21:15 when you say that the hello string starts at position 29? how do you calculate the 29?

[@larsbrinkhoff]

Sorry, should have explained that. There are five 7-bit characters in a 36-bit word. The first character is the most significant bits: 35-29. The PDP-10 byte instructions work with the lower number to specify a byte field, hence 29 points to the first character in the string.

[@GodEmperorSuperStar]

25:00 how are bytes sequenced in words? Why is octal 44 the byte before octal 35?

[@larsbrinkhoff]

The byte pointer specifies the byte location inside the word counting from 0 as the least significant bit. This is the location of the left end of the byte. Byte sequence only matters for the two incrementing byte instructions. You might guess they increment the byte posiition, but no, they subtract. So if byte position is 44 initially, ILDB will first decrement it to 35 and then use that updated position to load a byte from the word. The next byte will be at position 26. All numbers in octal. So you may think of bytes as stored left to right in a word.

[@GodEmperorSuperStar]

@@larsbrinkhoff Thanks. Is there a manual of PDP-10 opcodes and explanations of their functions?

[@larsbrinkhoff]

@@GodEmperorSuperStar Sure, this is from ITS originally: pdp10.nocrew.org/docs/instruction-set/pdp-10.html The full DEC manual is here: www.bitsavers.org/pdf/dec/pdp10/1982_ProcRefMan.pdf