[TUHS] Musings on PDP-7 Unix

[ This whole article is just a flight of fancy; feel free to ignore it or at least treat it as the whimsy that it is. ] The PDP-7 Unix system is the first step on the evolution of Unix as we know it. We have a snapshot of the system around the end of 1970/beginning of 1971 at http://www.tuhs.org/Archive/PDP-11/Distributions/research/McIlroy_v0/ and a reconstructed and working partial system at https://github.com/DoctorWkt/pdp7-unix PDP-7 Unix was a playground for Ken, Dennis and others to try out ideas and implementations, and it was quickly superseded by 1st Edition PDP-11 Unix. Details of how it evolved are at https://www.bell-labs.com/usr/dmr/www/hist.html and https://www.bell-labs.com/usr/dmr/www/chist.html All fine and good. However, I keep wondering, how far could they have taken Unix on the PDP-7 platform? The Kernel ---------- The reconstructed kernel only occupies 3070 words of 4096 words available, so there is room left for more code. There is already an alternative reconstruction where the "dd" concept has been replaced with the ".." concept (see https://github.com/DoctorWkt/pdp7-unix/tree/master/src/alt). PDP-7 Unix doesn't have the concept of absolute or relative filenames (e.g. /usr/bin/ls or a/b/c or ../../file), Could the nami kernel function be modified to do this? It would probably mean changing from two characters packed into a word to a single character per word (to make searching for '/' easier), and this would turn it into something more recognisable as Unix. What about pipes? They should not be too hard to implement. Even sixteen pipes with a 16-word buffer each would only be 256+ extra data words in the kernel. And a hundred words of code? There are only a few special devices in the kernel: ttyin, ttyout, keyboard, display, pptin, pptout. What about a disk block device? Was there a PDP-7 tape device, and if so, why not a tape driver and block device? Filesystem ---------- The implementation of the filesystem is, in some places, quite inefficient. The free block list is implemented as follows. In each block, there are 10 free block numbers then a pointer to the next part of the free list. However, each block can hold 64 block numbers, so why are only 10 free block numbers stored in each block? By using the whole of a block to store free block numbers, there would actually be more free blocks to use! Each i-node (size 12 words, 7 of which are direct or indirect pointers) has one word which holds a unique value. This doesn't seem to be used at all. If it was reused as a block pointer, this would allow files to be up to 8*64=512 (small) or 8*64*64=32768 (large) words in length, instead of 7*64=448 words (small) or 7*64*64=28672 (large) words. The system is set up to only use one side of the two-sided disk device. It looks like the other side was used to backup (snapshot) a working system in case of catastrophic filesystem corruption: they could simply copy the blocks from the "snapshot" side back to the working side. We could double the size of the filesystem quite easily. Macro Assembler --------------- The kernel is written using fairly tight assembly code, and there probably isn't a way to translate it into a high-level language. The PDP-7 has an arcane instruction set, and the existing assembler syntax is nothing special. What about a macro assembler that makes it easier to write code, especially readable code? Here are some ideas based on the existing kernel: u.rq := 8 ==> lac 8; dac u.rq function swap ==> swap: 0 { return; jmp swap i } subroutine .fork ==> fork: line 1 // i.e. not a function { line 1 } loop ==> 1: { } jmp 1b if (sad dm1) ==> sad dm1 { jmp 1f code1 code1 } else { 1: code2 code2 } betwen(o101,o132) ==> jms betwen; o101; o132 There are probably a bunch more that could be added. The aim is to make the control structures easier to read and write. The programmer still has to grok the PDP-7 instruction set. B (or other) Language --------------------- PDP-7 Unix has a B compiler while compiles source down to a virtual instruction set which is interpreted by the B interpreter. We have the B interpreter code, and Robert Swierczek managed to rewrite the B compiler, see https://github.com/DoctorWkt/pdp7-unix/blob/master/src/other/b.b At first glance, the PDP-7 architecture is not that amenable to high level languages, but it turns out that it is indeed possible to write a compiler for a C subset that targets the PDP-7, see https://github.com/DoctorWkt/a-compiler. So, could the B compiler be modified to actually output PDP-7 assembly code? If so, we could rewrite the utilities (cp, mv, ls etc.) in a high-level language and make the system easier to maintain. I would recommend treating int and char as the same and only storing one char per word. And then, even though the PDP-7 architecture doesn't support it, how hard would it be to add int/char types and bring the language one step closer to C? Conclusion ---------- All of this is pie in the sky. It can certainly be done, but a) who has the time and b) it would be a "tour de force", nothing really useful. But imagine if, at the beginning of 1970, Unix had a proper B or C compiler, utilities written in this high-level language, a kernel written in a semi-high level language, and a system with pipes and proper pathnames. Cheers, Warren