Re: [TUHS] Re: Porting Unix v6 to i386

Johnny Billquist wrote: Well, I tend to agree that there probably isn't really much difference in the difficulty of porting either V6 or V7 to an Intel x86 processor. The real deciding factor will (and should) be which version you *want* to port and why you want to port it in the first place. While the Intel architecture is very different from that of the PDP-11 it is possible to set up an IA-32 processor in such a way as to give you an environment which is quite similar to the one in which V7 expected to run. 16 bit protected mode can effectively give you a 64k+64k split i&d address space for both user and kernel mode and if you enable paging then you can map the underlying physical memory in 4096 byte pages wherever you want it to be. Once it's all set up correctly, most of the Intel MMU architecture can be ignored and pretty much all the kernel has to do is to manipulate 64 entries in a page table (16+16 for user i+d and 16+16 for kernel i+d). Rather than hack all of the code to build with a modern C compiler I would look seriously at the possibility of getting either pcc or the original Ritchie C compiler to generate x86 code. (This has, of course, been done before as anyone who ever used Xenix version 2.x on Intel machines will know - right down to 32 bit longs with PDP-11 word ordering ...) One possibility which actually isn't as bad as it sounds is to treat the pdp-11 assembler output of the C compiler as an intermediate language and have an additional compiler pass that effectively converted from pdp11 assembler to x86 assembler. A good convertor would do enough basic block analysis to be able to keep track of live registers and condition codes and would essentially recompile from pdp11 to x86 assembler. My guess is that with a little care it should be possible to keep the code expansion resulting from such a translation process to a minimum. Even a fairly simple minded conversion probably wouldn't be too bad.