Well that looks to me like a standard split I/D setup where each process, and the kernel, can have up to 64 kb of code and 64 kb of data. It will run 2.11bsd with a relatively straightforward porting effort -- direct translation of each code fragment to do something equivalent on your device.

What I discussed as another option (that I did on Z180 which did not have split I/D and hence had too small a logical address space to run 2.11bsd or any other reasonable unix)... was to increase the code limit past 64 kb by segmentation. Whilst this is difficult to do, it can be achieved without (much) compiler support. Code pointers become 3 bytes instead of 2, but luckily, code pointers are hardly used, and this limitation can be got around by using stubs.

You are right though, in that a natively running system that can freely utilize your 6 Mb or 8 Mb or whatever, certainly IS possible. The MS DOS C compilers for PC/XT, PC/AT etc, supported this. The "large" memory model had segmented code and data, any process could have multiple code and data segments. Pointers were 4 bytes (equivalent to 3 bytes in your case, a 16-bit offset and a 7-bit segment). However, pointer arithmetic did not carry into the segment field, so arrays and structs and the biggest "malloc" were limited to 64 kbytes. It was somewhat slower than the "small" memory model (2 byte pointers) and everything took more memory. The "huge" memory model removed the 64 kbyte limitation altogether via a software emulation of a flat address space (by doing carrying arithmetic whenever pointers, arrays or structs were derefenced) but was of course MUCH slower.

If your compiler supports these things, go ahead and use them and port 4.3BSD across, it would be great. I thought you were talking about a smaller system like 1 Mb, with 1 Mb you could support say 3 users and a kernel @ 256 kbytes each using a 2.11bsd-like split I/D system and a small memory model. They could run a reasonable toolchain consisting of a shell spawning cc spawning a compiler or linker (for example) and an editor, simultaneously, though it would be tight. In that case I would be concerned about speed and size and would recommend a "small" memory model. But if you have 6 Mb and your processor is relatively fast (say 8 MHz) and your compiler efficient, then I guess a "large" model would be reasonable. I like tiny unices, though :)

Nick