> When you say 4GB pointers, you mean physical addresses? It sounds > feasible, but I've never heard of anybody doing it. Basically, yes. It'd be a 4GB virtual address space, but could be backed by a real 4GB RAM if there was appropriate hardware. Otherwise it could be backed by swap. > This would give you fast pointer arithmetic at the expensive of much > slower indirection, due to having to frequently reload ds or es; I'd > imagine that there's more indirection than pointer arithmetic in the > average C program. Yeah, the trick to getting good performance would be compiler optimizations on the access strategies, with a more hard limit of really only having 2x64KB data windows in "view" at a time (without counting CS and SS). There wouldn't be many ways around that without the compiler performing caching strategies which would be on a whole new plane of insanity. The real killer for large applications wouldn't be the time for the kernel to adjust pointers, it'd be the data moving if there isn't RAM available to back the used memory. > Hmm... the compiler would be able to optimise constant indirections > (a->b), but not non-constant indirections (a[i]). The first case would > turn into a ds:[si+$n] indirection, but the latter case would have to do > explicit pointer arithmetic on the physical address a and then reload ds. Right, the optimization possible for a[i] type stuff would degrade quickly after 64KB size is reached. The same window could be reused only if the next data is still within it, but if the compiler doesn't know this ahead of time, there would be a penalty in checking if it's still ok. Some arithmetic could be done on the SI register but external code would have to be invoked to correct an arithmetic borrow/carry and fix the segment register/prepare data. >> tl;dr: I'd love me some nethack running on 8088. > > Get the overlaid real-mode version from here: > > http://www.nethack.org/v331/ports/download-msdos.html [totally off topic]: Well either way I'd have to build a RAM board to do nethack on this hardware, I'm mostly trying to find a good reason to build a big RAM board and have it useful for more than ramdisk or task switching DOS instances. On this machine there is well over 512KB of address space unused, so any chunk of the expansion memory could be mapped to each of the 4 segments by only having the hardware decode upper addresses based on 64KB mapping windows. x86-16 address -> ram board address mapping 0x4**** -> 0x0013**** mapping 0x5**** -> 0x0014**** DS=0x4367 would give a window into a 64KB that crosses ram board mapping boundaries The mapping hardware would just be a 4 address bit, 8 data bit RAM, for up to 24 bit hardware addressing. It starts to sound like a non-standard 286 without memory protection. I have an unexplainable thing for imagining up huge projects like this... Sorry if this is sending you CC messages, it shouldn't be, but I have to manually rewrite the address each time I reply. I might have forgot. -- To unsubscribe from this list: send the line "unsubscribe linux-8086" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
