12 Nov
2005
12 Nov
'05
7:18 p.m.
I believe you want this:
* 16-bit
* protected mode
* no paging
* 8 or 16 segments
* 128 kB of the memory
* no FPU
i may be wrong, but i don't think you want the segments. pdp-11
segments divided the address space into eight, well, segments. each
could be grow up or grow down and had a physical base and a limit.
intel segments don't work that way. the major difference is that the
segment selector, instead of being the upper most three bits of the
virtual address, is not even in the address space at all.
so the eight, for a pdp-11/40 say, or the sixteen for the 11/70 don't
really apply. instead just give each data segment the whole 64k
address space and it'll not klobber anybody else. just itself.
there's not really any reason not to use the whole 640K.
using a floppy to boot is a great idea.
i've written several boot
straps to boot from the floppy that configure the serial port, and put
the processor in 32-bit mode. it's tight, but not too hard. you have
a great advantage in that you're going to stick to 16 bit so you can
use the bios. 512 bytes in that case is tons of space.
512 is easy until you try to handle more than one machine. :-( next you'll need to write device drivers for
the screen/keyboard, the
clock, and the floppy. later you can write an ATA driver and get a
mbr and hard disk boot strap.
I think you need a 4 MB disk and a 10 MB disk. :-) if you want to do a 32-bit version, then things
are a little
different. but this should give you the v6 in about as little
work as can be.
You might use a 32-bit CPU in 16-bit mode. Then you could use
a 32-bit limit on the FS or GS segment for easier physical memory
access from the kernel. (use an opcode prefix as needed)