[TUHS] Redoing "V6on286" or porting V7...?

Brantley Coile brantley at coraid.com
Sat Nov 12 23:18:50 AEST 2005

> 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. :-(

didn't quit understand this.  the boot sector is a short driver for
the media you're booting.  the floopy or the harddisk.  it has to know
about the file system enough to pull out /boot.  it can use the
drivers in the bios.

>> 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. :-)

you're correct to imply that v6 takes up MUCH less space than other
systems.  it's amazing how small you can run this.  the root in the
6th edition system is only 1.4MB.  and that includes the normal
binaries, libraries and the source to the kernel.  it'll run great on
a floopy.

>> 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)

you could.  i still think that just plain old native mode would
be a great way to run 6th Ed.

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