On Mon, Jul 17, 2023 at 1:05 PM Paul Koning via cctalk
<[email protected]> wrote:
> > On Jul 17, 2023, at 12:51 PM, Ethan Dicks via cctalk
> > <[email protected]> wrote:
> >
> > On Mon, Jul 17, 2023 at 12:48 PM Ethan Dicks <[email protected]> wrote:
> >> From: http://www.chdickman.com/pdp11/pro380.txt
> >>
> >> "The RX50 floppy starts at track 1. Track 0 is logically placed after
> >> track 79. The sectors are...
> >
> > It should be "... interleaved 1, 3, 5, 7, 9, 0, 2, 4, 6, 8..." There
> > is no "sector 10" when counting from 0.
> >
> > -ethan
>
> I don't know what to make of that web page's description, it certainly
> doesn't have much connection to reality. Are those numbers supposed to
> represent the placement of logical sector numbers onto the physical track?
> If so, they show 5:1 interleave rather than 2:1 interleave. Or do they
> represent the physical sector numbers for consecutive logical sectors? If
> so, it seems to be backwards.
It looks like it represents how to go from a physical track to get
things back in the logical order that one would see from a PDP-11 or
VAX on real hardware - i.e., the boot block (if any) is the very first
thing you encounter.
I agree, it's not a 2:1 interleave in any classic sense.
> The actual algorithm is what I wrote in my previous email, which you can also
> find in my RSTSFLX tools. That has been tested against real world RX50
> floppies, and against the source code of the RT11 RX50 driver.
Chuck Dickman's algorithm is in lbn2rx50.c
#define RX50_TRACKS 80
#define RX50_SECTORS 10
int interleave[] = { 0, 2, 4, 6, 8, 1, 3, 5, 7, 9 };
track = lbn/RX50_SECTORS;
track = (track + 1)%RX50_TRACKS;
sector = lbn%RX50_SECTORS;
sector = (interleave[sector] + 2*(track - 1) + RX50_SECTORS)%RX50_SECTORS;
-ethan
