Imaging RX02 disks for simh (was: Re: MU-BASIC V2 and RT-11 V03B distribution disks.)

[Glen Slick]

On Sun, Mar 6, 2016 at 2:17 PM, Brian Walenz <thebri at gmail.com> wrote:
>
>
> http://www.dbit.com/putr/putr.asm has the following:
>
> ;
> ; RX01 interleave routine.
> ;
> ; bp    logical device rec
> ; ch    cylinder (0-75.)
> ; cl    logical sector (0-25.)
> ;
> ; On return:
> ; ch    cylinder (1-76.)
> ; cl    sector (1-26.)
> ;
> ; From RT-11 V04 DY.MAC:
> ;
> ; ISEC=(ISEC-1)*2
> ; IF(ISEC.GE.26) ISEC=ISEC-25
> ; ISEC=MOD(ISEC+ITRK*6,26)+1
> ; ITRK=ITRK+1
> ;
>

There is also yet another implementation of this in the 2.11BSD source
code file src\sys\pdpstand\rx.c

/*
 * rxfactr -- calculates the physical sector and physical
 * track on the disk for a given logical sector.
 * call:
 * rxfactr(logical_sector,&p_sector,&p_track);
 * the logical sector number (0 - 2001) is converted
 * to a physical sector number (1 - 26) and a physical
 * track number (0 - 76).
 * the logical sectors specify physical sectors that
 * are interleaved with a factor of 2. thus the sectors
 * are read in the following order for increasing
 * logical sector numbers (1,3, ... 23,25,2,4, ... 24,26)
 * There is also a 6 sector slew between tracks.
 * Logical sectors start at track 1, sector 1; go to
 * track 76 and then to track 0.  Thus, for example, unix block number
 * 498 starts at track 0, sector 25 and runs thru track 0, sector 2
 * (or 6 depending on density).
 */
static
rxfactr(sectr, psectr, ptrck)
    register int sectr;
    int *psectr, *ptrck;
{
    register int p1, p2;

    p1 = sectr / 26;
    p2 = sectr % 26;
    /* 2 to 1 interleave */
    p2 = (2 * p2 + (p2 >= 13 ?  1 : 0)) % 26;
    /* 6 sector per track slew */
    *psectr = 1 + (p2 + 6 * p1) % 26;
    if (++p1 >= 77)
        p1 = 0;
    *ptrck = p1;
}