OSWORD &7F - Single Density Disk Access
                =======================================

OSWORD &7F passes an 8271-style control block to the floppy disk controller
to access disks. Double-density DFSs also allow OSWORD &7F to access
double-density disks.

     On extry:
        XY+0    drive number %pxDDDdsd
        XY+1..4 data address
        XY+5    number of parameters
        XY+6    command
        XY+7... parameters
        XY+n    returned result

The drive number is expressed as a binary number %pxDDDdsd.
  p   : If 1, selects the previously-used drive and density
  DDD : select the drive density:
        %xx0=Acorn single density
        %xx1=Acorn double density
        %0xx=Watford current density
        %10x=Watford single density
        %11x=Watford double density
  dd  : physical drive number
  s   : side

Single density DFSs do not have any ability to select double density disk
access. Acorn DFSs select the density with b3, Watford DFSs select the
density with b4 and b5. To specify a density in a manner compatible with
both Acorn and Watford DFSs specify the density with both methods:

        %001000sd - to specify single density
        %001110sd - to specify double density

Most DFSs do not implement physical drives numbers larger than 1 (logical
drives greater than 3). Selecting the previously selected drive can only be
relied upon if DFS is the currently selected filing system. A drive number
of &FF should be passed to select the previously used drive.

No contents of the control block should be relied upon on exit other than
the result byte. Some systems update the control block, some systems do not.

Some DFSs, such as DFS 2.45 supplied in the Master MOS 3.50 ROM, only
respond to OSWORD &7F if DFS is the current filing system and fail silently
if any other filing system is selected.

DFS 2.24 in the Master MOS 3.20 ROM incorrectly passes the density bit in b3
to the drive control register b3 instead of b5, so OSWORD &7F cannot access
double density disks.

Commands
========
&4B=Write sectors,  &03 &4B Track Sector Count+Size
&53=Read sectors,   &03 &53 Track Sector Count+Size
&63=Format track,   &05 &63 Track ....
&69=Seek track.     &01 &69 Track

 Results returned are:
 &00=ok,
 &08=Clock error,
 &0A=Late DMA,
 &0C=CRC error in ID,
 &0E=CRC error in data,
 &10=Drive not ready,
 &12=Write protected,
 &14=Track 0 not found,
 &16=Write fault,
 &18=Sector not found,
 &FE=8271 command unknown.

If &10 'Drive not ready' is returned, it should be ignored and the call made again.

The following function can be used to perform an OSWORD &7F call that takes
three parameters, such as Write (&4B) and Read (&53). It requires
X%=>15-byte control block, Y%=X%DIV256.

DEFFNdisk(addr%,cmd%,drv%,trk%,sec%,num%,den%):LOCAL fs%
fs%=FNfs:IF fs%<>4:*FX143,18,4
REPEAT
  X%?0=drv%+den%*24+32:X%!1=addr%:X%?5=3
  X%?6=cmd%:X%?7=trk%:X%?8=sec%:X%?9=num%OR&20
  A%=127:CALL&FFF1:A%=X%?10
UNTIL A%<>&10:IF fs%<>4:OSCLI"FX143,18,"+STR$ fs%
=A%
DEFFNfs:LOCAL A%,E%,Y%:=(USR&FFDA)AND&FF