Move UD_ACLU to the end of the unit descriptor (Konamiman#72)

The improvement of the cluster chain allocation process performance
introduced a new unit descriptor field, UD_ACLU, in the middle
of the unit descriptor (between UD_MBYTE and UD_CDIR). This caused
severe problems when writing data (weird errors, computer crashes,
even data corruption) because somewhere in the kernel there's code
that assumed that these fields are stored consecutively in memory.

The fix consists of moving UD_ACLU to the end of the unit descriptor.

Fix calculation of cluster amount for FAT16 filesystems in FDISK (Kon…

…amiman#70)

The routine that calculates the amount of clusters for a new FAT16
filesystem in FDSIK has a bug that appears when creating a filesystem
with a maximum amount of clusters possible: a 16-bit variable overflows
and turns into 0 when it should actually be 65536.

This causes the sector count in the boot sector to be higher than
the actual size of the filesystem, and thus when writing to an almost
full partition a "Disk error writing" error may appear.

This commit fixes this by adding an intermediate calculation that
prevents the variable from overflowing.

Fix: EMUFILE feature of specifying the directory on "set" not working.

Cleanup/fix C tools compilation

v2.1 beta 2

- Nextor will now try to load MSXDOS2.SYS if NEXTOR.SYS is not found in the boot drive.

- The method for selecting partitions for automatic mapping has changed from requiring a NEXTOR.DAT file in the root directory to having the "active" flag set in the partition table.

- Now the first 9 partitions of a device will be scanned during the automatic mapping procedure, this includes extended partitions.

- FDISK allows to change the "active" flag of new and existing partitions.

- FDISK now always creates extended partitions, even if 4 or less partitions are defined.

- FDISK now creates FAT16 partitions with a partition type code of 14 (FAT16 LBA) instead of 6 (FAT16 CHS).

- The numeric keyboard can now be used both when booting and when changing disks in disk emulation mode.

- Russian keyboard is now properly recognized (numeric keys only).

- Introduced the boot key inverters.

- Introduced the one-time boot keys.

- Introduced the NEXBOOT.COM tool to set the RAM based one-time boot keys.

- Introduced the RAM based one-time disk emulation mode.

- The method to enter the old disk emulation mode (now named "persistent") has changed from requiring a NEXT_DSK.DAT file in the root directory to storing a pointer to the emulation data file in the partition table of the device.

- Pressing the 0 key at boot time will delete the pointer to the emulation data file in the partition table, thus permanently disabling the emulation mode - no need to manually do anything else.

- When Nextor is waiting for a disk key press after having pressed GRAPH in disk emulation mode, now you can press GRAPH again to cancel the disk change.

- The first Nextor kernel to boot now clears the screen before invoking the driver initialization.

- ARG is no longer used as temporary work area by the Nextor kernel, this should improve the compatibility of games in disk emulation mode.

- Fix: drive was remapped in case of error (even if it was retried successfully).

- Fix: boot sector checksum calculation had a bug that caused "Wrong disk" errors.

- Fix: Konamiman#1 pressing CTRL+STOP while Nextor was trying to load NEXTOR.SYS hanged the computer.

- Fix: Konamiman#23 computer hanged when booting with one single drive letter (e.g. when using single-device controller in a computer without internal disk drive).

- Fix: Konamiman#29 wrong stack management hangedd the computer when a file handle was read or written to a number of times.

- Fix: computer crashing when more than one Nextor kernel was present as soon as the extended BIOS hook was called (for example, when loading COMMAND2.COM).

- BREAKING CHANGE: The address of CODE_ADD, used by the CALLB0 routine, has changed to F1D0h (it was F84Ch).

- Fix: there was Nextor kernel code in the 1K free area in pages 0 and 3, so putting anything here caused problems, e.g. DOS 1 mode didn't work.

- _GPART now returns the status byte of the partition, and allows to retrieve the device sector number that holds the partition table entry instead of information about the partition.

v2.1 beta 2

- Nextor will now try to load MSXDOS2.SYS if NEXTOR.SYS is not found in the boot drive.

- The method for selecting partitions for automatic mapping has changed from requiring a NEXTOR.DAT file in the root directory to having the "active" flag set in the partition table.

- Now the first 9 partitions of a device will be scanned during the automatic mapping procedure, this includes extended partitions.

- FDISK allows to change the "active" flag of new and existing partitions.

- FDISK now always creates extended partitions, even if 4 or less partitions are defined.

- FDISK now creates FAT16 partitions with a partition type code of 14 (FAT16 LBA) instead of 6 (FAT16 CHS).

- The numeric keyboard can now be used both when booting and when changing disks in disk emulation mode.

- Russian keyboard is now properly recognized (numeric keys only).

- Introduced the boot key inverters.

- Introduced the one-time boot keys.

- Introduced the NEXBOOT.COM tool to set the RAM based one-time boot keys.

- Introduced the RAM based one-time disk emulation mode.

- The method to enter the old disk emulation mode (now named "persistent") has changed from requiring a NEXT_DSK.DAT file in the root directory to storing a pointer to the emulation data file in the partition table of the device.

- Pressing the 0 key at boot time will delete the pointer to the emulation data file in the partition table, thus permanently disabling the emulation mode - no need to manually do anything else.

- When Nextor is waiting for a disk key press after having pressed GRAPH in disk emulation mode, now you can press GRAPH again to cancel the disk change.

- The first Nextor kernel to boot now clears the screen before invoking the driver initialization.

- ARG is no longer used as temporary work area by the Nextor kernel, this should improve the compatibility of games in disk emulation mode.

- Fix: drive was remapped in case of error (even if it was retried successfully).

- Fix: boot sector checksum calculation had a bug that caused "Wrong disk" errors.

- Fix: Konamiman#1 pressing CTRL+STOP while Nextor was trying to load NEXTOR.SYS hanged the computer.

- Fix: Konamiman#23 computer hanged when booting with one single drive letter (e.g. when using single-device controller in a computer without internal disk drive).

- Fix: Konamiman#29 wrong stack management hangedd the computer when a file handle was read or written to a number of times.

- Fix: computer crashing when more than one Nextor kernel was present as soon as the extended BIOS hook was called (for example, when loading COMMAND2.COM).

- BREAKING CHANGE: The address of CODE_ADD, used by the CALLB0 routine, has changed to F1D0h (it was F84Ch).

- Fix: there was Nextor kernel code in the 1K free area in pages 0 and 3, so putting anything here caused problems, e.g. DOS 1 mode didn't work.

- _GPART now returns the status byte of the partition, and allows to retrieve the device sector number that holds the partition table entry instead of information about the partition.

v2.1 beta 2

- Nextor will now try to load MSXDOS2.SYS if NEXTOR.SYS is not found in the boot drive.

- The method for selecting partitions for automatic mapping has changed from requiring a NEXTOR.DAT file in the root directory to having the "active" flag set in the partition table.

- Now the first 9 partitions of a device will be scanned during the automatic mapping procedure, this includes extended partitions.

- FDISK allows to change the "active" flag of new and existing partitions.

- FDISK now always creates extended partitions, even if 4 or less partitions are defined.

- FDISK now creates FAT16 partitions with a partition type code of 14 (FAT16 LBA) instead of 6 (FAT16 CHS).

- The numeric keyboard can now be used both when booting and when changing disks in disk emulation mode.

- Russian keyboard is now properly recognized (numeric keys only).

- Introduced the boot key inverters.

- Introduced the one-time boot keys.

- Introduced the NEXBOOT.COM tool to set the RAM based one-time boot keys.

- Introduced the RAM based one-time disk emulation mode.

- The method to enter the old disk emulation mode (now named "persistent") has changed from requiring a NEXT_DSK.DAT file in the root directory to storing a pointer to the emulation data file in the partition table of the device.

- Pressing the 0 key at boot time will delete the pointer to the emulation data file in the partition table, thus permanently disabling the emulation mode - no need to manually do anything else.

- When Nextor is waiting for a disk key press after having pressed GRAPH in disk emulation mode, now you can press GRAPH again to cancel the disk change.

- The first Nextor kernel to boot now clears the screen before invoking the driver initialization.

- ARG is no longer used as temporary work area by the Nextor kernel, this should improve the compatibility of games in disk emulation mode.

- Fix: drive was remapped in case of error (even if it was retried successfully).

- Fix: boot sector checksum calculation had a bug that caused "Wrong disk" errors.

- Fix: Konamiman#1 pressing CTRL+STOP while Nextor was trying to load NEXTOR.SYS hanged the computer.

- Fix: Konamiman#23 computer hanged when booting with one single drive letter (e.g. when using single-device controller in a computer without internal disk drive).

- Fix: Konamiman#29 wrong stack management hangedd the computer when a file handle was read or written to a number of times.

- Fix: computer crashing when more than one Nextor kernel was present as soon as the extended BIOS hook was called (for example, when loading COMMAND2.COM).

- BREAKING CHANGE: The address of CODE_ADD, used by the CALLB0 routine, has changed to F1D0h (it was F84Ch).

- Fix: there was Nextor kernel code in the 1K free area in pages 0 and 3, so putting anything here caused problems, e.g. DOS 1 mode didn't work.

- _GPART now returns the status byte of the partition, and allows to retrieve the device sector number that holds the partition table entry instead of information about the partition.

Getting started guide updated

v2.1 beta 1

All the changes and fixes of v2.0.5 beta 1 and more:

* Addied the license file and the Markdown documentation.

* Fix Konamiman#3: RDBLK can't read files with read-only attribute

* Change in boot keys: now the key to request one single drive per driver is the 5 key, and CTRL is simply passed to MSX-DOS kernels as when booting with MSX-DOS.

* Added the "User is requesting reduced drive count" flag to the input of the DRV_INIT routine and the DRV_CONFIG routine.

Folder rename: 'msxdos25' to 'kernel', 'command25' to 'command'