+3DOS

Spectrum +3 Disk Operating System

+3DOS is the disk operating system of the Sinclair ZX Spectrum +2A, +3, +2B, and +3B. It is a CP/M compatible filesystem and is integrated into the +3 BASIC. The DOS routines are located in ROM2 (the +3DOS ROM) and the code for the Sinclair BASIC integration is located in ROM1 (the Syntax ROM). The +3DOS extensions to the BASIC are only present when running the +3 BASIC ROM. There is no provision for using +3DOS from ROM3 (the 48K BASIC ROM). In fact when 48 BASIC is selected from the main menu the ROM paging is locked and it is impossible to access the +3DOS ROM again until the spectrum has been rebooted.

Spectrum +2A/B

Despite the lack of disk interface hardware the Black +2 models contain the same Operating System ROMs as the +3. Therefore the +3DOS routines are all present. The ROMs detect the absence of the disk interface hardware and set the title of the boot menu to "128 +2A" as opposed to "128 +3". The RAMDisk (Drive M:) is available on the Black +2 machines but obviously the floppy drives (A: and B:) are not present. Attempting to access drive A: or B: will generate a "Drive not found" error. The default drive for load/save operations on the +2A/B is the tape (drive T:) however the CAT command (list drive catalogue) defaults to the RAMDisk (M:)

eXtended Disk Parameter Block

Each logical drive in +3DOS has an extended disk parameter block. This is a CP/M disk parameter block (DPB) with additional information to support different formats. When a disk is logged on the XDPB is initialised and set with the relevant settings for the disk format in use.

The Plus 3 manual describes the XDPB layout:

Bytes 0...1	SPT records per track
Byte 2		BSH log2(block size / 128)
Byte 3		BLM block size / 128 - 1
Byte 4		EXM extent mask
Bytes 5...6	DSM last block number
Bytes 7...8	DRM last directory entry number
Byte 9		AL0 directory bit map
Byte 10		AL1 directory bit map
Bytes 11...12	CKS size of checksum vector (bit 15 = permanent)
Bytes 13...14	OFF number of reserved tracks
Byte 15		PSH log2(sector size / 128)
Byte 16		PHM sector size / 128 - 1
Byte 17		Bits 0...1 Sidedness
			0 = Single sided
			1 = Double sided (alternating sides)
			2 = Double sided (successive sides)
		Bits 2...6 Reserved (set to 0)
		Bit 7 Double track
Byte 18		Number of tracks per side
Byte 19		Number of sectors per track
Byte 20		First sector number
Bytes 21...22	Sector size
Byte 23		Gap length (read/write)
Byte 24		Gap length (format)
Byte 25		Bit 7 Multi-track operation
			1 = multi-track
			0 = single track
		Bit 6 Modulation mode
			1 = MFM mode
			0 = FM mode
		Bit 5 Skip deleted data address mark
			1 = skip deleted data address mark
			0 = don't skip deleted address mark
		Bits 0...4 = 0
Byte 26		Freeze flag
			00h (0) = auto-detect disk format
			FFh (255) = don't auto-detect disk format

Byte 25 is normally set to 60h (96). Multi-track operation is not
recommended.

Setting the freeze flag (byte 26) prevents +3DOS from trying to
determine the format of a disk. This should be used when patching an
XDPB for a non-standard format.

It also gives examples of the XDPB for three different formats.

The AMSTRAD PCW range single track format (type 0) (As used by the +3):

36		SPT, records per track
3		BSH, block shift
7		BLM, block mask
0		EXM, extent mask
174		DSM, number of blocks - 1
63		DRM, number of directory entries - 1
C0h (192)	AL0, 2 directory blocks
00h (0)		AL1
16		CKS, size of checksum vector
1		OFF, reserved tracks
2		PSH, physical sector shift
3		PHM, physical sector mask

0		Single sided
40		Tracks per side
9		Sectors per track
1		First sector number
512		Sector size
42		Gap length (read/write)
82		Gap length (format)
60h (96)	MFM mode, skip deleted data address mark
0		Do auto select format

The AMSTRAD CPC range SYSTEM format (type 1)

36		SPT, records per track
3		BSH, block shift
7		BLM, block mask
0		EXM, extent mask
170		DSM, number of blocks - 1
63		DRM, number of directory entries - 1
C0h (192)	AL0, 2 directory blocks
00h (0)		AL1
16		CKS, size of checksum vector
2		OFF, reserved tracks
2		PSH, physical sector shift
3		PHM, physical sector mask

0		Single sided
40		Tracks per side
9		Sectors per track
41h (65)	First sector number
512		Sector size
42		Gap length (read/write)
82		Gap length (format)
60h (96)	MFM mode, skip deleted data address mark
0		Do auto select format

Finally the AMSTRAD CPC range DATA ONLY format (type 2)

36		SPT, records per track
3		BSH, block shift
7		BLM, block mask
0		EXM, extent mask
179		DSM, number of blocks - 1
63		DRM, number of directory entries - 1
C0h (192)	AL0, 2 directory blocks
00h (0)		AL1
16		CKS, size of checksum vector
0		OFF, reserved tracks
2		PSH, physical sector shift
3		PHM, physical sector mask

0		Single sided
40		Tracks per side
9		Sectors per track
C1h (193)	First sector number
512		Sector size
42		Gap length (read/write)
82		Gap length (format)
60h (96)	MFM mode, skip deleted data address mark
0		Do auto select format

The +3e ROMs

Garry Lancaster's +3e ROMs patch the XDPB to support the +3DOS filesystem on IDEDOS hard disk partitions.