Amstrad computers use standard CP/M 2 or CP/M 3 formats. The disc formats used also include automatic format detection systems.
Amstrad and Locomotive Software have made a number of non-CP/M systems which use the CP/M 2 disc format. These are:
Amstrad CP/M (and +3DOS) has an eXtended Disc Parameter Block (XDPB):
DEFW spt ;Number of 128-byte records per track DEFB bsh ;Block shift. 3 => 1k, 4 => 2k, 5 => 4k.... DEFB blm ;Block mask. 7 => 1k, 0Fh => 2k, 1Fh => 4k... DEFB exm ;Extent mask, see later DEFW dsm ;(no. of blocks on the disc)-1 DEFW drm ;(no. of directory entries)-1 DEFB al0 ;Directory allocation bitmap, first byte DEFB al1 ;Directory allocation bitmap, second byte DEFW cks ;Checksum vector size, 0 or 8000h for a fixed disc. ;No. directory entries/4, rounded up. DEFW off ;Offset, number of reserved tracks DEFB psh ;Physical sector shift, 0 => 128-byte sectors ;1 => 256-byte sectors 2 => 512-byte sectors... DEFB phm ;Physical sector mask, 0 => 128-byte sectors ;1 => 256-byte sectors, 3 => 512-byte sectors... ; ; (Amstrad extension starts here) ; DEFB sidedness ;Bits 0-1: 0 => Single sided ; 1 => Double sided, flip sides ; ie track 0 is cylinder 0 head 0 ; track 1 is cylinder 0 head 1 ; track 2 is cylinder 1 head 0 ; ... ; track n-1 is cylinder n/2 head 0 ; track n is cylinder n/2 head 1 ; 2 => Double sided, up and over ; ie track 0 is cylinder 0 head 0 ; track 1 is cylinder 1 head 0 ; track 2 is cylinder 2 head 0 ; ... ; track n-2 is cylinder 2 head 1 ; track n-1 is cylinder 1 head 1 ; track n is cylinder 0 head 1 ;Bit 6 set if the format is for a high-density disc ; (This is an extension in PCW16 CP/M, BIOS ; 0.09+. It is not an official part of the spec.) ;Bit 7 set if the format is double track. DEFB tracks/side DEFB sectors/track DEFB first physical sector number DEFW sector size, bytes DEFB uPD765A read/write gap DEFB uPD765A format gap DEFB MFM/Multitrack flags byte ;Bit 7 set => Multitrack else Single track ;Bit 6 set => MFM mode else FM mode ;Bit 5 set => Skip deleted data address mark DEFB freeze flag ;Set to nonzero value to force this format ;to be used - otherwise, attempt to determine ;format when a disc is logged in.
This simple system is used by CPC computers:
If the first physical sector is 41h, the disc is in System format, ie:
single sided, single track, 40 tracks, 9 sectors/track, 512-byte sectors, 2 reserved tracks, 1k blocks, 2 directory blocks, gap lengths 2Ah and 52h, bootable.
If the first physical sector is C1h, the disc is in Data format, ie:
single sided, single track, 40 tracks, 9 sectors/track, 512-byte sectors, no reserved tracks, 1k blocks, 2 directory blocks, gap lengths 2Ah and 52h, not bootable.
In addition to the above system, the PCW and Spectrum +3 can determine the format of a disc from a 16-byte record on track 0, head 0, physical sector 1:
CP/M pages
DEFB format number ;0 => SS SD, 3 => DS DD. Other values:
;bad format.
;1 and 2 are for the CPC formats, but those
;formats don't have boot records anyway.
DEFB sidedness ;As in XDPB
DEFB tracks/side
DEFB sectors/track
DEFB physical sector shift ;psh in XDPB
DEFB no. reserved tracks ;off in XDPB
DEFB block shift ;bsh in XDPB
DEFB no. directory blocks
DEFB read/write gap length
DEFB format gap length
DEFB 0,0,0,0,0 ;Unused
DEFB checksum fiddle byte ;Used to indicate bootable
;discs. Change this byte so that the 8-bit
;checksum of the sector is:
; 1 - sector contains a PCW9512 bootstrap
; 3 - sector contains a Spectrum +3 bootstrap
;255 - sector contains a PCW8256 bootstrap
;(the bootstrap code is in the remainder of the sector)
If all bytes of the spec are 0E5h, it should be assumed that the disc is a 173k PCW/Spectrum +3 disc, ie:
single sided, single track, 40 tracks, 9 sectors/track, 512-byte sectors, 1 reserved track, 1k blocks, 2 directory blocks, gap lengths 2Ah and 52h, not bootable.
I have extended the above "boot record" system in PCW16 CP/M (BIOS 0.09 and later). The extension is intended to allow a CP/M "partition" on a DOS-formatted floppy disc.
An extended boot sector (cylinder 0, head 0, sector 1) has the following characteristics:
| XDPB field | PCW 180k | CPC system | CPC data | PCW 720k | PCW16 1.4Mb |
|---|---|---|---|---|---|
| SPT | 24h | 24h | 24h | 24h | 48h |
| BSH | 3 | 3 | 3 | 4 | 5 |
| BLM | 7 | 7 | 7 | 0Fh | 1Fh |
| EXM | 0 | 0 | 0 | 0 | 1 |
| DSM | 0AEh | 0AAh | 0B3h | 164h | 164h |
| DRM | 3Fh | 3Fh | 3Fh | 0FFh | 0FFh |
| AL0 | 0C0h | 0C0h | 0C0h | 0F0h | 0C0h |
| AL1 | 0 | 0 | 0 | 0 | 0 |
| CKS | 10h | 10h | 10h | 40h | 40h |
| OFF | 1 | 2 | 0 | 1 | 1 |
| PSH | 2 | 2 | 2 | 2 | 2 |
| PHM | 3 | 3 | 3 | 3 | 3 |
| Sidedness | 0 | 0 | 0 | 81h | 0C1h |
| Cylinders | 28h | 28h | 28h | 50h | 50h |
| Sectors | 9 | 9 | 9 | 9 | 12h |
| 1st phys sec | 1 | 41h | 0C1h | 1 | 1 |
| Sec size | 200h | 200h | 200h | 200h | 200h |
| R/W gap | 2Ah | 2Ah | 2Ah | 2Ah | 1Bh |
| fmt gap | 52h | 52h | 52h | 52h | 54h |
| MFM mode | 60h | 60h | 60h | 60h | 60h |
| Freeze flag | 0 | 0 | 0 | 0 | 0FFh |