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:
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|
|1st phys sec||1||41h||0C1h||1||1|
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