Editing the CBIOS

Editing the CBIOS

The cbios is used to tell the CP/M operating system about your hardware. It contains routines (i.e. drivers) for your hardware. In my case I have software serial I/O and FRAM (i.e. flash) disks.

The only thing I have to change for the serial I/O is the interrupt vector (from RST6.5 to RST7.5).

The flash disk configurations and the read/write drivers.

The Disk Parameters

Using the CPM_2_0_System_Alteration_Guide,pdf as a guide I have recalculated the "standard floppy" for CP/M machines:

CP/M Disk Parameter Calcs
	Standard
Bytes/Sector	128	Record size
Sectors/Track	26
Tracks	77	Tracks

Disk Capacity	250.25	Disk Capacity kb
SYS	2	System Tracks
TRK	75	User Tracks
CAP	243.75	Storage Capacity kb
n	10	n = 10 to 14
BLS	1,024	BLS = 2**n

SPT	26	Sectors
BSH	3	BSH = n-7
BLM	7	BLM = 2**BSH - 1
EXM	0	EXM = 2**(BSH-3) - 1 if DSM<256
		EXM = 2**(BSH-4) - 1 if DSM>=256
DSM	242	DSM = maximum data block number
DRM	63	Directory Mask: DRM=BLS/16
BLOCKS	32
AL0	192
AL1	0

	Standard Floppy	DISK PARAMETER BLOCK:
SPT	26	SECTORS PER TRACK
BSH	3	BLOCK SHIFT FACTOR
BLM	7	BLOCK MASK
EXM	0	EXTENDED BLOCK MASK
DSM	242	DISK SIZE-1
DRM	63	DIRECTORY SIZE-1
AL0	192	ALLOC 0
AL1	0	ALLOC 1
CKS	16	CHECK SIZE
OFS	2	SYSTEM TRACK OFFSET

For the 8*256kb Flash I want 128 directory entries. Therefore I need a BLOCK size of 2048 bytes. Here are the parameters for the system disk A:

CP/M Disk Parameter Calcs
	256k System
Bytes/Sector	128	Record size
Sectors/Track	16
Tracks	128	Tracks

Disk Capacity	256	Capacity kb
SYS	4	System Tracks
TRK	124	User Tracks
CAP	248.00	Capacity kb
n	11	n = 10 to 14
BLS	2,048	BLS = 2**n

SPT	16	Sectors
BSH	4	BSH = n-7
BLM	15	BLM = 2**BSH - 1
EXM	1	EXM = 2**(BSH-3) - 1 if DSM<256
		EXM = 2**(BSH-4) - 1 if DSM>=256
DSM	247	DSM = maximum data block number
DRM	127	Directory Mask: DRM=BLS/16
BLOCKS	32
AL0	240
AL1	0

	256k System	DISK PARAMETER BLOCK:
SPT	16	SECTORS PER TRACK
BSH	4	BLOCK SHIFT FACTOR
BLM	15	BLOCK MASK
EXM	1	EXTENDED BLOCK MASK
DSM	247	DISK SIZE-1
DRM	127	DIRECTORY SIZE-1
AL0	240	ALLOC 0
AL1	0	ALLOC 1
CKS	0	NOT REMOVEABLE
OFS	4	SYSTEM TRACK OFFSET

And the same for the data disk (B):

	256k Data	DISK PARAMETER BLOCK:
SPT	16	SECTORS PER TRACK
BSH	4	BLOCK SHIFT FACTOR
BLM	15	BLOCK MASK
EXM	1	EXTENDED BLOCK MASK
DSM	255	DISK SIZE-1
DRM	127	DIRECTORY SIZE-1
AL0	240	ALLOC 0
AL1	0	ALLOC 1
CKS	0	NOT REMOVEABLE
OFS	0	SYSTEM TRACK OFFSET

Note: CP/M does not need to check the disks are the media (Flash) cannot be removed while the power is on.

Edits done, files compile, so next is make a disk image and to load the image onto the Flash chips.

AlanX

Update Schematic and PCB

PCB has Arrived

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