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Editing the CBIOS

A project log for My CP/M Version 1A

I decided to do a minor upgrade to My CP/M (Version 1). Swap RST6.5 for RST7.5 and increase the FRAM disk capacity from 32k to 256k.

agp.cooperagp.cooper 07/08/2020 at 06:430 Comments

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

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