PET index - DOS ROMs

Comparison

Here is a comparison table for the different DOS versions (Based on information from this drive_info.txt that comes with the VICE emulator

Model	1541	2031/4031	2040/3040	4040	8050	8250/1001
DOS version(s)	2.6	2.6	1.0/1.2	2.1/2.7	2.5/2.7	2.7
Disk format(s)	'2A'	'2A'	' '	'2A"/"2C"	"2C"	"2C"
Drives	1	1	2	2	2	2
Heads per drive	1	1	1	1	1	2
Storage capacity	170k	170k	340k	340k	1.05M	2.12M
Sequential files	168k	168k	168k	168k	521k	1.05M
Relative files	167k	167k	n/a	167k	183k/518k	1.04M
Buffer storage (kB)	2	2	4	4	4	4
Tracks per drive	35	35	35	35	77	2*77
Sectors per track	17-21	17-21	17-21	17-21	23-29	23-29
Bytes per block	256	256	256	256	256	256
Free blocks (single drive/all drives)	664	664	670/1340	664/1328	2052/4104	4133/8266
Directory and BAM	18	18	18	18	38/39	38/39
Directory entries/disk	144	144	144	144	224	224
Transfer rate (internal, kB/s))	40	40	40	40	40	40
Transfer rate (IEC/IEEE488, kB/s))	0.4	1.8	1.8	1.8	1.8	1.8
Access time (ms) track to track	30	30	30	30	5	5
Access time (ms) average	360	360	360	360	125	125
Revolutions/minute	300	300	300	300	300	300

The following table denotes the speed zones for the different formats:

Number of sectors per track

2040/3040 DOS1.x

1540/1541/4040

8050

8250

Tracks	#Sectors
1-17	21 (0-20)
18-24	20 (0-19)
25-30	18 (0-17)
31-35	17 (0-16)

Tracks	#Sectors
1-17	21 (0-20)
18-24	19 (0-18)
25-30	18 (0-17)
31-35	17 (0-16)

Tracks	#Sectors
1-39	29 (0-28)
40-53	27 (0-26)
54-64	25 (0-24)
65-77	23 (0-22)

Tracks	#Sectors
1-39	29 (0-28)
40-53	27 (0-26)
54-64	25 (0-24)
65-77	23 (0-22)
78-116	29 (0-28)
117-130	27 (0-26)
131-141	25 (0-24)
142-154	23 (0-22)

Note: the DOS2.7 4040 drive was a prototype.

DOS 1.x

The first DOS versions (1.0) were rushed out to get disk support for the PET machines. The first useable versions were the DOS 1.2 2040/3040 drives. These versions still lacked relative file support.

As the drives have two CPUs, one 6502 for the DOS and the IEEE488 and a 6504 for the disk controller (FDC), there are two ROM parts. The DOS ROM is 8 kByte in size, the FDC ROM is only 1 kByte, located in the 6530 RIOT chip. The size is so small that only the most basic hardware-related code is stored there. To format a disk, some code had to be copied to the shared RAM by the DOS CPU and was executed by the FDC CPU. Therefore, there are three ROM parts to discuss:

DOS1 ROM disassembly (recomment header file, comments transferred from the original CBM DOS2 source)
DOS1 FDC ROM disassembly (recomment header file, comments from the Commodore Inner Space Anthology)
DOS1 format code disassembly (recomment header file, comments from the Commodore Inner Space Anthology)

DOS 2.0/2.1

The second DOS version added support for relative files, and changed the disk format from 670 blocks free to 664 blocks free. It was directly used in the 4040 drives, but many 2040/3040 drives have been upgraded to these ROMs, as the hardware was otherwise the same.

The original Commodore DOS source code can be found in the CBM archive at the CBM archive.

DOS 2.5

The DOS 2.5 adds support for the large, quad density disk drives and is used in the 8050 disk drives.

DOS 2.6

DOS 2.6 is a modification of the 2.x DOS where the FDC code is also executed by the same DOS CPU. It is used in the VC1541 and 4031/2031 disk drives.

DOS 2.7

DOS 2.7 descends from 2.5, not 2.6. I.e. it still uses two CPUs, with dual drive support etc. It adds support for double sided disks, as well as super side sector for relative files. There are versions for the 8050 and the 8250 as well as the 1001 disk drives. A prototype version for the 4040 disk drive was never released. In fact the DOS2.7 probes the FDC and adjusts itself automatically to the 4040, 8050 or 8250 disk formats.

A disassembly can be found in the CBM archive.

8280 8" disk drive DOS

The 8" disk drives use a ROM that, according to the CBM archive, are very similar to DOS 2.7.

Harddisk ROMs

The hard disks have their own ROM. The source code can be found in the CBM archive.

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