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DOS322.PAT
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1986-09-23
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3KB
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57 lines
comment *
Re: DOS version compatibility with fixed disks.
When one formats a fixed disk with FORMAT.COM, a boot sector is written
on the partition that is formatted. The DOS version is written at the
beginning of this sector in ASCII characters. This is followed by the
BIOS parameter block that contains information about starting sector,
cluster size and so on.
When booting up, IBMBIO.COM reads the boot sector and decides whether
to use the information in the boot sector's parameter block on the
basis of the name of the DOS version it finds there. If it doesn't
recognize the name as a legal one, it uses a standard internal table
and ignores the parameter block's information.
If one has made any modifications to their DOS partition's format for
the purpose of reducing the cluster size (for example), then DOS will
acknowledge this change.
So far, so good. Now comes a new version of DOS. Will it recognize
a modification to the previous DOS's partition? Maybe. . .
DOS 2.0 and 2.1 both write "IBM 2.0" in their boot sectors.
DOS 3.0, 3.1 and 3.2 write their own respective version numbers as one
would expect.
DOS 2.0 and 2.1 do not read the name or the parameter block.
DOS 3.0 will accept a name of 2.0 or 3.0.
DOS 3.1 will accept a name of 2.0 or 3.1 but not 3.0.
DOS 3.2 will accept a name of 2.0, 3.0 or 3.2 but not 3.1.
Apparently, someone wasn't paying attention.
One cannot smoothly migrate from 3.0 to 3.1 or from 3.1 to 3.2.
No problem going from 3.0 to 3.2.
All DOS versions can completely corrupt the data on a disk whose format
is misinterpreted.
The patches below modify IBMBIO to accept disks formatted by preceeding
versions. Alternatively, one may use debug.com or other utility to change
the name in the boot sector to gain compatibility with a version or 2 but
this presupposes that a subset of versions will be used to access the disk.
*
PATCH IBMBIO.COM ; VERSION 3.2
COMMENT *
This patches DOS 3.2 to accept boot bpb from fixed disks formatted by
DOS 2.0, 3.0, 3.1 or 3.2 *
2F48
FROM DB 26,80,3E,0A,0,'0' ;CMP ES:[000A],'0'
DB 75,2 ;JNE TRY '2'
DB 0EBH,8 ;JMP GET FMT FROM BOOT
DB 26,80,3E,0A,0,'2' ;CMP ES:[000A],'2'
DB 75,0E2 ;JNE STANDARD TABLE
TO DB 26,80,3E,0A,0,'0' ;CMP ES:[000A],'0'
DB 72,0EC ;JB STANDARD TABLE
DB 90,90 ;NOPS
DB 26,80,3E,0A,0,'2' ;CMP ES:[000A],'2'
DB 77,0E2 ;JA STANDARD TABLE
;end of patch