of course you are right. but maybe you are missing the point why the original author (m2) wrote it exactly as it is: to save precious 2 bytes in the boot sector code
Tom am 6. Dezember 2009 um 00:32 schrieben Sie: > Hi, > the LBA detection of the FAT12/FAT16 boot sectors (both for the FreeDOS > and "OEM" kernels) could misdetect that a BIOS does support LBA if it > doesn't really. This is the used code (after calling Int13.41 for the boot > unit): >> shr cx, 1 >> ; CX must have 1 bit set >> sbb bx, 0aa55h - 1 >> ; tests for carry (from shr) too! >> jne read_normal_BIOS > This test succeeds if (cx&1) && (bx==AA55h) as intended but also succeeds > if (!(cx&1)) && (bx==AA54h). This is a highly unlikely case but > nonetheless an error which should be corrected. Example: >> sub bx, 0AA55h >> jne read_normal_BIOS >> shr cx, 1 >> jnc read_normal_BIOS > Note that the code after this expects bx to be zero, so sub must be used > here instead of cmp. I exchanged the order of the test for cx bit 0 (disk > access support) and the correct signature in bx since it's easier to > understand the code this way. (It would work the other way around as well.) > Regards, > Christian ------------------------------------------------------------------------------ Join us December 9, 2009 for the Red Hat Virtual Experience, a free event focused on virtualization and cloud computing. Attend in-depth sessions from your desk. Your couch. Anywhere. http://p.sf.net/sfu/redhat-sfdev2dev _______________________________________________ Freedos-kernel mailing list Freedos-kernel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/freedos-kernel
