Re: Basic Floppy Disk loader help.

Also you have problems with the limits of the floppy drive controller and
the chips it uses. DMA is a big one. Some older systems could not read
more than one physical track at a time, but most of those systems should be
in landfills by now. I think some would not even change heads on a single
read or write, but that might be for the 5.25 or even the 8" floppy drives.

"Jean-François Michaud" <spamtrap@xxxxxxxxxx> wrote in message
news:1135915163.385782.110730@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> Does anybody see why this bombs if i put 3Ch or above in
> nbSectorsTotal? This is a really old bug I can't seem to pinpoint. 3Ch
> would put us at 60 decimal which would be, if I calculated correctly,
> Track 1, head 1, sector 6. The code works fine below 3Ch.
>
> nbSectorsTotal: .word 0x003B
> startAddress: .word 0x0500
> trackToRead: .byte 0x00
> headToUse: .byte 0x00
>
> #______________________________________________________________________
> #
> # It is important to note the alternance of the tracks, head, sectors.
> # A high density floppy has 80 tracks ( 0-79 ), 2 heads ( 0-1 ) and
> # 18 sectors ( 1-18 )
> #
> # The value of these attributes alternates as if incrementing a 3 "bit"
> # binary counter, each "bit" ranging from their respective minimum
> # value to their maximum value:
> #
> # ( track, head, sector )
> #
> # sector 1 = ( 0, 0, 1 )
> # sector 2 = ( 0, 0, 2 )
> # ...
> # sector 19 = ( 0, 1, 1 )
> # sector 20 = ( 0, 1, 2 )
> # ...
> # sector 37 = ( 1, 0, 1 )
> # and so forth...
> #______________________________________________________________________
> #
> loadKernel:
>
> resetFloppyDrive:
> xor %ah, %ah # Zero to reset the floppy controler
> xor %dl, %dl
> int $0x13 # Controler reset
>
> readASector:
> mov $0x02, %ah # Function 0x02 of interrupt 0x13
> mov $0x00, %dl # Drive number ( 0x00 = A: 0x01 = B: 0x80 = hda
> 0x81 = hdb )
> mov $0x01, %al # Numbers of sectors to read
>
> movw $trackToRead, %si
> movb (%si), %ch # Track to read ( 0-79 tracks per floppy )
>
> movw $headToUse, %si
> mov (%si), %dh # Head to use ( 0-1 for a floppy )
>
> movw $sectorToStartFrom, %si
> mov (%si), %cl # Sector to start from ( 1-18 per head per track )
>
> xor %bx, %bx # Indirect addressing for %es
> mov %bx, %es # We change %es
>
> movw $startAddress, %bx
> movw (%bx), %bx # We finalize es:bx to situate ourselves at
> 0000:0500
>
> int $0x13 # We read the sectors
>
> sectorLogic:
> movw $sectorToStartFrom, %si # We want the next sector, so we adjust
> the
> movb (%si), %bl # sector to start from ( can't go beyond
> 0x12 )
> inc %bl
> movb %bl, (%si)
> cmp $0x13, %bl
> je headLogic
> jmp logicEnd
>
> headLogic:
> movb $0x01, %bl
> movb %bl, (%si)
>
> movw $headToUse, %si
> movb (%si), %bl # Track to read ( 0-79 tracks per floppy )
> inc %bl
> movb %bl, (%si)
> cmp $0x02, %bl
> je trackLogic
> jmp logicEnd
>
> trackLogic:
> xor %bx, %bx
> movb %bl, (%si)
>
> movw $trackToRead, %si
> movb (%si), %bl # Track to read ( 0-79 tracks per floppy )
> inc %bl
> movb %bl, (%si)
> cmp $0x50, %bl
> je readOverflowLogic
> jmp logicEnd
>
> readOverflowLogic:
> xor %bx, %bx
> movb %bl, (%si)
> jmp errorReadingFloppy
>
> logicEnd:
>
> movw $startAddress, %si # We just read one sector ( 512 bytes ) so we
> adjust the
> movw (%si), %bx # start address for the next sector to read
> addw $0x0200, %bx
> movw %bx, (%si)
>
> movw $nbSectorsTotal, %si # We decrement the counter and check if we
> have reached
> movw (%si), %bx # 0
> dec %bx
> movw %bx, (%si)
> cmp $0x0000, %bx
> jne readASector
>
> jc loadKernel # On error we try again
> ret
>
> Regards
> Jean-Francois Michaud
>


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