From: William Levak (wlevak_at_sdf.lonestar.org)
Date: 2007-08-25 01:33:50
On Fri, 24 Aug 2007, ruud.baltissen@abp.nl wrote:
>
> Hallo allemaal,
>
>
> A question for those who love to tinker with bits and bytes:
>
> At $FE89 you find the table with valid commands for the 1541. Followed
> with a table with the low bytes of the addresses where the code for
> these commands is found, followed by the high bytes of these addresses.
> Then you find some bytes used in the "Check syntax" routine, starting at
> $C268. This routine fills a variable, called IMAGE, with bits and these
> bits are checked later against the bytes mentioned above.
>
> ;** check input line
> ;
> ; Settings of IMAGE ($028B) after check:
> ;
> ; bit meaning
> ; --- -------------------------
> ; 7 found wildcard (Y=1) \
> ; 6 more then one file implied (Y=1) | file #1
> ; 5 Drive # specified |
> ; 4 filename given /
> ; 3 found wildcard (Y=1) \
> ; 2 more then one file implied (Y=1) | file #2
> ; 1 Drive # specified |
> ; 0 filename given /
>
> ...... <snip>
>
> L_C24C ora #$03 ; set bit 0 and 1
> eor IMAGE
> sta IMAGE ; flag syntax test
>
> L_C254 lda IMAGE
> ldx INSTRU
> and CmdAddrHB+4,X ; combine with checkbyte
> bne L_C260 ; faulty syntax, ->
>
> rts
>
> L_C260 sta ERWORD ; set error flag
> lda #$30
> jmp OutputErrorMsg ; 30, 'Syntax error'
>
>
>
> ;** table of command words
> [FE89]
> TblCommands
> .by "V" ; Validate
> .by "I" ; Initialize
> .by "D" ; D (back up, not used)
"D" is for "Duplicate". On dual drive units, the drive could copy an
entire disk on it's own. Also on dual drive units, most of the commands
could operate on a file on one drive and put the result on the other
drive. The command structure probably allowed for two files on all
commands as a simplified command structure. When Commodore introduced
single drive units, the disk operating system simply did not use the
second drive information.
wlevak@sdf.lonestar.org
SDF Public Access UNIX System - http://sdf.lonestar.org
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