ruud.baltissen_at_abp.nl
Date: 2007-08-24 09:55:26
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)
.by "M" ; Memory
.by "B" ; Block
.by "U" ; User
.by "P" ; Position
.by "&" ; USR execute
.by "C" ; Copy
.by "R" ; Rename
.by "S" ; Scratch
.by "N" ; New (Format)
;** low byte of command addresses
CmdAddrLB .BY <(Validate)
.BY <(Initialize)
.BY <(SyntaxError)
.BY <(Memory)
.BY <(Block)
.BY <(User)
.BY <(Position)
.BY <(USRexec)
.BY <(Copy)
.BY <(Rename)
.BY <(Scratch)
.BY <(New)
;** high byte of command addresses
[FEA1]
CmdAddrHB .BY >(Validate)
.BY >(Initialize)
.BY >(SyntaxError)
.BY >(Memory)
.BY >(Block)
.BY >(User)
.BY >(Position)
.BY >(USRexec)
.BY >(Copy)
.BY >(Rename)
.BY >(Scratch)
.BY >(New)
;** bytes for syntax check
.BY $51, $DD, $1C, $9E, $1C
;
; ^ ^
; | |
; +-------------------+---- why are these here?
;** file control methods
A_FEB2 .BY $52,$57,$41,$4D ; "RWAM"
My idea is that the first byte was used for the COPY command but
discarded later because the routines at $C8F0 uses its own method to
check IMAGE.
But where is the last byte used for? As you can see it is followed by
characters Read, Write, Append and Modify. No connection with these
IMHO.
At $F10F a buffer is selected for loading the BAM. The result is always
13. IIRC the BAM was always loaded at $0700. Can anybody shine a light
on this, please?
--
___
/ __|__
/ / |_/ Groetjes, Ruud
\ \__|_\
\___| URL: Ruud.C64.org
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