ruud.baltissen_at_abp.nl
Date: 2008-02-28 08:21:32
Hallo William,
> This looks to be a syntax check, with the table being masks
> to use with the check.
That was very clear to me. But have a look at this piece of code:
L_C254 lda IMAGE
ldx INSTRU
and TblSyntax-8,X ; combine with checkbyte
bne L_C260 ; faulty syntax, ->
My understanding from this bit of code is that X may vary from 8 to 12
because the table goes from $FEAD to and including $FEB1. But now have a
look at this code:
L_C17A stx INSTRU
cpx #$09 ; commandnumber < 9 ?
bcc L_C184 ; yes ->
jsr ChkSyntax ; test for R, S or N
L_C184 ldx INSTRU ; X = commandnumber
This code says to skip the ChkSyntax routine if X is smaller then 9.
This include the value 8. From $FE89 you'll find the table with the
characters that represent the various commands in plain ASCII.
ldx #$0B
L_C16A lda TblCommands,X ; compare commands
cmp CHAR ; with char
beq L_C17A ; found, ->
You see that the code starts with value 11, which represents New, and
can go down to 0, Validate. This means that 8 = Copy. So the question
is: why is Copy excluded from the Syntax check?
> in the structure table, STRUCT ($FEA5+)
The table starts at $FEAD, not at $FEA5. The first eight values simply
don't exist. The value you'll find at that place are the hi-bytes of the
address of the various commands.
--
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