From: William Levak (wlevak_at_sdf.lonestar.org)
Date: 2008-03-01 05:57:02
On Thu, 28 Feb 2008, ruud.baltissen@abp.nl wrote:
> 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?
Because the copy routine at $C8F0 does it's own syntax check.
> 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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> \ \__|_\
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