Re: Writing Assembler!

> >You can use lot of from compiler books when making the assembler
> >scanner and parser. In assemblers you should treat also end of lines
> >and spaces as tokens and not just skip them.
>
> Why that ? Spaces does not seem to mean anything for asm... Even
> worse: since the number of possible instructions and fields are so
> limited why not use a lookup table with all of it ? Anyway I am using
> it in the assembler I am building in QBASIC and it seems to work out
> great. Scanning and parsing seems to me a waste in this case ...


---------------


Spaces and eol's do in some assembler have meaning! For example the
old IBM assembler format which was copied to Motorola assemblers
has the following rules:


1. An identifier starting a line is a label. There after there may be a
      mnemonic or not.


2. If a space starts a line there is no label and the next identifier is
        a mnemonic.


3. A star "*" starting a line is a comment line.


Like:


*
* This is a comment
*
LABEL MOV R0,R1 Any comment text
MOV R2,R3
ETC.


So you have to know where is a start of a new line. You know if you
also know where was the last eol. Then you have to know if there is
a space in the begining of the line or not .. so you have to have also
the space token.


<EOL><IDENT> -> IDENT is a label
<EOL><SPACE><IDENT> -> IDENT is a mnemonic


But there are also other syntaxes .. they maybe don't need the spaces
and eols. Some assembly languages say that a label have to be followed
by a ":" .. so you know from that if it is a label or a mnemonic. You could
also check all the mnemonics and if the ident is not there .. it must be a
label.


But the eol is anyway meaningfull because assembly is always line
oriented. But if you already read everything as lines .. you already have
there a "pseudo eol" token.


JOJ
--

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