Subject: Re: Merlin
Newsgroups: comp.sys.apple2.programmer
From: dempson@actrix.gen.nz (David Empson)
Date: Thu, 6 May 1999 02:02:11 +1200
Message-ID: <1drcubi.n8p3n81cemy2oN@dempson.actrix.gen.nz>
References: <7gn8vo$h4d$1@nnrp1.dejanews.com>
Organization: Empsoft
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<bobryan9@my-dejanews.com> wrote:

> Thanks for the information about merlin!
> 
> Now, I would also like to make an area of bytes to be used for data,
> 
> Label   XXX   12345678             ;define 8 BYTES of data

There are several directives for this.  HEX lets you enter paired hex
digits, but I usually prefer DFB (define byte) because it is easier to
read, and lets you select the base.  The following two lines are
equivalent.

Label HEX  0102030405060708

Label DFB  1,2,3,4,5,6,7,8

You can also use DA (define address) to define 16-bit values (in the
standard low-byte-first order).

If you want to enter hexadecimal data with DFB or DA, use a $ prefix,
otherwise the value is decimal.

Another useful directive is DS (define space), which skips a designated
number of bytes.  I prefer to use this if the value in the location is
unimportant, e.g. a result or scratch variable which will be initialized
by the program before use.

e.g. for a hypothetical ProDOS MLI parameter block:

OpenPB   DFB 3               ; Parameter count
OpenPath DA  Pathname        ; Pointer to pathname to be opened
OpenBuf  DA  Buffer          ; Pointer to 512 byte page aligned buffer
OpenRef  DS  1               ; Reference number goes here

Pathname STR '/RAM5/TEMP'    ; Or whatever


The actual code to use this looks something like:

         JSR MLI             ; ProDOS call
         DFB $C8             ; OPEN
         DA  OpenPB          ; Parameter block pointer
         BCS OpenErr         ; Branch if error
* If no error, copy the reference number to the other parameter blocks
         LDA OpenRef
         STA ReadRef
         STA CloseRef
* etc.

-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand