On 16. 03. 2013 12:54, Masur Jonathan wrote:
Le 16.03.2013 01:13, Borut Ražem a écrit :
Helo Jonathan,
sdas and sdld are actually a fork of Alan Baldwin's ASXXXX assembler
(see http://shop-pdp.net/ashtml/asxxxx.htm) with some sdcc related
modifications. Only the targets supported by sdcc are included in
sdas/sdld but the documentation includes all ASXXXX targets, as you
already noticed, All sdcc targets except pics generate assembly code
primarily (or exclusively) for sdas / sdld. So the natural choice for
sdcc 6502 target wold be the ASXXXX assembler, which already include
support for 6502.
Thanks, this made things a little bit clearer. Did Alan Baldiwn's ASXXX
change much since the sdcc fork or is it still pretty much the same ? In
any case it sounds like it would be fairly trivial to port his version
of ASXXX to the SDAS fork.
There is ongoing project to resynchronize sdas/sdld with the latest
ASXXXX version: I did some work in this direction quite some time ago
mainly in the linker area an Maarten synced the assemblers. Amount of
deviation from the original also depends on the target. Maybe Leland
Morrison can give you much more in-deep information since he implemented
Rabbit 2000/3000/3000A support not so far ago.
There is an other component needed when implementing a new sdcc target:
the simulator. It is indispensable for testing the generated code and
for running regression tests. You can use any open source simulator you
can find, but the natural choice is ucsim, used by all sdcc targets
expect pics. Unfortunately uscim doesn't support 6502 yet, so this part
should be implemented too,
You are very right, in fact I tought that a hardware simulator using an
FPGA would be cooler, but in any case I think I can't avoid ucsim so
I'll have to see how it works for other CPUs first...
There is already dozen of very accurate 6502 simulators, Visual6502 is
very popular. Not to mention all the C64 and NES emulators.
The simulator should be:
- open source
- easily portable to all platforms used for sdcc regression testing (see
http://sdcc.sourceforge.net/snap.php)
- fully command line driven so that it can be used in automated
regression testing
Le 16.03.2013 00:38, Richard Gray a écrit :
> I'm dredging my memory, but back in the '80s this problem was
"solved" by a
> pseudo-instruction, LDAZ, meaning load zero-page. I wrote an
assembler myself
> for the 6502 back in the 1980's, and I seem to recall this is how I
did it. I
> don't think there's a reliable way of deducing whether a zero-page
instruction
> is appropriate otherwise.
The problem is that it is absolutely not compatible with the
standardized 6502 instruction set.
I already some assembler who handles it unconventional ways, like
lda general_variable
lda <zero_page_variable
for NESASM
and
lda zero_page_variable.b
lda general_variable.w
for WLA-DX
(it would actually default to zero-page and send an error if you ommit
the ".w" suffix when accedding a label which is not zero page - this is
not required if the address is a direct number greater than $100).
As far as I know, many assemblers in the '80s did not allow for
relocatable code (= no linker), and this was sovled with multple passes
in assembly :
Pass 1 : Parse all the variable declarations, labels, etc... and assign
them to a cathegory (zero page or not zero page)
Pass 2 : Parse all the code, now that we know where the labels are it's
possible to assemble the code. Since no relocatable code was used, they
just changed the PC with .org or similar directive and didn't ask
themselves more questions.
As you can see pass 1 is not exactly equal to assembling and pass 2 is
not equal to linking, as the code size is completely unknown on pass 1.
This worked fine in many cases, but caused problem if you wanted your
code to be relocatable, i.e. you copy it from ROM to RAM and execute
form RAM for example. Or in the Commodore 64, you copy the code from RAM
to the disk drive which has a completely different 6502 with a different
address space. In both case the code needs to be relocatable to work and
only a true linker can do this.
This can probably be solved with two pass assembly + one linking pass (3
passes in total). I'll have to check how CA65 and LD65 handles this problem.
In all cases, the closer the assembler is to the official syntax, the
better.
I also do not like that apparently ASXXX 6502 wants to use [] brackets
for indirection instead of the standard () brackets.
This is probably simple to change though.
This is probably because the parentheses are used for expressions. In
this case use of square brackets seems more appropriate to me. But I'm
only guessing...
P.S.: There is an other thing I want to mention here: there were many
attempts to add new targets to sdcc in it's history, but many of them
infamously failed. The problem is that adding a target is not a trivial
job: it requires a huge amount of work which is never finished. Many
developers started to work on new targets but then lost the interest or
didn't have enough time to continue. Such cases cause nothing else but
mess and problems to the whole sdcc project. So, if you have serious
intentions, we can integrate your work directly into the sdcc project.
If not it is better to create a totally separate project and eventually
merge it with the original sdcc if/when it is considered to be stable
enough. In this case your project should keep to be synchronized with
sdcc as much as possible in order to make the (potential) re-merge as
simple as possible.
Good luck,
Borut
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