sjasmpluss stupid n00b question

I am no expert in sjasmplus, but this is my understanding:
- In principle you'd do that with the "device" command, unfortunately sjasmplus only has spectrum-variant built-in devices. So, you'll have to do it manually.
- The easy way to do it manually is to replace the last line you have by this:

        ds 0xC000 - $

An "org" only changes the address that the labels you define afterwards will have, but it does not "create space". If you want to fill a space with 0s, you need to "define space" ("ds"). "$" refers to the current address, so "0xc000 - $" is whatever number of bytes you need to reach there.

I hope this helps!

ps: nice user name

And why would you need to pad the cart with 0's ?

Yes, but in Sjasm it's not necessary.
When you define the page length, it automatically pads the page to make the correct length.

defpage 0,8000h,04000h
page 0

will define a code page of 4000h in length, and pads the empty part with 0's.
And I suppose that Sjasmplus does the same.

"raw" option: either the mentioned `ds 0xC000-$` or if you need to reach power of two, then you can also `ALIGN 0x4000` or `ALIGN 0x4000,0xFF` to fill with ones (also `ds` takes second argument with filler value) (but not ALIGN 0xC000 - that's not power of two).

`OUTPUT` option http://z00m128.github.io/sjasmplus/documentation.html#po_output :

    OUTPUT "t.bin"
    SIZE 0x1000
    db 0,1,2,3,4,5,6,7
    OUTEND

`DEVICE` option http://z00m128.github.io/sjasmplus/documentation.html#po_device :
- the ZX 48/128/... "devices" have memory paging per 16kiB pages, and taint the virtual memory with fake ZX sysvars upon init (should not matter if you overwrite the memory with your content for cart), total memory depends on particular device seleced (from 64kiB up to 8MiB) (ie. don't worry about "ZXSPECTRUM" name, if the device has configuration which is useful also for MSX, you can use it, should work well and if not, please report any issues)
- the ZX Next device has memory paging per 8kiB pages (all zeroed, no pre-init with anything), total memory 2MiB. Thinking about it, this device does actually have one more extra, it enables Z80N instruction extensions, which are useless outside of Spectrum Next, but if you don't use the extra instructions, the "device" is as generic as "noslot64k"
- "noslot64k" device has paging per 64kiB pages (all zeroed), total memory 2MiB, no other extra, just plain block of memory to fill with code/data

Then you can `SAVEDEV` ( http://z00m128.github.io/sjasmplus/documentation.html#po_sav... ) whole cart in one go, if you have everything layed out in the virtual-device memory as designed (the current snapshot of virtual memory is saved by SAVEDEV, so you put it after all the code and data specified/incbin, not at beginning of file).

For first two option you may be interested also in --longptr option in case you want 24bit label values working with full cart size (in case of 64ki+ carts), and you can technically produce any size of cart.

The device option brings extra support for paging, during assembly time the MMU directive may be handy, and in the code you can then use stuff like `ld a,$$label` to get page number where the label was defined, etc.

(all of this does apply to z00m's fork of sjasmplus)

So there are many possible ways how to do it, may depend on your personal preference what you want to use and also how much time you want to spend giving docs quick glance to learn about various directives and operators.

If you feel like there's anything seriously missing hampering the ability to produce MSX carts (AFAIK the sjasmplus should be usable for this, I'm not aware of any problem, but I don't do MSX dev, so I may just not have idea), let me know what the issue is. Cheers.