Creating a MSX ROM in C with SDCC

You can use the MSX Template Pack to get a quick start.

I have followed the same path of starting a project in the 8bit workshop IDE using msxbios, and then running out of space, and then I have used the MSX Template Pack (MSX.ROM.Application in particular) and transferred my code across to it, and can make 16K ROMs that work in OpenMSX and on my real MSX1 Panasonic CF-2700.
But I have now again run out of space and have been trying to make a 32K ROM. I have made them with both with the msxromcrt0.rel of the MSX,ROM.Application and have also substituted in crt0_MSX32kROM4000 (from https://github.com/mvac7/SDCC_startup_MSX32kROM4000) and hex2bin duly makes 32k ROMs, but then they do not work in openMSX or on my real machine. (They go to the MSX BASIC version 1.0 screen, instead of my cool looking Mode 2 intro screen for my game).
My game is called Blastoid. Hopefuily it will be an entry in MSXDEV22.
The make file runs these:

sdcc --code-loc 0x0000403E --data-loc 0xC000 -mz80 --no-std-crt0 --opt-code-size --disable-warning 196 ".\RELEASE\\objs\msxromcrt0.rel" ".\RELEASE\\objs\msxbios.rel" ".\RELEASE\\objs\Blastoid.rel" -I"." -o ".\RELEASE\\objs\BLASTOID.IHX"

hex2bin -e ROM -l 8000 ".\RELEASE\\objs\BLASTOID.IHX"

In the Application_Settings.txt file:
; crt0 features
GLOBALS_INITIALIZER _ON
RETURN_TO_BASIC _OFF
STACK_HIMEM _ON
SET_PAGE_2 _ON
;----------------------------------------------------------
; ROM settings
ROM_SIZE 32k
FILESTART 0x4000 ; Typically, 0x4000 for 32kb ROM or 0x8000 for 16kb ROM
CALL_EXPANSION _OFF
DEVICE_EXPANSION _OFF
BASIC_PROGRAM 0

What am I missing or not understanding about 32K ROMs? Anything I should try?

Thanks. That will be great.
I've zipped it up here.
https://www.dropbox.com/s/6qk4hq874lypcs2/Blastoid.MSX_ROM_A...
There is one source file, levels.h with 2 versions of it. One has some static char arrays commented out to bring it under the 16K boundary. The other makes it go over. levels.h presently makes it go over. So make\make RELEASES builds it.
I'll delete it once you've got it from dropbox, so let me know when you have.
Cheers.

Hi again MattyT,

I just made a Quick test and the game seems to work fine with both levels.h files, under DEBUG and under RELEASE modes. I tested under several MSX configurations in OpenMSX and they all worked. There is a good chance of me doing something wrong though...

Do you want to contact me directly? You may find my email on the cover page of the MSX Template Pack Manual. From there we can find a more efficient way to communicate (WhatsApp/Telegram).

So I'm running into an interesting issue here with SDCC ROM creation using the Fr3el project methods (which work flawlessly otherwise). I think I may have an understanding of it but I want to check that I get it and ask for a workaround.

For my testing and experimentation I'm using an 8k ROM that starts at 0x4000. In theory that means that RAM from 0x6000 straight through to the end (minus the stack) is available for use. Everything I've done bears that out, and I can read and write to those without a problem.

Where things get interesting is if I want to write to addresses BELOW my ROM. Let's say I want to put 64 bytes of data at 0x1000, which to my understanding is outside of the trampolines to the BIOS code. If I try writing to that address, it doesn't stick. Between yesterday and today I've determined that my understanding of how the BIOS works is flawed, but I just want to make sure.

What I think is happening is that the BIOS ROM is, just like my own ROM, overlaid on top of 0x0000-0x3FFF. So without paging out the BIOS, that low RAM is inaccessible. Am I right so far?

Let's say then that I *do* want to have access to that low RAM through straight assembly. I get that it might not be advisable due to the fact that I'm going to be using interrupts, but is it possible? And if it is, what assembly would I use to do it (with the appropriate DI and EI commands around it, of course)?