[C] 32K ROM limitations in openMSX

I don't know much about sdcc, but it seems to me it might be a case of not setting the ROM/RAM pages correctly at cartridge initialization. At least, in a cartridge coded in assembler, you would have to add some special initialization code if your ROM is larger than 16KB. It might work in some configurations due to the way ROM/RAM pages are set at startup.

See, for example: https://www.msx.org/wiki/Develop_a_program_in_cartridge_ROM#...

You mean these configurations?

;----------------------------------------------------------
; ApplicationSettings.txt - by Danilo Angelo, 2020
;
; MSX ROM project general configuration.
;----------------------------------------------------------

;----------------------------------------------------------
; Project Type
PROJECT_TYPE ROM

;----------------------------------------------------------
; crt0 features
GLOBALS_INITIALIZER _ON _ON: Includes routine to initialize global variables; _OFF: Don't include
RETURN_TO_BASIC _OFF ; _ON: Returns to basic; _OFF: Don't return
STACK_HIMEM _ON ; _ON: Sets stack to HIMEM (RETURN_TO_BASIC *MUST* be _OFF); _OFF: Don't set
SET_PAGE_2 _ON ; _ON: Enables page 2 on the same slot/subslot as page 1 (useful for 32kb ROMs on pages 1 and 2); _OFF: Leaves page 2 as is

;----------------------------------------------------------
; ROM settings
ROM_SIZE 32k ; 16k/32k
FILESTART 0x4000 ; Typically, 0x4000 for 32kb ROM or 0x8000 for 16kb ROM
CALL_EXPANSION _OFF ; _ON: expands BASIC's CALL instruction; _OFF: Don't expand
DEVICE_EXPANSION _OFF ; _ON: creates BASIC's device; _OFF: Don't create
BASIC_PROGRAM 0 ; 0 -> no BASIC program; other values: BASIC program address

;----------------------------------------------------------
; BASIC CALL expansion
;
; NOT MANDATORY!
; Use this to extend the funcionality of
; BASIC's CALL command
;
; BASIC usage example (for the standard app)
; 10 CALL CMD1RINT "Oi."
; 20 CALL CMD2RINT "Ola."
; 30 CALL CMD1
; 40 PRINT "Hi."
; 50 CALL CMD2
; 60 PRINT "Hello."
;
CALL_STATEMENT CMD1
CALL_STATEMENT CMD2

;----------------------------------------------------------
; BASIC device expansion
;
DEVICE DEV

;----------------------------------------------------------
; Code segment (.area _CODE) start address
; If you don't know what you are doing,
; LEAVE IT COMMENTED for automatic setting.
;CODE_LOC 0x8100

;----------------------------------------------------------
; Data segment (.area _DATA) start address
; 0 -> data segment immediately after code
; other value -> data segment address
; If you don't know what you are doing,
; LEAVE IT AT 0xC000 (page 3)
DATA_LOC 0xC000

In z88dk, the default settings will create a 32K ROM. I don't know much about sdcc, but it should be just as easy as z88dk in making 32k rom files too.

You probably have a wrong setting somewhere.

Also, a correct 32K ROM should be able to work on most MSX configurations, with or without mirroring.

This seems to be right.

In your [DEBUG/RELEASE]\obj folder, there's, a *.MAP file (after compiling the project). You can inspect it to check where the compiler put the several segments of your project, and where each and every symbol is. That can give some insight of what's happening. Please check this file and see if it makes sense.

Are you sure your template handle the case where the cartridge is in a secondary slot? I'm asking this because the first version of my crt0 for 32 KB cartridge didn't handle this case well. ^^
You have to switch the page 3's slot to cartridge slot before changing the secondary slot register in FFFFh (SLTSL), then reset the page 3's slot to its initial value (and of course do not use the call-stack or system variables in the meantime).
See the INIT_P1_TO_P2 macro for an exemple.