C64 cartridge images - how to do them?

There exist a number of specific structures for CRT files. Do you already know the structure you would like your image to use ?

The possible cartridge types supported by CRT files are listed here: ar.c64.org/wiki/CRT_ID

I suppose it's a lot easier to take a CRT file already prepared and only replace the content of the ROM files.

For example, if you want a custom Fastload CRT then just take the CRT file for the real Fastload cart and change the content of the ROM image (noted CHIP in the CRT file hex: 43 48 49 50 )

Then use the following structure:

example: 
00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F ASCII

----------------------------------------------- ----------------

0000: 43 48 49 50 00 00 20 10 00 00 00 00 80 00 20 00 CHIPúúúúúúúú€úúú

Bytes:$0000-0003 - Contained ROM signature "CHIP" (note there can be more

than one image in a .CRT file)

0004-0007 - Total packet length ($00002010, ROM image size and

header combined) (high/low format)

0008-0009 - Chip type

0 - ROM

1 - RAM, no ROM data

2 - Flash ROM

000A-000B - Bank number ($0000 - normal cartridge)

000C-000D - Starting load address (high/low format)

000E-000F - ROM image size in bytes (high/low format, typically

$2000 or $4000)

0010-xxxx - ROM data

I assume it's much easier to modify and existing CRT file than to create one from scratch.

OK, first results.

Tested were both Z80 separately and in both C128 modes - 128 and 64.
Long time known thing is that calling missing Z80 never produces crash, for example - internal Z80 is not accessible while is SCPU turned on or in 64 mode​.
The same is for external Z80 - if is not present, code continues normally without crash.

I created two separate routines for each Z80 - internal increment VIC border, external increment VIC paper.
I tried them in all possible combinations with SCPU, CP/M cartridge, 64 mode etc. - all was working correctly.

Then I started Dual Z80 routine.
Yesterday routine starts and hanging computer. This was surprise 'cause before using handling code after switch from internal Z80 to external Z80 it always crashed.
It was looking like code attempts to switch and continue, but hanging is not ok.
This night I tried similar code.
Routine starts in 8502 code to switch to internal Z80, then internal Z80 attempts switch to external Z80 and then continues with border increment.
So, I was waiting for starting increment VIC paper by external Z80 and by internal Z80 increment VIC border at once.
It works not - looks like switch to external Z80 is not succesfull. Ok, this can be waited, but why happened problems before?
I was searching in resources and found that expansion port pin 7 allows to activate external Z80, there is CP/M cartridge connected and this is reason why can work with SCPU too. Running with SCPU requires only slow down to 1MHz while running Z80 code, only for switch sync. Internal Z80 is not possible to access with SCPU for hardware connection and in 64 mode disabling MMU.
But here's Q - expansion port, especially pin 7, is maybe accessible by internal Z80, why to throw out hardware which can run with Z80...
If is accessible it needs to find where and then switch it by that way. This switch produces signal to 8502 (or 6502) to turn off, but it was not designed to produce signal to turn off internal Z80 - so this fight I lost not yet.
Many times is easy to find CP/M memory map - for your info - C128's CP/M3 handles whole 128K - many times I had to read that 64K only. And finding differences between CP/M memory map and 8502 memory map is not true for Z80 between 8502 - CP/M is operating system, example:
CP/M uses for 40 column screen attributes area $1000-$1300 etc., but if you want to POKE by Z80 code any character to 40 column screen you must to use area $0400-$07E7 (by default).
So comparing CP/M and 8502 memory maps is mismatch. It needs to compare Z80 between 8502 map.
And differently is RAM mapped by internal and external Z80 - internal uses same addressing, but external Z80 has whole RAM shifted $1000 lower - and accessing switch there is $CE00 instead 8502/internal Z80 $DE00 - so external Z80 see pin 7, again reason why to search...
Here it looks like no source to get where is mapped exp. port - or I can to use also not IO opcodes and try it again. Also 64 mode has not accessible MMU and other differences can make it not accessible, but CP/M cartridge I was using first time with C128 - I was warned that it's impossible to use and my results were succesfull and rewritten words in one SuperCPU FAQ page...
Very important is in Z80 assembler to use correct opcodes - for IO IN and OUT and normally LD...
I will experiment - this fight is not finished, not yet has winner.

If anyone has any knowledge in mapping C128's RAM and exp. port for Z80, suggestions and each help are welcome.
Please don't forgive that this topic was started for help with C64 cartridge.
So, thank you all.

Miro