FW 3.12a - Comal CRT producing garbled text in Graphic Screen

[Schwefelholz]

Testing some new software written in Comal 80 2.01 I found issues in displaying text in a graphic screen, only when a CRT image of Comal is attached in the U64 or the UII+:

This issue does not come up when a physical original Comal cartridge or even a Kung Fu Flash using the exactly same CRT image is being used. In that case, the screen is displayed like this:

Also, the very same CRT image is working in VICE.

So, from my perspective, it looks as if the Comal CRT image is somehow badly handled by the U64 and UII+ cores.

[markusC64]

I have solved the problem by comparing U2+ source code and VICE.
This diff fixes the problem, but is only useful as a POC, because it would change all other OCEAN 16k modules.
Anyway, this tells you what the problem is: Bits 6 and 7 from the writing in $DExx must be observed as I have recoded it there:

commit e63a78bb85d31792cb246784a0ba8b4375ce2895
Author: markusC64 <markusC64@localhost>
Date:   Mon Aug 25 21:29:14 2025 +0200

    WIP: Just a test, not to be used productive

diff --git a/fpga/cart_slot/vhdl_source/all_carts_v5.vhd b/fpga/cart_slot/vhdl_source/all_carts_v5.vhd
index c1991ee..1a62bf2 100644
--- a/fpga/cart_slot/vhdl_source/all_carts_v5.vhd
+++ b/fpga/cart_slot/vhdl_source/all_carts_v5.vhd
@@ -340,9 +340,10 @@ begin
                 if io_write='1' and io_addr(8)='0' then -- DE00 range
                     -- variant sets max number of banks, 000 = 4, 001 = 8, 011 = 16, 111 = 32
                     bank_bits(18 downto 14) <= io_wdata(4 downto 0) and (variant & "11"); -- max 32 banks of 16K
+                    mode_bits(1 downto 0) <= io_wdata(7 downto 6);
                 end if;
-                game_n    <= '0';
-                exrom_n   <= '0';
+                game_n    <= mode_bits(0);
+                exrom_n   <= mode_bits(0) and mode_bits(1);
                 serve_rom <= '1';
                 rom_mode  <= "01"; -- 16K banks```

[markusC64]

@GideonZ Can you please look at this bug?