VGA_MEM_FIX

[vruppert]

I have now applied a set of fixes for the VGA memory management, but I left them disabled until we have made some more tests. It should fix the Debian / GRUB issue (#257) with both Bochs VBE and Cirrus. If you have some old DOS games, please uncomment the VGA_MEM_FIX define in vgacore.h to test the new code. When we have no regression the new code could be enabled for the planned release 2.8.

[Vort]

This is how Jazz Jackrabbit looks with VGA_MEM_FIX:

[vruppert]

Now I found out what's going on. The game uses modeX (mode 13h with chain4 disabled). When playing the game it switches between planar4 writes and odd/even writes with "chain odd/even" turned off. The newly implemented odd/even mode works correctly in text mode, but it has "chain odd/even" turned on. The other mode is not yet implemented. I'll have a look how to handle the address remapping correctly.

[vruppert]

Applied a temporary fix for this case. Since the planar mode was used before I added odd/even mode, falling back to planar mode should be okay for now.

[Vort]

Ok, Jazz Jackrabbit looks good now.
Next problems are with text mode in Commander Keen 5:

[Vort]

Another problem: Windows XP start to produce crash with Cirrus card and VGA_MEM_FIX:
cursor_address is 0 here and bx_gui->cursor_address is 65535. size of the buffer is 32768:

>	bochs.exe!bx_gui_c::text_update_common(unsigned char * old_text, unsigned char * new_text, unsigned short cursor_address, bx_vga_tminfo_t * tm_info) Line 1237	C++
 	bochs.exe!bx_vgacore_c::update() Line 1749	C++
 	bochs.exe!bx_svga_cirrus_c::update() Line 1219	C++
 	bochs.exe!bx_vgacore_c::vga_timer_handler(void * this_ptr) Line 2547	C++
 	bochs.exe!bx_virt_timer_c::periodic(unsigned __int64 time_passed, bool mode) Line 165	C++
 	bochs.exe!bx_virt_timer_c::advance_virtual_time(unsigned __int64 time_passed, bool mode) Line 323	C++
 	bochs.exe!bx_virt_timer_c::timer_handler(bool mode) Line 435	C++
 	bochs.exe!bx_virt_timer_c::pc_system_timer_handler_0(void * this_ptr) Line 536	C++
 	bochs.exe!bx_pc_system_c::countdownEvent() Line 382	C++
 	bochs.exe!bx_pc_system_c::tickn(unsigned int n) Line 117	C++
 	bochs.exe!BX_CPU_C::cpu_loop() Line 125	C++
 	bochs.exe!bx_begin_simulation(int argc, char * * argv) Line 1051	C++
 	bochs.exe!bx_real_sim_c::begin_simulation(int argc, char * * argv) Line 930	C++
 	bochs.exe!win32_ci_callback(void * userdata, ci_command_t command) Line 778	C++
 	bochs.exe!bx_real_sim_c::configuration_interface(const char * ignore, ci_command_t command) Line 922	C++
 	bochs.exe!bxmain() Line 336	C++
 	bochs.exe!main(int argc, char * * argv) Line 555	C++

[vruppert]

This one should be fixed now. For the Commander Keen issue I need to do some more investigation, The startup screen uses contradictory text mode settings. The new code uses the graphics controller setting for the odd/even mode, but the related setting of the setting of the sequencer is different. I need to find out how to handle this.

[vruppert]

Update: I just tried Commander Keen with Qemu, since most of the new code is based on it's implementation. It shows the same issues as Bochs with VGA_MEM_FIX. The DOSBox display looks correct, but I don't really understand it's code. It seems to use the "chain odd/even" bit only. I finally had a look at the PCem source, but I didn't test it yet. It uses the other two odd/even related switches - one for the read case and the other for the write case. If odd/even mode is detected it's way, it rotates the address bits by two bits. I think we have to find the right way ourselves after some more testing.

[Vort]

Another source to look at is VirtualBox:
https://www.virtualbox.org/browser/vbox/trunk/src/VBox/Devices/Graphics/DevVGA.cpp

[vruppert]

Thanks for pointing this out. I have now applied some new code based on the VirtualBox implementation. It seems to be a fixed version of the Qemu code. I have not yet added the address shift by two instead of one, since I have not yet tested it with Debian / GRUB. The startup screen of Commander Keen is now okay, but the game itself has issues that need to be observed. Jazz Jackrabbit also looks okay after this change.

[Vort]

Several more games I tested had no additional problems.
Scroll problems with non-0 update_freq persists, but they are not related to recent changes.

[Vort]

One more problem is with Dangerous Dave in the Haunted Mansion, image flickers:

bochs_dave2.mp4

[vruppert]

I have currently no idea what can go wrong here. It looks like a timing issue that appears only with VGA_MEM_FIX. It is a planar VGA mode and there seem to be no writes in other modes. When I add BX_INFOs in mem_write() to find out what's going on the flicker doesn't seem to happen.

[Vort]

Game stopped glitching like that after I changed vgamem_mask from BX_VGA_THIS s.memsize - 1 to 0x3FFFF.

[vruppert]

I have now added a variable that will be modified in VBE mode for 4-bpp support.