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PC-98 CG MMIO fix for "Plants of Dragon" by ASCII. Not entirely sure if this fix reflects real PC-98 hardware, but it fixes the corrupted characters (custom char font 0xxx56 codes) in the game. Apparently that part of the CG cannot be read/written the full 16 bits, and if you try, you get 8 bits at a time anyway?

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This commit is contained in:
Jonathan Campbell 2025-04-17 23:42:40 -07:00
parent d43fd90a16
commit 0eaed9299c
1 changed files with 26 additions and 6 deletions
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32
src/hardware/vga_memory.cpp
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@ -1604,13 +1604,33 @@ class VGA_PC98_CG_PageHandler : public PageHandler {
uint8_t readb(PhysPt addr) override {
/* uses the low 12 bits and therefore does not need PAGING_GetPhysicalAddress() */
uint8_t high = a1_font_load_addr & 0xff;
if((high >= 0x09 && high <= 0x0b) || (high >= 0x0c && high <= 0x0f) || (high >= 0x58 && high <= 0x5f)) {
if(addr & 1) {
return pc98_font_char_read(a1_font_load_addr,(addr >> 1) & 0xF, (a1_font_char_offset & 0x20) ? 0 : 1);
} else {
return 0;
}
if ((a1_font_load_addr & 0x007E) == 0x0056 && (a1_font_load_addr & 0xFF00) != 0x0000) {
/* FIXME: Is there something about NEC PC-98 CG hardware where the custom char cells
* cannot be read as full 16-bit values like the rest of the CG, but can only be
* raad 8 bits at a time using port 0xA5 to select the half, and if you issue a
* 16-bit read, you get the 8-bit value twice?
*
* Test case: Planets of Dragon by ASCII. We have to emulate like this or the custom
* char cells it uploads are shown corrupted.
*
* For these char cells the game renders using LODSW / SUB AL,AL / STOSW for the first
* 8 bits and then LODSW / SHR AX,8 / STOSW for the second 8 bits. That will render
* corrupt graphics without this oddball code.
*
* Question: What does real PC-9821 hardware do? Does later hardware fix this, uh, quirk?
* The file dates in the HDI image are from 1994.
*
* The game appaers to write the custom cells, use them in the text layer AND rely on
* drawing them on the graphics layer. */
return pc98_font_char_read(a1_font_load_addr,(addr >> 1) & 0xF,(a1_font_char_offset & 0x20) ? 0 : 1);
}
else if ((high >= 0x09 && high <= 0x0b) || (high >= 0x0c && high <= 0x0f) || (high >= 0x58 && high <= 0x5f)) {
if(addr & 1)
return pc98_font_char_read(a1_font_load_addr,(addr >> 1) & 0xF, (a1_font_char_offset & 0x20) ? 0 : 1);
else
return 0;
}
return pc98_font_char_read(a1_font_load_addr,(addr >> 1) & 0xF,addr & 1);
}
void writeb(PhysPt addr,uint8_t val) override {