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Force 8bpp highres SVGA modes to "bypass" the shifter entirely

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The current implementation still uses the shifter forced to Shift 256 mode to keep things generic, but we can always opt for a separate scanline renderer.
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GreaseMonkey
2023-11-23 10:04:04 +13:00
parent d19fe700f5
commit 81d285b883
1 changed files with 14 additions and 7 deletions
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21
src/video/vid_svga_render.c
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@@ -462,20 +462,27 @@ svga_render_indexed_gfx(svga_t *svga, bool highres, bool combine8bits)
const bool attrblink = ((svga->attrregs[0x10] & 0x08) != 0);
// The following is likely how it works on an IBM VGA - that is, it works with its BIOS.
// But on an S3 Trio, mode 13h is broken - seemingly accepting the address shift but ignoring the increment.
// Forcing it to use incbypow2=0, incevery=1, loadevery=1 makes it behave.
// But on some cards, certain modes are broken.
// - S3 Trio: mode 13h (320x200x8), incbypow2 given as 2 treated as 0
// - ET4000/W32i: mode 2Eh (640x480x8), incevery given as 2 treated as 1
const bool forcepacked = combine8bits && (svga->force_old_addr || svga->packed_chain4);
// SVGA cards with a high-resolution 8bpp mode may actually bypass the VGA shifter logic.
// - HT-216 (+ other Video7 chipsets?) has 0x3C4.0xC8 bit 4 which, when set to 1, loads bytes directly, bypassing the shifters.
const bool highres8bpp = combine8bits && highres;
const bool dwordload = ((svga->seqregs[0x01] & 0x10) != 0);
const bool wordload = ((svga->seqregs[0x01] & 0x04) != 0) && !dwordload;
const bool wordincr = ((svga->crtc[0x17] & 0x08) != 0);
const bool dwordincr = ((svga->crtc[0x14] & 0x20) != 0) && !wordincr;
const bool dwordshift = ((svga->crtc[0x14] & 0x40) != 0);
const bool wordshift = ((svga->crtc[0x17] & 0x40) == 0) && !dwordshift;
const uint32_t incbypow2 = (combine8bits && (svga->force_old_addr || svga->packed_chain4)) ? 0 : (dwordshift ? 2 : wordshift ? 1 : 0);
const uint32_t incevery = (combine8bits && (svga->force_old_addr || svga->packed_chain4)) ? 1 : (dwordincr ? 4 : wordincr ? 2 : 1);
const uint32_t loadevery = (combine8bits && (svga->force_old_addr || svga->packed_chain4)) ? 1 : (dwordload ? 4 : wordload ? 2 : 1);
const uint32_t incbypow2 = forcepacked ? 0 : (dwordshift ? 2 : wordshift ? 1 : 0);
const uint32_t incevery = forcepacked ? 1 : (dwordincr ? 4 : wordincr ? 2 : 1);
const uint32_t loadevery = forcepacked ? 1 : (dwordload ? 4 : wordload ? 2 : 1);
const bool shift2bit = ((svga->gdcreg[0x05] & 0x60) == 0x20 );
const bool shift4bit = ((svga->gdcreg[0x05] & 0x40) == 0x40 );
const bool shift4bit = ((svga->gdcreg[0x05] & 0x40) == 0x40 ) || highres8bpp;
const bool shift2bit = ((svga->gdcreg[0x05] & 0x60) == 0x20 ) && !shift4bit;
const int dwshift = highres ? 0 : 1;
const int dotwidth = 1 << dwshift;