[Platform] [PC-98] Gaiji upload

Will come in handy for various research programs… 👀

Part of P0232, funded by [Anonymous].

platform/x86real/pc98/font.cpp

@@ -0,0 +1,46 @@
+#include "platform.h"
+#include "x86real.h"
+#include "pc98.h"
+#include "planar.h"
+#include "shiftjis.hpp"
+#include "platform/x86real/pc98/font.hpp"
+
+// The PC-98 character generator can be accessed in two ways:
+//
+// 1) I/O ports (0xA1, 0xA3, 0xA5, 0xA9)
+// 2) BIOS function INT 18h AH=14h (reading) / AH=18h (writing gaiji)
+//
+// The BIOS variant is both simpler to use *and* faster, as the BIOS implements
+// it in the optimal way for the specific hardware we run on:
+//
+// • At worst, it's an optimized ASM version of the I/O port code we would
+//   write ourselves (as seen in the PC-9801VX / PC-9801RA BIOS).
+// • At best, it directly reads from and writes to memory after mapping the JIS
+//   code point to an address, which is always preferable to port I/O (as seen
+//   in the PC-98HA BIOS).
+// • Emulators directly implement it in native code.
+
+void font_gaiji_write(
+	const dot_rect_t(GLYPH_FULL_W, GLYPH_H)* gaiji,
+	uint16_t count,
+	uint8_t id
+)
+{
+	// Yeah, that copy is kind of silly, but we don't want to force that
+	// awkward 16×17 format onto the caller.
+	font_glyph_kanji_t tmp;
+
+	jis_t jis = (0x7680 + id);
+	for(uint16_t i = 0; i < count; i++) {
+		tmp.dots = *gaiji;
+		_BX = FP_SEG(&tmp);
+		_CX = FP_OFF(&tmp);
+		_DX = jis;
+		_DX &= 0xFF7F;
+		_AH = 0x1A;
+		geninterrupt(0x18);
+
+		gaiji++;
+		jis++;
+	}
+}

platform/x86real/pc98/font.hpp

@@ -16,3 +16,19 @@ struct font_glyph_ank_8x16_t : public font_glyph_header_t {
 struct font_glyph_kanji_t : public font_glyph_header_t {
 	dot_rect_t(GLYPH_FULL_W, GLYPH_H) dots;
 };
+
+// Writes user-defined glyphs to the given offset inside the PC-98 gaiji RAM.
+// The 256 possible gaiji IDs are mapped onto JIS X 0208 codepoints as follows:
+//
+// 	((0x7680 + id) & 0xFF7F)
+//
+// Note that not all PC-98 models support all 256 gaiji codepoints. IDs 0x00
+// and 0x80 are particularly unlikely to be supported, as they collide with
+// ASCII 0x56 (V) and 0x57 (W) when written into TRAM. The safest IDs range
+// from 0x21 to 0x5F inclusive, which correspond to the gaiji range from the
+// first PC-9801 models that supported the feature (PC-9801E/F/M).
+void font_gaiji_write(
+	const dot_rect_t(GLYPH_FULL_W, GLYPH_H)* gaiji,
+	uint16_t count,
+	uint8_t first_id
+);