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Clarify what the EGA switch bits mean

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Jonathan Campbell 2024-12-04 20:20:44 -08:00
parent e93f0fbdd3
commit 22328353be
1 changed files with 22 additions and 1 deletions
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23
src/ints/int10.cpp
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@ -1241,7 +1241,28 @@ static void INT10_Seg40Init(void) {
if (IS_EGAVGA_ARCH) {
real_writeb(BIOSMEM_SEG,BIOSMEM_VIDEO_CTL,0x60);
real_writeb(BIOSMEM_SEG,BIOSMEM_CHAR_HEIGHT,16);
real_writeb(BIOSMEM_SEG,BIOSMEM_SWITCHES,(!IS_VGA_ARCH && ega200)?0xF8:0xF9);
// NTS: This is direclty from the configuration switches. The bits are inverted,
// 1=OFF 0=ON.
//
// sw1 sw2 sw3 sw4 sw4321
// ----------------------------------------------------------------------
// ON ON ON ON 0000 Color 40x25 EGA
// OFF ON ON ON 0001 Color 80x25 EGA
// ON OFF ON ON 0010 Enhanced Display Emulation Mode
// OFF OFF ON ON 0011 Enhanced Display Hi Res Mode
// ON ON OFF ON 0100 Monochrome (CGA is primary 40x25)
// OFF ON OFF ON 0101 Monochrome (CGA is primary 80x25)
// ON OFF OFF ON 0110 Color 40x25 EGA (MDA secondary)
// OFF OFF OFF ON 0111 Color 80x25 EGA (MDA secondary)
// ON ON ON OFF 1000 Enhanced Display Emulation Mode (MDA secondary)
// OFF ON ON OFF 1001 Enhanced Display Hi Res Mode (MDA secondary)
// ON OFF ON OFF 1010 Monochrome (CGA is secondary 40x25)
// OFF OFF ON OFF 1011 Monochrome (CGA is secondary 80x25)
//
// [http://hackipedia.org/browse.cgi/Computer/Platform/PC%2c%20IBM%20compatible/Video/EGA/IBM/IBM%20Enhanced%20Graphics%20Adapter%20%281984%2d08%2d02%29%2epdf]
//
// Anything else is not valid.
real_writeb(BIOSMEM_SEG,BIOSMEM_SWITCHES,0xFF^((!IS_VGA_ARCH && ega200)?0x8:0x6));
// Set the pointer to video save pointer table
real_writed(BIOSMEM_SEG, BIOSMEM_VS_POINTER, int10.rom.video_save_pointers);
}