If you tell an EGA card that it's connected to a monitor capable of 350-line modes by setting the appropriate switches on the card itself, it will by default use these 350-line modes for its text mode (using the 14-scanline character set instead of the 8-scanline one, yielding higher fidelity text).<\/p>\n
But sometimes you want a 200-line text mode. In particular, there is an obscure 160x100 16 colour mode of the CGA which was obtained by using 80-column text mode, filling the text characters with the \"left vertical bar\" or \"right vertical bar\" characters (221 and 222 repectively), disabling blinking and setting up the CRTC for 100 visible rows of 2-scanline characters. This was used by some Windmill Software games and a few others. With VGA you can do the same thing with 400-line text mode (by using 100 visible rows of 4-scanline characters).<\/p>\n
But how do you do it with EGA? One way is to program all the registers directly (as one must do on CGA) using the timings, sync polarity and palette from 200-line graphics modes and all other settings from the 350-line text modes. As mentioned in yesterday's post<\/a>, we must use the palette 0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x10, 0x11, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17 instead of the usual 0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x14, 0x07, 0x38, 0x39, 0x3a, 0x3b, 0x3c, 0x3d, 0x3e, 0x3f because the monitor will interpret bit 4 as intensity instead of secondary green in 200-line modes.<\/p>\n Another way (which may be either more or less compatible with clone EGA cards) is to fool the BIOS into thinking a 200-line monitor is connected. The EGA BIOS reads the card's switches only once at boot time and then stores the results in BIOS memory at 0x40:0x88 and uses this value instead of the hardware value at mode-setting time. The low nybble of the value at this location is 3 or 9 for 350-line monitors and the corresponding values for 200-line monitors are 2 and 8 respectively. So an alternate algorithm is to check the byte at this address, decrement the low nybble if it's 3 or 9, store it back, do the \"int 0x10\" to set text mode, set the Maximum Scan Line Register to 1, disable blink and fill the text characters with a vertical bar.<\/p>\n