VGA_GC_READ_MAP_SELECT
reg[3] = vga_get_grc(VGA_REG_BASE, VGA_GC_READ_MAP_SELECT);
vga_set_grc(VGA_REG_BASE, VGA_GC_READ_MAP_SELECT, 0x02);
vga_set_grc(VGA_REG_BASE, VGA_GC_READ_MAP_SELECT, reg[3]);
REG_WRITE1(sc, VGA_GC_ADDRESS, VGA_GC_READ_MAP_SELECT);