From d996226ded0190b96cce09169291cc696a390832 Mon Sep 17 00:00:00 2001 From: tangbinzy Date: Wed, 13 Mar 2024 02:51:28 +0000 Subject: [PATCH] vga: fix incorrect line height in 640x200x2 mode mainline inclusion commit 37e7b86766244b62a406747bb78e049390d0b528 category: bugfix --------------------------------------------------------------- When in CGA modes, QEMU wants to ignore the maximum scan field (bits 0..4) of the maximum scan length register in the CRTC. It is not clear why this is needed---for example, Bochs ignores bit 7 instead. The issue is that the CGA modes are not detected correctly, and in particular mode 6 results in multi_scan==3 according to how SeaBIOS programs it. The right way to check for CGA graphics modes is to check whether bit 13 of the address is special cased by the CRT controller to achieve line interleaving, i.e. whether bit 0 of the CRTC mode control register is clear. Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1020 Reported-by: Korneliusz Osmenda Signed-off-by: Paolo Bonzini Signed-off-by: tangbinzy --- hw/display/vga.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/hw/display/vga.c b/hw/display/vga.c index 9d1f66af40..33765148d9 100644 --- a/hw/display/vga.c +++ b/hw/display/vga.c @@ -1514,9 +1514,10 @@ static void vga_draw_graphic(VGACommonState *s, int full_update) force_shadow = true; } + /* bits 5-6: 0 = 16-color mode, 1 = 4-color mode, 2 = 256-color mode. */ shift_control = (s->gr[VGA_GFX_MODE] >> 5) & 3; double_scan = (s->cr[VGA_CRTC_MAX_SCAN] >> 7); - if (shift_control != 1) { + if (s->cr[VGA_CRTC_MODE] & 1) { multi_scan = (((s->cr[VGA_CRTC_MAX_SCAN] & 0x1f) + 1) << double_scan) - 1; } else { -- 2.27.0