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Re: [PATCH 3/8] vga: mask addresses in non-VESA modes to 256k


From: BALATON Zoltan
Subject: Re: [PATCH 3/8] vga: mask addresses in non-VESA modes to 256k
Date: Sun, 31 Dec 2023 17:13:42 +0100 (CET)

On Sun, 31 Dec 2023, Paolo Bonzini wrote:
This allows setting the start address to a high value, and reading the
bottom of the screen from the beginning of VRAM.  Commander Keen 4
("Goodbye, Galaxy!") relies on this behavior.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
hw/display/vga-helpers.h | 9 +++++----
hw/display/vga.c         | 3 +++
2 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/hw/display/vga-helpers.h b/hw/display/vga-helpers.h
index 10e9cfd40a0..83b9a15604a 100644
--- a/hw/display/vga-helpers.h
+++ b/hw/display/vga-helpers.h
@@ -108,7 +108,7 @@ static void vga_draw_line2(VGACommonState *vga, uint8_t *d,
    plane_mask = mask16[vga->ar[VGA_ATC_PLANE_ENABLE] & 0xf];
    width >>= 3;
    for(x = 0; x < width; x++) {
-        data = vga_read_dword_le(vga, addr);
+        data = vga_read_dword_le(vga, addr & (VGA_VRAM_SIZE - 1));
        data &= plane_mask;
        v = expand2[GET_PLANE(data, 0)];
        v |= expand2[GET_PLANE(data, 2)] << 2;
@@ -144,7 +144,7 @@ static void vga_draw_line2d2(VGACommonState *vga, uint8_t 
*d,
    plane_mask = mask16[vga->ar[VGA_ATC_PLANE_ENABLE] & 0xf];
    width >>= 3;
    for(x = 0; x < width; x++) {
-        data = vga_read_dword_le(vga, addr);
+        data = vga_read_dword_le(vga, addr & (VGA_VRAM_SIZE - 1));
        data &= plane_mask;
        v = expand2[GET_PLANE(data, 0)];
        v |= expand2[GET_PLANE(data, 2)] << 2;
@@ -177,7 +177,7 @@ static void vga_draw_line4(VGACommonState *vga, uint8_t *d,
    plane_mask = mask16[vga->ar[VGA_ATC_PLANE_ENABLE] & 0xf];
    width >>= 3;
    for(x = 0; x < width; x++) {
-        data = vga_read_dword_le(vga, addr);
+        data = vga_read_dword_le(vga, addr & (VGA_VRAM_SIZE - 1));
        data &= plane_mask;
        v = expand4[GET_PLANE(data, 0)];
        v |= expand4[GET_PLANE(data, 1)] << 1;
@@ -209,7 +209,7 @@ static void vga_draw_line4d2(VGACommonState *vga, uint8_t 
*d,
    plane_mask = mask16[vga->ar[VGA_ATC_PLANE_ENABLE] & 0xf];
    width >>= 3;
    for(x = 0; x < width; x++) {
-        data = vga_read_dword_le(vga, addr);
+        data = vga_read_dword_le(vga, addr & (VGA_VRAM_SIZE - 1));
        data &= plane_mask;
        v = expand4[GET_PLANE(data, 0)];
        v |= expand4[GET_PLANE(data, 1)] << 1;
@@ -242,6 +242,7 @@ static void vga_draw_line8d2(VGACommonState *vga, uint8_t 
*d,
    palette = vga->last_palette;
    width >>= 3;
    for(x = 0; x < width; x++) {
+        addr &= VGA_VRAM_SIZE - 1;
        PUT_PIXEL2(d, 0, palette[vga_read_byte(vga, addr + 0)]);
        PUT_PIXEL2(d, 1, palette[vga_read_byte(vga, addr + 1)]);
        PUT_PIXEL2(d, 2, palette[vga_read_byte(vga, addr + 2)]);
diff --git a/hw/display/vga.c b/hw/display/vga.c
index 3bd357d72c2..b1660bdde67 100644
--- a/hw/display/vga.c
+++ b/hw/display/vga.c
@@ -47,6 +47,9 @@ bool have_vga = true;
/* 16 state changes per vertical frame @60 Hz */
#define VGA_TEXT_CURSOR_PERIOD_MS       (1000 * 2 * 16 / 60)

+/* Address mask for non-VESA modes.  */
+#define VGA_VRAM_SIZE                   262144

(256 * KiB) maybe?

+
/*
 * Video Graphics Array (VGA)
 *




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