From cb6d24b838632e1269a5e4bb4e83425061f5d853 Mon Sep 17 00:00:00 2001
From: "Juan J. Martinez" <jjm@usebox.net>
Date: Thu, 8 Jun 2023 22:11:44 +0100
Subject: Detect the VGA card before setting mode 0x13

---
 src/main.c |  6 +++++-
 src/vga.c  | 12 +++++++++++-
 src/vga.h  |  2 +-
 3 files changed, 17 insertions(+), 3 deletions(-)

(limited to 'src')

diff --git a/src/main.c b/src/main.c
index c0e7dc3..672a478 100644
--- a/src/main.c
+++ b/src/main.c
@@ -17,7 +17,11 @@ int main(int argc, char *argv[])
     atexit(keyb_free);
 
     /* set VGA 320x200, 256 col */
-    set_mode(0x13);
+    if (!set_mode(0x13))
+    {
+        fprintf(stderr, "ERROR: failed to init the VGA card\n");
+        return 1;
+    }
 
     set_palette(binary_palette_start);
 
diff --git a/src/vga.c b/src/vga.c
index e90ebba..6b093c0 100644
--- a/src/vga.c
+++ b/src/vga.c
@@ -25,9 +25,17 @@ void close_framebuffer()
     __djgpp_nearptr_disable();
 }
 
-void set_mode(uint8_t mode)
+uint8_t set_mode(uint8_t mode)
 {
     __dpmi_regs regs = { 0 };
+
+    /* detect VGA card */
+    regs.x.ax = 0x1a00;
+    __dpmi_int(0x10, &regs);
+    if (regs.h.al != 0x1a)
+        return 0;
+
+    memset(&regs, 0, sizeof(regs));
     regs.x.ax = mode;
     __dpmi_int(0x10, &regs);
 
@@ -57,6 +65,8 @@ void set_mode(uint8_t mode)
         outportw(0x3d4, 0x8e11); /* restore retrace */
         enable();
     }
+
+    return 1;
 }
 
 void wait_vsync()
diff --git a/src/vga.h b/src/vga.h
index a185974..8dae83d 100644
--- a/src/vga.h
+++ b/src/vga.h
@@ -14,7 +14,7 @@ typedef struct {
 uint8_t open_framebuffer();
 void close_framebuffer();
 
-void set_mode(uint8_t mode);
+uint8_t set_mode(uint8_t mode);
 void wait_vsync();
 
 /* the palette is expected to be 8 bit per color, and will be converted to VGA's 6 bit per color */
-- 
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