Add VESA initiation code to Stage 1 bootloader (Issue #37)

Author: jabedude

src/stage_1.s

@@ -7,6 +7,32 @@
 # the bootloader from disk, and jumps to stage_2.
 
 _start:
+    # Try VESA
+    mov ax, 0x9000
+    mov es, ax
+    mov di, 0
+    mov ax, 0x4f00
+    int 0x10
+    cmp ax, 0x004f
+    jne zero
+    # Check VESA version 2 or 3
+    mov ax, es:[di+4]
+    cmp ax, 0x0300
+    je init_vesa
+    cmp ax, 0x0200
+    jne zero
+    # Hard coded 800x600x16bit mode; this gets us the PHY-addr of the FB
+init_vesa:
+    mov ax, 0x4f01
+    mov cx, 0x114
+    mov di, VESAInfo
+    int 0x10
+
+    mov ax, 0x4f02
+    mov bx, 0x4114
+    int 0x10
+
+zero:
     # zero segment registers
     xor ax, ax
     mov ds, ax
@@ -35,7 +61,6 @@ enable_a20:
     out 0x92, al
 enable_a20_after:
 
-
 enter_protected_mode:
     # clear interrupts
     cli
@@ -231,3 +256,6 @@ dap_start_lba:
 
 .org 510
 .word 0xaa55 # magic number for bootable disk
+
+VESAInfo:
+    .space  256, 0