get drive geometry

boot0.s

@@ -1,9 +1,17 @@
+; MEMORY LAYOUT
+; 0x0000 - 0x7000: stack
+; 0x7000 - 0x7c00: globals
+;   0x7000 - 0x7001: boot drive number
+;   0x7001 - 0x7002: number of hard disk drives
+;   0x7002 - 0x7003: 
+; 0x7c00 - 0x7e00: boot sector
+
 ; BIOS puts our boot sector at 0000:7c00
 org 0x7c00  
 ; We're (probably) in real mode
 bits 16 
 
-  ; Disable interrupts, fuck knows what an interrupt would do right now
+  ; Disable interrupts
   cli
 
   xor ax, ax
@@ -11,41 +19,45 @@ bits 16
   mov ds, ax
   mov es, ax
 
-  ; Put the stack base at the address of our program text (stack will grow high->low away from it).
-  ; We're not doing anything with a stack right now but who knows we might want to later
+  ; Put stack base at 0x7000. Stack grows high->low, so we'll grow away from our program text at
+  ; 0x7c00. We reserve the range [0x7000,0x7c00) for globals.
   mov ss, ax
-  mov sp, 0x7c00
+  mov sp, 0x7000
 
   ; Segment for VGA (0xb800 * 16 = 0xb8000)
   mov ax, 0xb800
   mov fs, ax
 
+  mov di, 0x7000
+
+  ; Store boot drive number
+  mov ds:[di], dl
+
+  ; Get drive geometry
+  mov di, 0x00
+  mov ah, 0x08
+  int 0x13
+  jb panic
+  mov ds:[di + 1], dl
+
   ; Set VGA mode
   ; https://mendelson.org/wpdos/videomodes.txt
-  mov ah, 0x00
-  mov al, 0x03
+  mov ax, 0x0003
   int 0x10
 
-  ; Reveal the true nature of jen
-  xor bx, bx
-  mov ax, 17
-loop:
-  cmp ax, bx
-  jle done
-  lea si, [womble + bx]
-  mov dh, [si]
-  mov di, bx
-  shl di, 1
-  mov byte fs:[di], dh
-  mov byte fs:[di + 1], 0x1f
-  inc bx
-  jmp loop
-
-done:
+  mov word fs:[0x0000], 0xc048
+  mov word fs:[0x0002], 0xc069
+  mov word fs:[0x0004], 0xc021
   hlt
 
-womble:
-  db "jen is a womble!"
+panic:
+  mov ax, 0x0003
+  int 0x10
+  mov word fs:[0x0000], 0x4f46
+  mov word fs:[0x0002], 0x4f41
+  mov word fs:[0x0004], 0x4f49
+  mov word fs:[0x0006], 0x4f4c
+  hlt
 
   ; TODO: enable A20