kernel: Generalize startup code into main

kernel/main.asm

@@ -1,3 +1,53 @@
+; assumptions about the starting envionment
+cpu 8086
+org 0x0000
+
+_startup:
+  xor ax, ax ; trash ax
+  mov ax, cs
+  cmp ax, 0x0000 ; MBR or NBP, CS=0000 IP=7C00
+  je .mbr
+  cmp ax, 0x07C0 ;  MBR on weird BIOS, CS=07C0 IP=0000
+  je .mbr
+.dos:
+  ; ROP because call only works on immediate or memory
+  mov ax, cs
+  push ax ; return CS
+  mov ax, .dosret
+  add ax, 0x0100
+  push ax ; return IP
+  mov ax, cs
+  add ax, 0x0010
+  push ax ; main CS
+  mov ax, main
+  push ax ; main IP
+  retf
+.dosret:
+  int 0x20
+  jmp .halt
+.mbr:
+  push si ; index of partition table entry
+  push ds ; segment for ^
+  push dx ; dl might contain drive number
+  push bx ; possible offset for pxeenv+ struct
+  push es ; segment for ^
+
+  mov ax, 0x0050
+  mov es, ax
+  mov di, 0x0000
+
+  xor ax, ax
+  mov ds, ax
+  mov si, 0x7C00
+
+  mov cx, _reloc_end
+  rep movsb
+
+  call 0x0050:main
+.halt:
+  hlt
+  jmp .halt
+
 main:
   mov ax, cs
   mov ds, ax
@@ -10,10 +60,12 @@ main:
 
   retf
 
-%include "kernel/heap.asm"
-%include "kernel/intr.asm"
-%include "kernel/debug.asm"
-%include "kernel/kprintf.asm"
+%include "heap.asm"
+%include "intr.asm"
+%include "debug.asm"
+%include "kprintf.asm"
+
+_reloc_end:
 
 align 16
 heap: