Make setup generic for both NBP and DOS startup

kernel/main.asm

@@ -2,15 +2,13 @@ main:
   mov ax, cs
   mov ds, ax
 
-  call debug_init
   call heap_init
+  call ivt_backup
+  call debug_init
 
-  mov ax, 18
+  int 0x2E
-  call malloc
 
-.endloop:
+  retf
-  hlt
-  jmp .endloop
 
 %include "kernel/heap.asm"
 %include "kernel/intr.asm"
@@ -19,4 +17,3 @@ main:
 
 align 16
 heap:
-  dw 0

nbp.asm

@@ -4,29 +4,55 @@ org 0x0000
 ; es:bx		pxeenv+ structure
 ; ss:sp		functional stack
 
-; during PXE, CS=0000 and IP=7C00
 _startup:
   xor ax, 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:
+  mov ax, cs
+  push ax ; return CS
+  mov ax, .dosret
+  add ax, 0x0100
+  push ax ; return IP
 
-  push cx ; byte counter
+  mov ax, cs
-  push si ; source offset
+  add ax, 0x0010
-  push di ; destination offset
+  push ax ; main CS
-  push es ; destination segment
+  mov ax, main
-
+  push ax ; main IP
+  retf
+.dosret:
+  int 0x20
+  int 0x18
+.mbr:
+  mov ax, 0x0050
   mov ds, ax
-  mov es, ax
+  mov [0xFFFE], ss ; save current stack configuration onto new stack
+  mov [0xFFFC], sp
+  mov ss, ax
+  mov sp, 0xFFFC
 
+  push dx
+  push bx ; possible far ptr to pxeenv+ struct
+  push es
+
+  mov es, ax ; 0x0050
+  mov di, 0x0000
+
+  xor ax, ax
+  mov ds, ax
   mov si, 0x7C00
-  mov di, 0x0500
+
   mov cx, _reloc_end
   rep movsb
 
-  pop es
+  call 0x0050:main
-  pop di
+.halt:
-  pop si
+  hlt
-  pop cx
+  jmp .halt
-
-  jmp 0x0050:main
 
 %include "kernel/main.asm"