fatvbr: correctly setup segment registers

boot/fatvbr.asm

@@ -63,6 +63,7 @@ init:
   mov ax, (psp >> 4)
   mov ss, ax
   xor sp, sp
+  push sp
 
   ; Relocate from [prog] to [self]
   xor ax, ax
@@ -110,16 +111,25 @@ main:
   mov bp, 0x3332
   jc error
 
-  ; setup int 19h call at 0000, and push its address to stack
+  ; setup int 19h call if prog returns
   mov WORD [psp], 0x19CD
-  push sp
 
-  ; clear out registers and jump into target
+  ; clean registers
   xor ax, ax
   xor cx, cx
   xor dx, dx
   xor bx, bx
+
+  ; restore boot drive number
   mov dl, [fdc.drv]
+
+  ; setup segment registers
+  push ss
+  pop ds
+  push ss
+  pop es
+
+  ; jump into target
   jmp 0x07B0:0x0100
 
 ; Read sectors from disk