Improve memory position handling

boot/fat.asm

@@ -1,6 +1,5 @@
 %define segment   0x00100
 %define self      (0x7C00-(segment<<4)) ; 1 sector
-%define prog      (RAM-0x1000) ; 4K at the end for OS
 
 cpu 8086
 org self
@@ -58,7 +57,7 @@ main:
   ; Stack grows down from 64k
   mov ax, cs
   mov ss, ax
-  mov sp, prog
+  mov sp, BDOS
   mov ds, ax
   mov es, ax
 
@@ -67,7 +66,7 @@ main:
   mov ax, 1
   mov cx, 8
   mov dx, 0
-  mov bx, prog
+  mov bx, BDOS
 
 .loop:
   push ax
@@ -106,8 +105,8 @@ main:
 
   mov dl, [fdc.drv]
 
-  ; jump into kernel
-  jmp segment:(prog+0x0800)
+  ; jump into bios cold boot entry point
+  jmp segment:BIOS
 
 error:
   mov ax, bp