use e820 to get available memory

boot0.s

@@ -3,8 +3,9 @@
 ; --------------------------------------------------------------------
 ; R | 0x000000 - 0x000400: real-mode interrupt vector table
 ; R | 0x000400 - 0x000500: bios data area
-; U | 0x000500 - 0x007000: stack
-; U | 0x007000 - 0x007c00: globals
+; U | 0x000500 - 0x004000: stack
+; U | 0x004000 - 0x006a00: globals
+; U | 0x006a00 - 0x007c00: memory map
 ; U | 0x007c00 - 0x007e00: boot sector
 ; U | 0x007e00 - 0x080000: conventional usable memory
 ; R | 0x080000 - 0x0a0000: extended bios data area (maximum possible size)
@@ -29,7 +30,7 @@ main:
   mov ds, ax
   mov es, ax
 
-  ; Put the stack base at 0x7000.
+  ; Put the stack base at 0x4000.
   ; Stack grows high->low, so we'll grow away from our globals and program text.
   mov ss, ax
   mov bp, STACK_BASE