use e820 to get available memory

defines.s

@@ -1,8 +1,14 @@
 %define BOOT0_LOADPOINT 0x7c00
 %define BOOT1_LOADPOINT 0x8200
 
-%define GLOBALS    0x7000
-%define STACK_BASE GLOBALS
+%define MEMMAP          0x6a00
+%define MEMMAP_END      BOOT0_LOADPOINT
+%define MEMMAP_ENT_SIZE 24
+%define MEMMAP_CAP      ((MEMMAP_END - MEMMAP) / MEMMAP_ENT_SIZE)
+
+%define GLOBALS     0x4000
+%define GLOBALS_END MEMMAP
+%define STACK_BASE  GLOBALS
 
 %define EBDA_START 0x080000
 
@@ -25,6 +31,9 @@
 %define INTEL_8042_CMD_CONTROLLER_OUT_PORT_READ  0xd0
 %define INTEL_8042_CMD_CONTROLLER_OUT_PORT_WRITE 0xd1
 
+; ASCII string "SMAP", which is used by e820 as a magic number argument and return value.
+%define E820_MAGIC 0x534d4150
+
 ; boot0 base stack frame variable offsets / globals
 ; (we use the same offsets once we copy the variables to the globals section)
 ; -------------------------------------------------------------------------------------------------
@@ -53,6 +62,7 @@
 ; ------------------------------------------------------------------------------------------------
 %define VGA_COL                0x1c
 %define TEXTBUF_LINE           0x1e
+%define MEMMAP_ENTRIES         0x20
 
 %macro fnstart 0
   push bp