update memory layout table

boot0.s

@@ -1,10 +1,20 @@
 ; MEMORY LAYOUT
-; 0x0000 - 0x7000: stack
-; 0x7000 - 0x7c00: globals
-;   0x7000 - 0x7001: boot drive number
-;   0x7001 - 0x7002: number of hard disk drives
-;   0x7002 - 0x7003: 
-; 0x7c00 - 0x7e00: boot sector
+; R = reserved, U = usable
+; --------------------------------------------------------------------
+; R | 0x000000 - 0x000400: real-mode interrupt vector table
+; R | 0x000400 - 0x000500: bios data area
+; U | 0x000500 - 0x007000: stack
+; U | 0x007000 - 0x007c00: globals
+;   |   0x007000 - 0x007001: boot drive number
+;   |   0x007001 - 0x007002: number of hard disk drives
+;   |   0x007002 - 0x007003: 
+; U | 0x007c00 - 0x007e00: boot sector
+; U | 0x007e00 - 0x080000: conventional usable memory
+; R | 0x080000 - 0x0a0000: extended bios data area (maximum possible size)
+; R | 0x0a0000 - 0x0c0000: video memory
+; R | 0x0c0000 - 0x0c8000: video bios
+; R | 0x0c8000 - 0x0f0000: bios expansiosn
+; R | 0x0f0000 - 0x100000: motherboard bios
 
 ; BIOS puts our boot sector at 0000:7c00
 org 0x7c00