use e820 to get available memory

boot1.s

@@ -247,8 +247,14 @@ main:
   mov ax, msg_a20_enabled
   call vga_println
   
+  call mem_detect
+  jc .mem_detect_fail
+
   hlt
 
+.mem_detect_fail:
+  panic PANIC_TYPE_MEM_DETECT
+
 
 ; Print a panic message then terminate.
 ; Arguments:
@@ -704,6 +710,74 @@ enable_a20_intel_8042:
   ret
 
 
+mem_detect:
+  %if MEMMAP_CAP < 1
+  %error "memmap buffer too small"
+  %endif
+
+  fnstart
+  push es
+
+  mov word [GLOBALS + MEMMAP_ENTRIES], 0
+
+  ; Memset the memmap buffer to 0.
+  xor ax, ax
+  mov es, ax
+  mov di, MEMMAP
+  mov cx, (MEMMAP_END - MEMMAP)
+  rep stosb
+  
+  ; Entry number (will be incremented for us)
+  xor ebx, ebx
+  ; Buffer (es:di)
+  mov di, MEMMAP
+  mov dword [di + 0x20], 0x1
+  ; Entry size
+  mov ecx, MEMMAP_ENT_SIZE
+  ; Magic number
+  mov edx, E820_MAGIC
+  ; Detect memory
+  mov eax, 0xe820
+  clc
+  int 0x15
+  jc .fail
+  cmp eax, E820_MAGIC
+  jne .fail
+
+  inc word [GLOBALS + MEMMAP_ENTRIES]
+
+.loop:
+  cmp word [GLOBALS + MEMMAP_ENTRIES], MEMMAP_CAP
+  jae .done
+
+  add di, MEMMAP_ENT_SIZE
+  mov dword [di + 0x20], 0x1
+  mov ecx, MEMMAP_ENT_SIZE
+  mov eax, 0xe820
+  clc
+  int 0x15
+  ; Carry flag will be set if we'd already reached the end of the entries.
+  jc .done
+
+  inc word [GLOBALS + MEMMAP_ENTRIES]
+
+  test ebx, ebx
+  jz .done
+
+  jmp .loop
+
+.fail:
+  stc
+  jmp .return
+
+.done:
+  clc
+
+.return:
+  pop es
+  fnret
+
+
 msg_boot1_loaded db "boot1 loaded. hello!", 0
 msg_a20_enabled  db "a20 enabled", 0
 msg_a20_disabled db "a20 not enabled", 0
@@ -746,15 +820,18 @@ table_reg_msgs:
   dw msg_reg_flags
   dw 0
 
-msg_panic_generic db "generic panic", 0
-msg_panic_a20     db "failed to enable a20 line", 0
+msg_panic_generic    db "generic panic", 0
+msg_panic_a20        db "failed to enable a20 line", 0
+msg_panic_mem_detect db "failed to detect available memory", 0
 
 panic_type_msgs:
-  PANIC_TYPE_GENERIC equ ($ - panic_type_msgs) / 2
+  PANIC_TYPE_GENERIC    equ ($ - panic_type_msgs) / 2
   dw msg_panic_generic
-  PANIC_TYPE_A20     equ ($ - panic_type_msgs) / 2
+  PANIC_TYPE_A20        equ ($ - panic_type_msgs) / 2
   dw msg_panic_a20
-  PANIC_TYPE_MAX     equ ($ - panic_type_msgs) / 2
+  PANIC_TYPE_MEM_DETECT equ ($ - panic_type_msgs) / 2
+  dw msg_panic_mem_detect
+  PANIC_TYPE_MAX        equ ($ - panic_type_msgs) / 2
   dw 0
 
 boot1_magic dd BOOT1_MAGIC