load stage 2 partition start and end

boot0.s

@@ -203,8 +203,31 @@ bits 16
   jmp .loop_find_stage2
 
 .found_stage2:
-  ; TODO: we've found the right entry, now we need to parse it, load the sector and jump to it
   push ax ; address of the GPT entry for stage 2
+  mov si, ax
+
+  ; Load partition LBA start.
+  mov eax, [si + 0x20]
+  mov ebx, [si + 0x24]
+  ; Ensure it fits in 16 bits.
+  or ebx, ebx
+  jnz .panic
+  cmp ebx, 0xffff
+  ja .panic
+  ; Load partition LBA end.
+  mov ecx, [si + 0x28]
+  mov edx, [si + 0x2c]
+  ; Assert that the end LBA is greater than the start LBA, so we have at least one sector to load.
+  ; TODO: is the end LBA inclusive?
+  or edx, edx
+  jnz .stage2_end_lba_ok
+  cmp eax, ecx
+  jae .panic
+.stage2_end_lba_ok:
+
+  mov bx, 0x8200
+  call .read_lba
+  ; TODO: jump to the sector we just read
 
   mov al, [0x7e00]
   mov byte fs:[0x0000], al