Start detect memory

README.md

@@ -25,6 +25,7 @@ Minimal operating systems to learn low level programming.
             1.  [keyboard loop](bios_keyboard_loop.S)
         1.  [disk load](bios_disk_load.S)
             1.  [disk load 2](bios_disk_load2.S)
+        1.  [detect memory](bios_detect_memory.S)
     1.  [Initial state](initial_state.S)
     1.  [reboot](reboot.S)
     1.  [Not testable in userland](not-testable-in-userland.md)

bios_detect_memory.S

@@ -0,0 +1,29 @@
+/*
+# Detecte memory
+
+# int 15
+
+http://wiki.osdev.org/Detecting_Memory_%28x86%29
+
+Determine how much memory you've got, and how much of it is low memory.
+
+This is important in particular so that you can start your stack there
+when you enter protected mode, since the stack grows down.
+
+In 16-bit mode, it does not matter much,
+since most modern machines have all addressable memory there.
+
+`int 15` returns a list:
+each time you call it a new memory region is returned.
+
+## Low memory
+
+TODO what is it?
+*/
+
+#include "common.h"
+BEGIN
+    mov $0xE820, %eax
+    int $0x15
+
+    hlt

common.h

@@ -285,7 +285,18 @@ end:
     pop %eax
 .endm
 
-/* Print a 32-bit register in hex. */
+/*
+Print a 32-bit register in hex.
+
+Sample usage:
+
+    mov $12345678, %eax
+    VGA_PRINT_REG <%eax>
+
+Expected output on screen:
+
+    12345678
+*/
 .macro VGA_PRINT_REG reg=<%eax>
     LOCAL loop
     push %eax

segment_registers.S

@@ -20,7 +20,7 @@ Rationale of the registers:
 
 TODO: this does seem to have special properties as used by string instructions.
 
-- used by the `int 13h` BIOS disk read service
+- BIOS calls: `int 13h` disk read, `int 15` memory detection
 
 ## FS