Enable A20 at bootloader stage 2

Author: scopeInfinity

Makefile

@@ -101,7 +101,7 @@ binaries: $(bt_stage1) $(bt_stage2) $(kernel_core) $(rm_static)
 # Note: Relying on default value doesn't guarantee bin size.
 SECTOR_COUNT_BT_STAGE1 = 1
 SECTOR_COUNT_SHARED_LIBRARY = 1
-SECTOR_COUNT_BT_STAGE2 = 11
+SECTOR_COUNT_BT_STAGE2 = 12
 SECTOR_COUNT_KERNEL = 47
 
 SECTOR_START_BT_STAGE1 = 0

src/bootloader/stage2.asm

@@ -8,6 +8,7 @@ extern entry_stage
 extern gdtr
 global enter_protected_mode
 global label_exit
+global call_int_0x15
 
 [SECTION .text]
         mov eax, 0x0
@@ -43,6 +44,16 @@ global label_exit
         HLT
         JMP label_exit
 
+    call_int_0x15:
+        push ebp
+        mov ebp, esp
+
+        mov ax, [ebp+0x08]
+        int 0x15
+
+        pop ebp
+        ret
+
 
 [SECTION .data]
     bl_stage_2             db      "Bootloader: Stage 2"

src/bootloader/stage2.c

@@ -16,6 +16,30 @@ char DIGIT_TO_HEX[] = "0123456789ABCDEF";
 
 extern void enter_protected_mode();
 extern void label_exit();
+extern unsigned short call_int_0x15(unsigned short ax);
+
+void enable_a20() {
+    int ax;
+    ax = call_int_0x15(0x2403);
+    if (!ax) {
+        print_log("BIOS A20-gate not supported");
+        label_exit();
+    }
+
+    ax = call_int_0x15(0x2402);
+    if (ax==1) {
+        print_log("BIOS A20-gate already enabled");
+        return;
+    }
+
+    ax = call_int_0x15(0x2401);
+    if (!ax) {
+        print_log("BIOS A20-gate enabling attempt failed");
+        label_exit();
+    }
+
+    print_log("BIOS A20-gate enabled");
+}
 
 char *get_memdump_8byte(void *address) {
     static char shared_memdump[17];
@@ -62,6 +86,7 @@ void entry_stage() {
     load_static_library();
     load_kernel();
 
+    enable_a20();
     populate_gdt_table(&gdtr, gdt_table, GDT_TABLE_SIZE, 0);
 
     // Enter_protected_mode never returns.