Kernel: Stop booting and print if PAE is not supported by the processor

We currently require PAE and not having it causes us to crash.
This turns that crash into an error message.

1 files changed, 39 insertions, 16 deletions

diff --git a/Kernel/Arch/x86/common/Boot/boot.S b/Kernel/Arch/x86/common/Boot/boot.S
index 6e5f5fc779..c5e6f81081 100644
--- a/Kernel/Arch/x86/common/Boot/boot.S
+++ b/Kernel/Arch/x86/common/Boot/boot.S
@@ -118,6 +118,16 @@ start:
 */
 print_and_halt:
 
+/*  from now on, we don't really care about booting because we are missing required CPU features such as PAE or long mode.
+    the flow from now is like so:
+    1. Copy all necessary parts to low memory section in RAM
+    2. Jump to that section
+    3. In that section we do:
+        a. exit protected mode to pure 16 bit real mode
+        b. load the "<missing feature> is not supported" String, call the BIOS print to screen service
+        c. halt
+*/
+
 .equ COPIED_STRING_LOCATION, 0x400
 .equ GDT_REAL_MODE_LOCATION, 0x45000
 .equ EXITING_PROTECTED_MODE_CODE_LOCATION, 0x10000
@@ -214,6 +224,9 @@ gdt_table_real_mode_end:
 no_long_mode_string:
     .asciz "Your computer does not support long mode (64-bit mode). Halting!"
 
+no_pae_string:
+    .asciz "Your computer does not support PAE. Halting!"
+
 kernel_image_too_big_string:
     .asciz "Error: Kernel Image too big for memory slot. Halting!"
 
@@ -307,28 +320,32 @@ real_start:
     hlt
 
 kernel_not_too_large:
-
-
-#if ARCH(X86_64)
-    /* test for long mode presence, save the most important registers from corruption */
+    /* test for PAE presence, save the most important registers from corruption */
     pushl %eax
     pushl %edx
     pushl %ebx
 
+    movl $0x1, %eax       /* PAE presence is in CPUID input 0x1 */
+    cpuid
+    testl $(1 << 6), %edx /* Test if the PAE-bit, which is bit 6, is set in the edx register. */
+    jnz pae_supported     /* If the bit is not set, there is no PAE capability. */
+
+    /* Since there is no PAE capability, halt with an error message */
+    movl $(no_pae_string - KERNEL_BASE), %esi
+    pushl %esi
+    call print_and_halt
+    /* We should not return, but just in case, halt */
+    hlt
+
+
+#if ARCH(X86_64)
+pae_supported:
     movl $0x80000001, %eax
     cpuid
     testl $(1 << 29), %edx   /* Test if the LM-bit, which is bit 29, is set in the edx register. */
     jnz long_mode_supported             /* If LM-bit is not enabled, there is no long mode. */
 
-    /*  from now on, we don't really care about booting because we don't have long mode supported.
-         the flow from now is like so:
-         1. Copy all necessary parts to low memory section in RAM
-         2. Jump to that section
-         3. In that section we do:
-            a. exit protected mode to pure 16 bit real mode
-            b. load the "Long mode is not supported" String, call the BIOS print to screen service
-            c. halt
-    */
+    /* Since there is no long mode, halt with an error message */
     movl $(no_long_mode_string - KERNEL_BASE), %esi
     pushl %esi
     call print_and_halt
@@ -336,14 +353,21 @@ kernel_not_too_large:
     hlt
 
 
-/* If long mode is supported, continue with booting the system */
+/* If both PAE and long mode is supported, continue with booting the system */
 
-.code32
 long_mode_supported:
     /* restore the pushed registers and continue with booting */
     popl %ebx
     popl %edx
     popl %eax
+#else
+/* If PAE is supported, continue with booting the system */
+
+pae_supported:
+    /* restore the pushed registers and continue with booting */
+    popl %ebx
+    popl %edx
+    popl %eax
 #endif
 
     /* We don't know where the bootloader might have put the command line.
@@ -545,4 +569,3 @@ long_mode_supported:
 loop:
     hlt
     jmp loop
-