rom: Enter protected mode exactly how the SDM says

Also gets rid of any issues with the reset vector code overflowing.

Signed-off-by: Joe Richey <joerichey@google.com>

Author: josephlr

src/asm/rom.s

@@ -64,28 +64,30 @@ rom32_start:
     jmpl *%eax
 
 .code16
-rom16_protected:
-    # We are now in 16-bit protected mode, To enter 32-bit protected mode, we
+rom16:
+    # Order of instructions from Intel SDM 9.9.1 "Switching to Protected Mode"
+    # Step 1: Disable interrupts
+    cli
+
+    # Step 2: Load the GDT
+    # We are currently in 16-bit real mode. To enter 32-bit protected mode, we
     # need to load 32-bit code/data segments into our GDT. The gdt32 in ROM is
     # at too high of an address (4 GiB - offset) for the data segment to reach.
     # So, we load gdt32 via the 16-bit code segement, using a 16-bit address.
     movw  $gdt32_ptr_addr16, %bx
     lgdtl %cs:(%bx)
 
-    # Set CS to a 32-bit segment and jump to 32-bit code.
-    ljmpl $(code32_desc - gdt32_start), $rom32_addr32
-
-.align 16
-reset_vector: # 0xffff_fff0
-    # This code must be 16 bytes or less, so be careful when adding anyting.
-    cli
-
-    # Set CRO.PE (Protected Mode Enable)
+    # Step 3: Set CRO.PE (Protected Mode Enable)
     movl %cr0, %eax
     orb  $0b00000001, %al # Set bit 0
     movl %eax, %cr0
 
-    jmp rom16_protected
+    # Step 4: Far JMP to change execution flow and serializes the processor.
+    # Set CS to a 32-bit segment and jump to 32-bit code.
+    ljmpl $(code32_desc - gdt32_start), $rom32_addr32
 
 .align 16
+reset_vector: # 0xffff_fff0
+    jmp rom16
+.align 16
 rom_end: # 0x1_0000_0000