Update rt0 code to check for multiboot support and call kernel.Kmain

We still keep the required main func in stub.go to prevent the compiler
from optimizing the code out. We also force the compiler not to inline
the call to kernel.Kmain so we can find the symbol in the generated .o
file.

arch/x86/asm/rt0.s

@@ -24,17 +24,29 @@ section .text
 bits 32
 align 4
 
+MULTIBOOT_MAGIC equ 0x36d76289
+
+err_unsupported_bootloader db '[rt0] kernel not loaded by multiboot-compliant bootloader', 0
+err_kmain_returned db '[rt0] kMain returned; halting system', 0
+
 ;------------------------------------------------------------------------------
 ; Kernel arch-specific entry point
 ;
 ; The boot loader will jump to this symbol after setting up the CPU according
 ; to the multiboot standard. At this point:
+; - A20 is enabled
 ; - The CPU is using 32-bit protected mode
 ; - Interrupts are disabled
 ; - Paging is disabled
+; - EAX contains the magic value ‘0x36d76289’; the presence of this value indicates
+;   to the operating system that it was loaded by a Multiboot-compliant boot loader
+; - EBX contains the 32-bit physical address of the Multiboot information structure
 ;------------------------------------------------------------------------------
 global _rt0_entry
 _rt0_entry:
+	cmp eax, MULTIBOOT_MAGIC
+	jne unsupported_bootloader
+
 	; Initalize our stack by pointing ESP to the BSS-allocated stack. In x86,
 	; stack grows downwards so we need to point ESP to stack_top
 	mov esp, stack_top
@@ -48,19 +60,60 @@ _rt0_entry:
 	mov dword [g0_stack_lo], stack_bottom
 	mov dword [g0_stackguard0], stack_bottom
 
-	extern main.main
-	call main.main
+	; push multiboot info ptr to the stack and call the kernel entrypoint
+	push ebx
+	extern kernel.Kmain
+	call kernel.Kmain
+
+	; kmain should never return
+	mov edi, err_kmain_returned
+	call write_string
 
 	; Main should never return; halt the CPU
+halt:
 	cli
 	hlt
+
+unsupported_bootloader:
+	mov edi, err_unsupported_bootloader
+	call write_string
+	jmp halt
 .end:
 
+;------------------------------------------------------------------------------
+; Write the NULL-terminated string contained in edi to the screen using white
+; text on red background.  Assumes that text-mode is enabled and that its
+; physical address is 0xb8000.
+;------------------------------------------------------------------------------
+write_string:
+	push eax
+	push ebx
+
+	mov ebx,0xb8000
+	mov ah, 0x4F
+next_char:
+	mov al, byte[edi]
+	test al, al
+	jz done
+
+	mov word [ebx], ax
+	add ebx, 2
+	inc edi
+	jmp next_char
+
+done:
+	pop ebx
+	pop eax
+	ret
+
 ;------------------------------------------------------------------------------
 ; Load GDT and flush CPU caches
 ;------------------------------------------------------------------------------
 
 _rt0_load_gdt:
+	push eax
+	push ebx
+
 	; Go code uses the GS register to access the TLS. Set the base address
 	; for the GS descriptor to point to our tls0 table
 	mov eax, tls0
@@ -84,6 +137,8 @@ update_descriptors:
 	mov ax, GS_SEG
 	mov gs, ax
 
+	pop ebx
+	pop eax
 	ret
 
 ;------------------------------------------------------------------------------