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-rw-r--r--kernel/src/boot.asm65
1 files changed, 65 insertions, 0 deletions
diff --git a/kernel/src/boot.asm b/kernel/src/boot.asm
index a10b14c..de832f8 100644
--- a/kernel/src/boot.asm
+++ b/kernel/src/boot.asm
@@ -5,10 +5,75 @@ bits 32
start:
mov esp, stack_top
+ call check_multiboot
+ call check_cpuid
+ call check_long_mode
+
; print 'OK'
mov dword [0xb8000], 0x2f4b2f4f
hlt
+check_multiboot:
+ cmp eax, 0x36d76289
+ jne .no_multiboot
+ ret
+.no_multiboot
+ mov al, "0"
+ jmp error
+
+check_cpuid:
+ ; Check if CPUID is supported by attempting to flip the ID bit (bit 21)
+ ; in the FLAGS register. If we can flip it, CPUID is available.
+
+ ; Copy EFLAGS in to EAX via stack
+ pushfd
+ pop eax
+
+ ; Save the current flags
+ mov ecx, eax
+
+ ; Flip the ID bit
+ xor eax, 1 << 21
+
+ ; push eax to eflags
+ push eax
+ popfd
+
+ ; Copy EAX to FLAGS via the Stack
+ pushfd
+ pop eax
+
+ ; Restore FLAGS from the old version stored in ECX (i.e. flipping the
+ ; ID bit back if it was ever flipped).
+ push ecx
+ popfd
+
+ ; Check if the ID was changed
+ cmp eax, ecx
+ je .no_cpuid
+ ret
+.no_cpuid:
+ mov al, "1"
+ jmp error
+
+check_long_mode:
+ ; test if extended processor info in available
+ mov eax, 0x80000000 ; implicit argument for cpuid
+ cpuid ; get highest supported argument
+ cmp eax, 0x80000001 ; it needs to be at least 0x80000001
+ jb .no_long_mode ; if it's less, the CPU is too old for long mode
+
+ ; extended info about long mode
+ mov eax, 0x80000001 ; argument for the cpuid function
+ cpuid ; Cpu id
+ test edx, 1 << 29 ; check long mode availablity
+ jz .no_long_mode
+ ret
+
+.no_long_mode
+ mov al, "2"
+ jmp error
+
; Prints 'ERR: ' and the given error code to the screen and halts
; parameter: error code letter (ascii) in al
error: