Check for magic bytes before booting a disk's boot sector

boot.asm

@@ -1,14 +1,26 @@
 ; disk booting routines
-; these are only used during booting, they are not exposed via the jump table
 
 ; load the boot sector of disk 0 to 0x00000800 and jump to it
 ; inputs:
 ; none
 ; outputs:
-; none (doesn't return)
+; none (returns if disk 0 is not bootable)
 start_boot_process:
-    mov r0, 0x80003000 ; command to read a sector from disk 0 into memory
+    ; read sector 0 to 0x800
-    mov r1, 0x80002000 ; command to set the location of the buffer
+    mov r0, 0
+    mov r1, 0
+    mov r2, 0x00000800
+    call read_sector
+
+    ; check for the bootable magic bytes
+    cmp [0x000009FC], 0x523C334C
+    ifnz ret
+
+    ; now clean up and jump to the loaded binary
+    call boot_cleanup
+    mov rsp, SYSTEM_STACK ; reset stack pointer
+    mov r0, 0             ; booting from disk id 0
+    jmp 0x00000800
 
     ; read sector 0 to 0x800
     out r1, 0x00000800
@@ -28,4 +40,37 @@ boot_cleanup:
     mov r0, BACKGROUND_COLOR
     call fill_background
 
+    movz.8 r0, 0
+    movz.8 r1, 0
+    movz.8 r2, 0
+    movz.8 r3, 0
+    movz.8 r4, 0
+    movz.8 r5, 0
+    movz.8 r6, 0
+    movz.8 r7, 0
+    movz.8 r8, 0
+    movz.8 r9, 0
+    movz.8 r10, 0
+    movz.8 r11, 0
+    movz.8 r12, 0
+    movz.8 r13, 0
+    movz.8 r14, 0
+    movz.8 r15, 0
+    movz.8 r16, 0
+    movz.8 r17, 0
+    movz.8 r18, 0
+    movz.8 r19, 0
+    movz.8 r20, 0
+    movz.8 r21, 0
+    movz.8 r22, 0
+    movz.8 r23, 0
+    movz.8 r24, 0
+    movz.8 r25, 0
+    movz.8 r26, 0
+    movz.8 r27, 0
+    movz.8 r28, 0
+    movz.8 r29, 0
+    movz.8 r30, 0
+    movz.8 r31, 0
+
     ret