Create basic MCB at boot

boot/kernel.asm

@@ -3,6 +3,7 @@
 	jmp 0x07C0:init
 
 	%include "inc/bpb.asm"
+	%include "kernel/mcb.asm"
 
 banner:	db "rdos", 0xA, 0xD, 0
 
@@ -39,8 +40,6 @@ entry:	push bp
 	mov ss, ax
 	xor sp, sp
 
-	int 3
-
 	; Jump back to userspace
 ujump:	cli
 	mov ax, es
@@ -233,9 +232,8 @@ init:	cli
 
 	call dnconv
 	call select
-	int 3
-	mov ax, [cs:bpb+bpb_ss]
-	int 3
+
+	call mcb_init
 
 main:	push cs
 	pop ds

kernel/mcb.asm

@@ -0,0 +1,40 @@
+	; First MCB block is always at 00540
+mcb_first:
+	mov ax, 0x54
+	mov ds, ax
+	ret
+
+mcb_init:
+	call mcb_first
+	mov dx, ds
+	inc dx
+	xor si, si
+	int 0x12
+	mov cl, 6
+	shl ax, cl
+	sub ax, dx
+	mov byte [si+0], 'Z'
+	mov word [si+1], 0
+	mov word [si+3], ax
+	mov dx, cs
+	call mcb_find
+	int 3
+	ret
+
+	; IN	dx	segment we look for
+	; OUT	ds	segment where mcb starts
+mcb_find:
+	call mcb_first
+	xor si, si
+.loop:	stc
+	adc ax, [si+3]
+	cmp dx, ax
+	jc .found
+	cmp byte [si+0], 'Z'
+	jz .fail
+	mov ds, ax
+	jmp .loop
+.fail:	stc
+	ret
+.found:	clc
+	ret