Implement MCB splitting

Makefile

@@ -48,6 +48,8 @@ vga11.com: cp437.bin
 %.bs: boot/%.asm
 	$(NASM) $(NASM_ARGS) -o $@ $<
 
+kernel.bs: kernel/*.asm
+
 # Special case: variations of FAT vbr
 fat1.bs: boot/fat.asm
 	$(NASM) $(NASM_ARGS) -DFAT12 -DCHS -o $@ $<

kernel/mcb.asm

@@ -17,8 +17,36 @@ mcb_init:
 	mov word [si+1], 0
 	mov word [si+3], ax
 	mov dx, cs
+	call mcb_split
+	mov word [si+1], 8
+	ret
+
+	; split mcb block into two
+	; IN	dx	segment of second block
+mcb_split:
 	call mcb_find
-	int 3
+	dec dx
+	; Calculate size of first block
+	; dx = dx - ds - 1
+	stc
+	mov ax, ds
+	sbb dx, ax
+	;
+	push dx
+	; swap old size with new size
+	xchg dx, [si+3]
+	; if current mcb is last, its not anymore
+	mov cl, 'M'
+	xchg cl, byte [si+0]
+	; advance DS to begin of first block
+	pop ax
+	mov ds, ax
+	stc
+	sbb dx, ax
+	; write MCB data
+	mov byte [si+0], cl
+	mov word [si+1], 0
+	mov word [si+3], dx
 	ret
 
 	; IN	dx	segment we look for