From 6d38a313c7dd960770af3703c554bf9795ed356b Mon Sep 17 00:00:00 2001
From: Ain <41307858+nero@users.noreply.github.com>
Date: Fri, 17 Apr 2020 23:33:42 +0200
Subject: [PATCH] Implement MCB splitting

---
 Makefile       |  2 ++
 kernel/mcb.asm | 30 +++++++++++++++++++++++++++++-
 2 files changed, 31 insertions(+), 1 deletion(-)

diff --git a/Makefile b/Makefile
index d564f76..467cf36 100644
--- a/Makefile
+++ b/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 $@ $<
diff --git a/kernel/mcb.asm b/kernel/mcb.asm
index 08a8601..3d089a8 100644
--- a/kernel/mcb.asm
+++ b/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