diff --git a/Makefile b/Makefile
index 01be640..2573c13 100644
--- a/Makefile
+++ b/Makefile
@@ -13,8 +13,10 @@ ASFLAGS = -plosgff
 SRC = \
 src/crt0.s \
 src/bios.s \
+src/_mulint.s \
 src/random.c \
 src/console.c \
+src/convert.c \
 
 OBJ_1 = $(notdir $(SRC:.c=.rel))
 OBJ_2 = $(notdir $(OBJ_1:.s=.rel))
diff --git a/include/convert.h b/include/convert.h
new file mode 100644
index 0000000..949603d
--- /dev/null
+++ b/include/convert.h
@@ -0,0 +1,5 @@
+#pragma once
+
+#include "types.h"
+
+uint32_t uint16_to_bcd(uint16_t value);
diff --git a/include/types.h b/include/types.h
index 42e764e..680d0d5 100644
--- a/include/types.h
+++ b/include/types.h
@@ -1,7 +1,9 @@
 #pragma once
 
-typedef signed   char    int8_t;
-typedef short    int     int16_t;
+typedef signed char       int8_t;
+typedef signed short int  int16_t;
+typedef signed long  int  int32_t;
 
-typedef unsigned char    uint8_t;
-typedef unsigned short   uint16_t;
+typedef unsigned char       uint8_t;
+typedef unsigned short int  uint16_t;
+typedef unsigned long  int  uint32_t;
diff --git a/src/_mulint.s b/src/_mulint.s
new file mode 100644
index 0000000..145e61f
--- /dev/null
+++ b/src/_mulint.s
@@ -0,0 +1,73 @@
+;--------------------------------------------------------------------------
+;  _mulchar.s
+;
+;  Copyright (C) 2000, Michael Hope
+;
+;  This library is free software; you can redistribute it and/or modify it
+;  under the terms of the GNU General Public License as published by the
+;  Free Software Foundation; either version 2, or (at your option) any
+;  later version.
+;
+;  This library is distributed in the hope that it will be useful,
+;  but WITHOUT ANY WARRANTY; without even the implied warranty of
+;  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+;  GNU General Public License for more details.
+;
+;  You should have received a copy of the GNU General Public License 
+;  along with this library; see the file COPYING. If not, write to the
+;  Free Software Foundation, 51 Franklin Street, Fifth Floor, Boston,
+;   MA 02110-1301, USA.
+;
+;  As a special exception, if you link this library with other files,
+;  some of which are compiled with SDCC, to produce an executable,
+;  this library does not by itself cause the resulting executable to
+;  be covered by the GNU General Public License. This exception does
+;  not however invalidate any other reasons why the executable file
+;   might be covered by the GNU General Public License.
+;--------------------------------------------------------------------------
+
+.area   _CODE
+
+.globl	__mulint
+
+__mulint:
+        pop     af
+        pop     bc
+        pop     de
+        push    de
+        push    bc
+        push    af
+
+	;; 16-bit multiplication
+	;;
+	;; Entry conditions
+	;; bc = multiplicand
+	;; de = multiplier
+	;;
+	;; Exit conditions
+	;; hl = less significant word of product
+	;;
+	;; Register used: AF,BC,DE,HL
+__mul16::
+	xor	a,a
+	ld	l,a
+	or	a,b
+	ld	b,#16
+
+        ;; Optimise for the case when this side has 8 bits of data or
+        ;; less.  This is often the case with support address calls.
+        jr      NZ,2$
+        ld      b,#8
+        ld      a,c
+1$:
+        ;; Taken from z88dk, which originally borrowed from the
+        ;; Spectrum rom.
+        add     hl,hl
+2$:
+        rl      c
+        rla                     ;DLE 27/11/98
+        jr      NC,3$
+        add     hl,de
+3$:
+        djnz    1$
+        ret
diff --git a/src/convert.c b/src/convert.c
new file mode 100644
index 0000000..3b2d8b1
--- /dev/null
+++ b/src/convert.c
@@ -0,0 +1,65 @@
+#include "convert.h"
+
+uint32_t uint16_to_bcd(uint16_t value) __naked {
+	value;
+
+	__asm
+		ld iy, #2
+		add iy, sp
+
+		ld h, 1(iy)
+		ld l, 0(iy)
+
+		ld bc, #16*256+0
+		ld de, #0
+	loop:
+		add hl, hl
+
+		ld a, e
+		adc a, a
+		daa
+		ld e, a
+
+		ld a, d
+		adc a, a
+		daa
+		ld d, a
+
+		ld a, c
+		adc a, a
+		daa
+		ld c, a
+
+		djnz loop
+
+		ex de, hl
+		ld e, c
+
+		ret
+	; Bin2Bcd:
+		; LD BC, 16*256+0 ; handle 16 bits, one bit per iteration
+		; LD DE, 0
+		; cvtLoop:
+		; ADD HL, HL
+
+		; LD A, E
+		; ADC A, A
+		; DAA
+		; LD E, A
+
+		; LD A, D
+		; ADC A, A
+		; DAA
+		; LD D, A
+
+		; LD A, C
+		; ADC A, A
+		; DAA
+		; LD C, A
+
+		; DJNZ cvtLoop
+
+		; EX HL, DE
+		; RET
+	__endasm;
+}