diff --git a/2019/day1/part1_part2.s b/2019/day1/part1_part2.s
index 8a4970c..3019934 100644
--- a/2019/day1/part1_part2.s
+++ b/2019/day1/part1_part2.s
@@ -50,7 +50,7 @@ end:	.long _binary_2019_day1_input_txt_end
 	.global _parse_int
 	.align 4
 _parse_int:
-	/* r0 = pointer to text ; r1 = number */
+	/* r8 = pointer to text ; r1 = number */
 	mov	#0,r1	/* r1: accumulated number */
 parse_int_loop:
 	mov.b 	@r8+,r2
diff --git a/2019/deps.mk b/2019/deps.mk
index db6295e..e4c7086 100644
--- a/2019/deps.mk
+++ b/2019/deps.mk
@@ -1,2 +1,2 @@
-2019/day1/part1.elf: 2019/day1/part1_part2.o 2019/day1/sample.txt.o 2019/day1/input.txt.o
+2019/day1/part1_part2.elf: 2019/day1/part1_part2.o 2019/day1/sample.txt.o 2019/day1/input.txt.o
 	$(DEFAULT_LINK)
diff --git a/2020/day2/input.txt b/2020/day2/input.txt
new file mode 100644
index 0000000..95e3e8b
--- /dev/null
+++ b/2020/day2/input.txt
@@ -0,0 +1,1000 @@
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diff --git a/2020/day2/sample.txt b/2020/day2/sample.txt
new file mode 100644
index 0000000..fe19c03
--- /dev/null
+++ b/2020/day2/sample.txt
@@ -0,0 +1,3 @@
+1-3 a: abcde
+1-3 b: cdefg
+2-9 c: ccccccccc
diff --git a/2020/day2/solution.s b/2020/day2/solution.s
new file mode 100644
index 0000000..071a122
--- /dev/null
+++ b/2020/day2/solution.s
@@ -0,0 +1,134 @@
+	.global _start
+_start:
+	mova	stack_top,r0
+	mov	r0,r15
+
+	/* part1 */
+	mova	_part1,r0
+	mov 	r0,r14
+	mova	solve,r0
+	jsr	@r0
+	nop
+	/* move part1 answer to GBR */
+	ldc	r13,gbr
+
+	/* part2 */
+	mova	_part2,r0
+	mov 	r0,r14
+	mova	solve,r0
+	jsr	@r0
+	nop
+	/* move part2 answer to VBR */
+	ldc	r13,vbr
+
+	trapa	#0
+
+	/* return number of valid passwords in r13 */
+	.balign 4
+solve:
+	sts.l	pr,@-r15
+
+	mov.l	start,r8
+	mov.l	end,r9
+
+	/* password is from r1 to r8, excluding r8; next line is at r8 */
+	mov	#0,r13
+line_loop:
+	mova	_parse_line,r0
+	jsr	@r0
+	nop
+	jsr	@r14
+	nop
+	cmp/eq	r8,r9
+	bf/s	line_loop
+	add	r1,r13
+
+	lds.l	@r15+,pr
+	rts
+	nop
+
+	/* return: valid as `1` in r1 */
+	/* r10: test position 1 */
+	/* r11: test position 2 */
+	/* r12: test character */
+	.balign 4
+_part2:
+	add	#-1,r1		/* offset for 1-based indexing */
+	mov	r10,r0
+	mov.b 	@(r0,r1),r3	/* r3: character */
+	cmp/eq	r3,r12
+	movt	r2
+	mov	r11,r0
+	mov.b 	@(r0,r1),r3	/* r3: character */
+	cmp/eq	r3,r12
+	movt	r1
+	rts
+	xor	r2,r1
+
+	/* return: valid as `1` in r1 */
+	/* r10: count >= lower bound */
+	/* r11: count <= upper bound */
+	/* r12: test character */
+	.balign 4
+_part1:
+	mov	#0,r2 		/* r2: count */
+part1_loop:
+	cmp/eq	r1,r8
+	bt	part1_break
+	mov.b	@r1+,r3		/* r3: character */
+	cmp/eq	r3,r12
+	movt	r0
+	bra	part1_loop
+	add	r0,r2
+part1_break:
+	/* lower bound */
+	cmp/hs	r10,r2		/* Rn ≥ Rm (unsigned) ; r2 >= r10 */
+	movt	r0
+	cmp/hs	r2,r11		/* Rn ≥ Rm (unsigned) ; r11 >= r2 */
+	movt	r1
+	rts
+	and	r0,r1
+
+	.balign 4
+_parse_line:
+	/* save PR */
+	sts.l	pr,@-r15
+
+	mov.l	parse_base10_ptr,r0
+	jsr	@r0
+	nop
+	mov	r1,r10 		/* r10: first number */
+	mov.l	parse_base10_ptr,r0
+	jsr	@r0
+	nop
+	mov	r1,r11 		/* r11: second number */
+
+	mov.b	@r8+,r12 	/* r12: character */
+	/* skip colon character ; skip space character */
+	add	#2,r8
+	/* advance until newline */
+	mov	r8,r1
+newline_loop:
+	mov.b	@r8+,r0
+	cmp/eq	#0xa,r0
+	bf	newline_loop
+
+	/* restore PR */
+	lds.l	@r15+,pr
+	rts
+	nop
+
+	.balign 4
+parse_base10_ptr: /* r8 = pointer to text ; r1 = returned number */
+	.long	_parse_base10
+
+	.balign 4
+start:	.long _binary_2020_day2_input_txt_start
+end:	.long _binary_2020_day2_input_txt_end
+
+	.balign 4
+stack_bot:
+	.rept 16
+	.long 0
+	.endr
+stack_top:
diff --git a/2020/deps.mk b/2020/deps.mk
new file mode 100644
index 0000000..c21c53f
--- /dev/null
+++ b/2020/deps.mk
@@ -0,0 +1,11 @@
+2020_DAY2_OBJ = \
+	2020/day2/solution.o \
+	2020/day2/sample.txt.o \
+	2020/day2/input.txt.o \
+	common/parse_base10.o
+
+2020/day2/solution.elf:
+2020/day2/solution.elf: $(2020_DAY2_OBJ)
+	$(DEFAULT_LINK)
+
+ALL_ELF += 2020/day2/solution.elf
diff --git a/Makefile b/Makefile
index 7586fd6..21fddf4 100644
--- a/Makefile
+++ b/Makefile
@@ -28,6 +28,9 @@ CFLAGS += -ffreestanding -nostdlib
 DEPFLAGS = -MMD -MP
 LDFLAGS += --no-warn-execstack
 
+ALL_ELF =
+all:
+
 %.o: %.c
 	$(CC) $(CARCH) $(CC_ENDIAN) $(CFLAGS) $(OPT) $(DEBUG) $(DEPFLAGS) -MF ${<}.d -c $< -o $@
 
@@ -75,3 +78,6 @@ clean:
 MAKEFLAGS += --no-builtin-rules
 
 include 2019/deps.mk
+include 2020/deps.mk
+
+all: $(subst .elf,.bin,$(ALL_ELF))
diff --git a/common/parse_base10.s b/common/parse_base10.s
new file mode 100644
index 0000000..7478906
--- /dev/null
+++ b/common/parse_base10.s
@@ -0,0 +1,22 @@
+	.section .text.parse_base10
+
+	.global _parse_base10
+_parse_base10:
+	/* r8 = pointer to text ; r1 = number */
+	mov	#0,r1	/* r1: accumulated number */
+parse_base10_loop:
+	mov.b 	@r8+,r2
+	add	#-48,r2 /* ascii '0' */
+	cmp/pz	r2	/* Rn >= 0 (signed)  (r2 >= 0 ) */
+	bf/s	parse_base10_rts
+	mov	#9,r4	/* const 9 */
+	cmp/ge	r2,r4	/* Rn >= Rm (signed) ( 9 >= r2) */
+	bf/s	parse_base10_rts
+	mov	#10,r4	/* const 10 */
+	mulu.w	r1,r4
+	sts	macl,r1
+	bra	parse_base10_loop
+	add	r2,r1
+parse_base10_rts:
+	rts
+	nop