diff --git a/c64/day01.asm b/c64/day01.asm
index f9d33be..d5c7b46 100644
--- a/c64/day01.asm
+++ b/c64/day01.asm
@@ -1,16 +1,12 @@
 // vim: filetype=kickass
 
+*=* "Day 01"
 day01:
-    move_16_imm($03, $04, str_input)
-    jsr write_string
-
-    move_16_imm($03, $04, day01_input)
-    jsr write_string
-
-    move_16_imm($03, $04, str_converted)
+    move_16_imm($03, $04, str_title)
     jsr write_string
 
     move_16_imm($03, $04, day01_input)
+    move_16_imm($25, $26, buffer)
     ldy #0
 
 !line:
@@ -26,45 +22,132 @@ day01:
     i16_mul10($22, $23)
 
     lda ($03), y
-
     sec
     sbc #'0'
 
     i16_i8_add_a($22, $23)
 
     iny
+    bne !digit-
+
+    inc $04
     jmp !digit-
 
 !break:
     sty $24
 
-    lda $23
-    jsr print_hex_0x
+    ldy #0
     lda $22
-    jsr print_hex
+    sta ($25), y
+    iny
+    lda $23
+    sta ($25), y
 
-    jsr print_newline
-    jsr print_carriage_return
+    lda #2
+    i16_i8_add_a($25, $26)
 
     ldy $24
     iny
-
+    bne !+
+    inc $04
+!:
     lda ($03), y
     bne !line-
 
+    move_16_imm($25, $26, buffer)
+
+    move_16_imm($03, $04, str_parse_done)
+    jsr write_string
+
+    // Add every number to every other number
+!outer:
+    move_16_imm($27, $28, buffer)
+
+    // $03..$04 = buffer[i]
+    ldy #0
+    lda ($25), y
+    sta $03
+    iny
+    lda ($25), y
+    sta $04
+
+    dey
+    lda ($25), y
+    bne !not_zero+
+    iny
+    lda ($25), y
+    bne !not_zero+
+    .break
+    jmp !error+
+!not_zero:
+
+    // Subtract from 2020 to see what number we need
+    i16_imm_i16_sub($fb, $fc, 2020, $03, $04)
+
+!inner:
+    // $fd..$fe = buffer[j]
+    ldy #0
+    lda ($27), y
+    sta $fd
+    iny
+    lda ($27), y
+    sta $fe
+
+    dey
+    lda ($27), y
+    bne !not_zero+
+    iny
+    lda ($27), y
+    bne !not_zero+
+
+    i16_inc($25, $26)
+    jmp !outer-
+!not_zero:
+
+    // j++
+    lda #2
+    i16_i8_add_a($27, $28)
+    i16_i16_cmp_bne($fb, $fc, $fd, $fe, !inner-)
+
 !done:
+    // Multiply the results together
+    sec
+    jsr multiply_16bit_unsigned
+
+    move_16_imm($03, $04, str_calc_done)
+    jsr write_string
+
+    lda $25
+    sta udivmod32_dividend + 3
+    lda $24
+    sta udivmod32_dividend + 2
+    lda $23
+    sta udivmod32_dividend + 1
+    lda $22
+    sta udivmod32_dividend + 0
+    jsr print_decimal_u32
+
+    rts
+
+!error:
+    move_16_imm($03, $04, str_no_result)
+    jsr write_string
     rts
 
 
-day01_input:
-    .import text "../rust/inputs/day01_example"
-    .byte 0
-
-str_input:
-    .text "# Input"
+str_title:
+    .text "# Day 01, part 1:"
     .byte '\n', 0
 
-str_converted:
+str_parse_done:
+    .text "* Finished parsing"
+    .byte '\n', 0
+
+str_calc_done:
+    .text "* Finished calculating"
+    .byte '\n', 0
+
+str_no_result:
     .byte '\n'
-    .text "# Converted"
+    .text "! Could not find a result"
     .byte '\n', 0
diff --git a/c64/main.asm b/c64/main.asm
index 6873104..aca4b8c 100644
--- a/c64/main.asm
+++ b/c64/main.asm
@@ -4,6 +4,7 @@
 
     BasicUpstart2(main)
 
+* = $8000
     .align $100
 * = * "Square table"
 square1_lo:
@@ -15,16 +16,17 @@ square2_lo:
 square2_hi:
     .fill 512, 0
 
-* = * "Day 1 input"
+* = * "Day 01 Input"
 day01_input:
     .import text "../rust/inputs/day01"
     .byte 0
+* = * "Day 01 Input End"
 
 //* = $8000 "Cartridge ROM"
 
 //* = $a000 "Basic ROM"
 
-* = $c000
+* = $c000 "Main program"
 .const SCREEN_RAM = $0400
 .const COLOR_RAM = $d800
 
@@ -56,17 +58,8 @@ main:
 
     jsr generate_multiplication_tables
 
-    // Test division by 10
-    lda #$cc
-    sta udivmod32_dividend + 0
-    lda #$28
-    sta udivmod32_dividend + 1
-    lda #$f9
-    sta udivmod32_dividend + 2
-    lda #$07
-    sta udivmod32_dividend + 3
+    jsr day01
 
-    jsr print_decimal_u32
 
 !loop:
     jmp !loop-
diff --git a/c64/math.asm b/c64/math.asm
index e4b6be8..bf8ecaf 100644
--- a/c64/math.asm
+++ b/c64/math.asm
@@ -1,5 +1,7 @@
 // vim: filetype=kickass
 
+*=* "Math Routines"
+
 .const T1=$03
 .const T2=$fd
 .const PRODUCT=$22
diff --git a/c64/screen.asm b/c64/screen.asm
index d668586..eea294e 100644
--- a/c64/screen.asm
+++ b/c64/screen.asm
@@ -1,5 +1,7 @@
 // vim: filetype=kickass
 
+*=* "Screen Routines"
+
 //
 // clear_screen
 //
@@ -141,6 +143,7 @@ write_string:
     jsr print_hex
 }
 
+// Destroys: a, x, y, $03
 print_hex_0x:
     sta zp_temp