diff --git a/c64/main.asm b/c64/main.asm
index ba9a370..6873104 100644
--- a/c64/main.asm
+++ b/c64/main.asm
@@ -2,12 +2,29 @@
 
 #import "math.inc"
 
-* = $0801                               // BASIC start address (#2049)
-.byte $0d,$08,$dc,$07,$9e,$20,$34,$39   // BASIC loader to start at $c000...
-.byte $31,$35,$32,$00,$00,$00           // puts BASIC line 2012 SYS 49152
+    BasicUpstart2(main)
+
+    .align $100
+* = * "Square table"
+square1_lo:
+    .fill 512, 0
+square1_hi:
+    .fill 512, 0
+square2_lo:
+    .fill 512, 0
+square2_hi:
+    .fill 512, 0
+
+* = * "Day 1 input"
+day01_input:
+    .import text "../rust/inputs/day01"
+    .byte 0
+
+//* = $8000 "Cartridge ROM"
+
+//* = $a000 "Basic ROM"
 
 * = $c000
-
 .const SCREEN_RAM = $0400
 .const COLOR_RAM = $d800
 
@@ -32,20 +49,33 @@
 main:
     jsr clear_screen
 
-    // lda #0
-    // sta $d020
-    // sta $d021
+    // Swap out BASIC and kernal
+    sei
+    lda %00110000
+    sta $01
 
-    jsr day01
+    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 print_decimal_u32
 
 !loop:
-    // inc $d020
     jmp !loop-
 
 
 buffer:
-    .fill $0100, 0
+    .fill $0400, 0
 
 
 #import "day01.asm"
 #import "screen.asm"
+#import "math.asm"
diff --git a/c64/math.asm b/c64/math.asm
index 5eb93e1..e4b6be8 100644
--- a/c64/math.asm
+++ b/c64/math.asm
@@ -1,3 +1,5 @@
+// vim: filetype=kickass
+
 .const T1=$03
 .const T2=$fd
 .const PRODUCT=$22
@@ -282,18 +284,32 @@ generate_multiplication_tables:
 //
 // udivmod32
 //
-// TODO consistent name
-// TODO document input, output, destroyed
+// 32-bit unsigned integer div/mod routine.
+// Based on https://bisqwit.iki.fi/story/howto/bitmath/
+//
+// TODO consistent name
+// TODO could probably be optimized further
+//
+// Input:
+//   - udivmod32_dividend: LSB of the dividend.
+//   - udivmod32_divisor: LSB of the divisor.
+//
+// Output:
+//   - udivmod32_result: LSB of the division result.
+//   - udivmod32_remainder: LSB of the modulo result.
+//
+// Destroys: a, $20..$2b
 //
-.const udivmod32_dividend = $10 // 11,12,13
-.const udivmod32_divisor = $14 // 15,16,17
-.const udivmod32_result = $18 // 19,1a,1b
-.const udivmod32_remainder = $1c // 1d,1e,1f
-
 .const scaled_divisor = $20 // ..23
-.const multiple = $28 // ..2b
-.const temp = $2b // ..2f
+.const multiple = $24 // ..27
+.const temp = $28 // ..2b
 udivmod32:
+    lda #0
+    sta udivmod32_result + 0
+    sta udivmod32_result + 1
+    sta udivmod32_result + 2
+    sta udivmod32_result + 3
+
     // if (divisor == 0) {
     lda udivmod32_divisor + 0
     bne !if_end+
@@ -322,12 +338,12 @@ udivmod32:
     // remainder = dividend;
     lda udivmod32_dividend + 0
     sta udivmod32_remainder + 0
-    lda udivmod32_dividend + 0
-    sta udivmod32_remainder + 0
-    lda udivmod32_dividend + 0
-    sta udivmod32_remainder + 0
-    lda udivmod32_dividend + 0
-    sta udivmod32_remainder + 0
+    lda udivmod32_dividend + 1
+    sta udivmod32_remainder + 1
+    lda udivmod32_dividend + 2
+    sta udivmod32_remainder + 2
+    lda udivmod32_dividend + 3
+    sta udivmod32_remainder + 3
 
 
     // uint32_t multiple = 1;
@@ -338,25 +354,12 @@ udivmod32:
     sta multiple + 2
     sta multiple + 3
 
-    // uint32_t result = 0;
-    sta udivmod32_result + 0
-    sta udivmod32_result + 1
-    sta udivmod32_result + 2
-    sta udivmod32_result + 3
 
-
-    // while (scaled_divisor < dividend) {
+    // while (!(scaled_divisor & 0x80000000)) {
 !while_start:
-    sec
-    lda scaled_divisor + 0
-    sbc udivmod32_dividend + 0
-    lda scaled_divisor + 1
-    sbc udivmod32_dividend + 1
-    lda scaled_divisor + 2
-    sbc udivmod32_dividend + 2
-    lda scaled_divisor + 3
-    sbc udivmod32_dividend + 3
-    bcs !while_end+
+    lda #$80
+    and scaled_divisor + 3
+    bne !while_end+
 
     //     scaled_divisor <<= 1
     asl scaled_divisor + 0
@@ -382,15 +385,19 @@ udivmod32:
     lda udivmod32_remainder + 0
     sbc scaled_divisor + 0
     sta temp + 0
+
     lda udivmod32_remainder + 1
     sbc scaled_divisor + 1
     sta temp + 1
+
     lda udivmod32_remainder + 2
     sbc scaled_divisor + 2
     sta temp + 2
+
     lda udivmod32_remainder + 3
     sbc scaled_divisor + 3
     sta temp + 3
+
     bcc !if_end+
 
     //         remain -= scaled_divisor;
@@ -403,6 +410,24 @@ udivmod32:
     lda temp + 3
     sta udivmod32_remainder + 3
 
+    //         result += multiple;
+    clc
+    lda udivmod32_result + 0
+    adc multiple + 0
+    sta udivmod32_result + 0
+
+    lda udivmod32_result + 1
+    adc multiple + 1
+    sta udivmod32_result + 1
+
+    lda udivmod32_result + 2
+    adc multiple + 2
+    sta udivmod32_result + 2
+
+    lda udivmod32_result + 3
+    adc multiple + 3
+    sta udivmod32_result + 3
+
     //     }
 !if_end:
 
diff --git a/c64/math.inc b/c64/math.inc
index a613bb0..7f61353 100644
--- a/c64/math.inc
+++ b/c64/math.inc
@@ -1,4 +1,8 @@
 // vim: filetype=kickass
+.const udivmod32_dividend = $10 // 11,12,13
+.const udivmod32_divisor = $14 // 15,16,17
+.const udivmod32_result = $18 // 19,1a,1b
+.const udivmod32_remainder = $1c // 1d,1e,1f
 
 .macro i8_div5_a() {
     sta zp_temp
@@ -95,6 +99,47 @@
     i16_i16_add(lo, hi, $fd, $fe)
 }
 
+.macro u32_mul2(lsb) {
+    asl lsb + 0
+    rol lsb + 1
+    rol lsb + 2
+    rol lsb + 3
+}
+
+.macro u32_mul10(lsb) {
+    u32_mul2(lsb)
+
+    lda lsb + 0
+    sta $2c + 0
+    lda lsb + 1
+    sta $2c + 1
+    lda lsb + 2
+    sta $2c + 2
+    lda lsb + 3
+    sta $2c + 3
+
+    u32_mul2(lsb)
+    u32_mul2(lsb)
+
+    i32_i32_add(lsb, $2c)
+}
+
+.macro i32_i32_add(dst, src) {
+    clc
+    lda dst + 0
+    adc src + 0
+    sta dst + 0
+    lda dst + 1
+    adc src + 1
+    sta dst + 1
+    lda dst + 2
+    adc src + 2
+    sta dst + 2
+    lda dst + 3
+    adc src + 3
+    sta dst + 3
+}
+
 // Destroys: a, dst_lo, dst_hi
 .macro i16_i8_add_a(dst_lo, dst_hi) {
     clc
diff --git a/c64/screen.asm b/c64/screen.asm
index 458ba4e..d668586 100644
--- a/c64/screen.asm
+++ b/c64/screen.asm
@@ -52,9 +52,7 @@ print_newline:
     sta cursor_pointer_lo
     bcc !done+
     inc cursor_pointer_hi
-
 !done:
-    rts
 
 
 //
@@ -136,6 +134,13 @@ write_string:
     rts
 
 
+.macro i16_print_hex_0x(lo, hi) {
+    lda hi
+    jsr print_hex_0x
+    lda lo
+    jsr print_hex
+}
+
 print_hex_0x:
     sta zp_temp
 
@@ -180,3 +185,122 @@ print_hex:
 !hex_chars:
     .text "0123456789ABCDEF"
 
+
+print_decimal_u16:
+    ldy #6
+
+    // Null-terminate the buffer
+    lda #0
+    sta !buffer+, y
+
+!loop:
+    u16_div10()
+
+    lda $24
+    sta $22
+
+    lda $25
+    sta $23
+
+    i16_mul10($22, $23)
+
+    i16_i16_sub($22, $23, $03, $04, $22, $23)
+
+    lda $24
+    sta $03
+    lda $25
+    sta $04
+
+    lda $22
+    clc
+    adc #'0'
+
+    sta !buffer+ - 1, y
+
+    dey
+    bne !loop-
+
+    // Find the first digit
+    ldy #0
+
+!:
+    lda !buffer+, y
+    iny
+    cmp #'0'
+    beq !-
+    dey
+    tya
+
+    clc
+    adc #<(!buffer+)
+    sta $03
+    lda #0
+    adc #>(!buffer+)
+    sta $04
+    jsr write_string
+
+    rts
+
+
+//
+// Input:
+//   - udivmod32_dividend - Number to print
+print_decimal_u32:
+    ldy #11
+
+    // Null-terminate the buffer
+    lda #0
+    sta !buffer+, y
+
+    lda #0
+    sta udivmod32_divisor + 3
+    sta udivmod32_divisor + 2
+    sta udivmod32_divisor + 1
+    lda #10
+    sta udivmod32_divisor + 0
+
+!loop:
+    jsr udivmod32
+
+    lda udivmod32_remainder
+    clc
+    adc #'0'
+
+    sta !buffer+ - 1, y
+
+    lda udivmod32_result + 0
+    sta udivmod32_dividend + 0
+    lda udivmod32_result + 1
+    sta udivmod32_dividend + 1
+    lda udivmod32_result + 2
+    sta udivmod32_dividend + 2
+    lda udivmod32_result + 3
+    sta udivmod32_dividend + 3
+
+    dey
+    bne !loop-
+
+    // Find the first digit
+    ldy #0
+
+!:
+    lda !buffer+, y
+    iny
+    cmp #'0'
+    beq !-
+    dey
+    tya
+
+    clc
+    adc #<(!buffer+)
+    sta $03
+    lda #0
+    adc #>(!buffer+)
+    sta $04
+    jsr write_string
+
+    rts
+
+
+!buffer:
+    .fill 12, 0