diff --git a/aoc.mk b/aoc.mk
index 0524a3c..c9ed242 100644
--- a/aoc.mk
+++ b/aoc.mk
@@ -11,6 +11,7 @@ OBJ = \
 DREAMCAST_OBJ = \
 	runner_dreamcast.o \
 	input_dreamcast.o \
+	sh7091_scif.o \
 	$(LIB)/holly/core.o \
 	$(LIB)/holly/region_array.o \
 	$(LIB)/holly/background.o \
diff --git a/day10/input.txt b/day10/input.txt
new file mode 100644
index 0000000..483f666
--- /dev/null
+++ b/day10/input.txt
@@ -0,0 +1,54 @@
+034565433432321821235456556798712438943432345432101010
+125674344341210930012367349897601123876501656543238923
+545981265210523841112398232098532014101665437654147014
+436780378019654756501454101123442125012878328956056985
+521091489328765661498760130210356916763969012347841076
+342698456410644570187321221202197809854054521078932987
+434784387569653089298435430143087612870123654169410673
+525601295678762120387589347834561543965434783254321034
+510567856789854871456678956921670123457650199165890321
+654356943889903962345665067830589894398743278056785410
+701245012958712653434784167845678765210634565410156789
+898434321867012344321093254976104543231023876321943089
+686321780154102101287765323987267650122012989423872176
+567980691233283237898894015416398532143606788714565765
+408974500543098946907623234505458545014545698601159854
+312363217632127655416510107612369876105894332589018345
+221457898565634562323421458733478983276701201478121230
+100656967874567641032102369021098744589856782363210121
+789345450923498634543001078112321654678345891054308701
+654212321012654521694210981205430903543232654143489610
+503403430156787610784300870376567812654121743267876523
+012567567343898525695421889487854925780010893210965432
+123498788210123436786438976598943056891234984921014101
+019567699101898345877567887678762147878765675870123012
+108754543210567212968900194589054038969454566767636323
+205610121056750303453213293032123229454323879098545438
+014981232347841219874984782143210110301012978121654569
+123672654338932306705675676154560121212307665430743478
+298543789221069455012310145069430430925408578989832387
+567010176109178764563498232178521567876519454567601296
+432123485458769853074567310123678106898323343218970345
+012034394367458102189601489433439256765401252102181234
+123465210210343012078732676512508349850106769843098321
+054896990101212043896543216109612556743219856784587410
+969887889654302158987894107898743445654340145697656531
+878791078745983467856765545677654354567854234548545621
+745622363215676500345653231987980123498960129639343210
+034215454301876211256570120345678154323870038721658343
+122100189321945308967981011234569067212721801290569012
+543893276430932457898876323233432178008934980387378143
+456764345567801966501895400145567889127645673456269654
+369632105478765873432689312176876901234560012562158765
+278543254309894569546576543089985432321071239873043210
+103450569212723278637434010034394321810980548954560127
+894321078112010198728922121165223010901289687643073438
+765697151003434587017213033278112987874398796542189569
+056788769876524326501304544569001056965834321233675678
+145679458985015415432987693432102345216945610344569547
+230989347034376102341056780540989104307898700123678632
+321078234123289211652346791691071234487465410878766781
+989165106000105690789905882782360943296578321969215690
+076234245612234782349814943485456850123489100154304385
+145210238763145671456723876596306765012101256701235276
+234300149854078980365432101487217898721032349810120123
diff --git a/day10/input.txt.h b/day10/input.txt.h
new file mode 100644
index 0000000..4b2963f
--- /dev/null
+++ b/day10/input.txt.h
@@ -0,0 +1,15 @@
+#pragma once
+
+#include <stdint.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+extern uint32_t _binary_day10_input_txt_start __asm("_binary_day10_input_txt_start");
+extern uint32_t _binary_day10_input_txt_end __asm("_binary_day10_input_txt_end");
+extern uint32_t _binary_day10_input_txt_size __asm("_binary_day10_input_txt_size");
+
+#ifdef __cplusplus
+}
+#endif
diff --git a/day10/sample1.txt b/day10/sample1.txt
new file mode 100644
index 0000000..cada9b3
--- /dev/null
+++ b/day10/sample1.txt
@@ -0,0 +1,8 @@
+89010123
+78121874
+87430965
+96549874
+45678903
+32019012
+01329801
+10456732
diff --git a/day10/sample1.txt.h b/day10/sample1.txt.h
new file mode 100644
index 0000000..6e8e1bc
--- /dev/null
+++ b/day10/sample1.txt.h
@@ -0,0 +1,15 @@
+#pragma once
+
+#include <stdint.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+extern uint32_t _binary_day10_sample1_txt_start __asm("_binary_day10_sample1_txt_start");
+extern uint32_t _binary_day10_sample1_txt_end __asm("_binary_day10_sample1_txt_end");
+extern uint32_t _binary_day10_sample1_txt_size __asm("_binary_day10_sample1_txt_size");
+
+#ifdef __cplusplus
+}
+#endif
diff --git a/day10/solution.c b/day10/solution.c
new file mode 100644
index 0000000..ce13c55
--- /dev/null
+++ b/day10/solution.c
@@ -0,0 +1,110 @@
+#include <stdint.h>
+
+#include "parse.h"
+#include "cartesian.h"
+#include "printf.h"
+#include "memory.h"
+#include "array.h"
+
+struct offset {
+  int x;
+  int y;
+};
+
+int walk_trailhead(const char * input,
+                   int depth,
+                   int stride,
+                   int width, int height,
+                   int x, int y,
+                   char * reachable)
+{
+  if (depth == 9) {
+    reachable[y * width + x] = 1;
+    return 1;
+  }
+
+  const static struct offset offset[] = {
+    { 1,  0},
+    {-1,  0},
+    { 0,  1},
+    { 0, -1},
+  };
+
+  int rating = 0;
+  for (int i = 0; i < (sizeof (offset)) / (sizeof (offset[0])); i++) {
+    int new_x = x + offset[i].x;
+    int new_y = y + offset[i].y;
+    if (cartesian_inside(width, height, new_x, new_y)) {
+      char c = input[new_y * stride + new_x];
+      int new_depth = parse_base10_digit(c);
+      if (new_depth == depth + 1) {
+        rating += walk_trailhead(input,
+                                 new_depth,
+                                 stride,
+                                 width, height,
+                                 new_x, new_y,
+                                 reachable);
+      }
+    }
+  }
+
+  return rating;
+}
+
+struct rating_score {
+  int rating;
+  int score;
+};
+
+struct rating_score find_trailheads(const char * input, int length,
+                                    int stride,
+                                    int width, int height
+                                    )
+{
+  int x = 0;
+  int y = 0;
+
+  char reachable[width * height];
+  int total_score = 0;
+  int total_rating = 0;
+
+  for (int i = 0; i < length; i++) {
+    if (input[i] == '0') {
+      memory_set_char(reachable, 0, width * height);
+      int rating = walk_trailhead(input, 0, stride, width, height, x, y, reachable);
+      total_rating += rating;
+      int score = array_sum_char(reachable, width * height);
+      total_score += score;
+    }
+    if (input[i] == '\n') {
+      x = 0;
+      y += 1;
+    } else {
+      x += 1;
+    }
+  }
+
+  return (struct rating_score){total_rating, total_score};
+}
+
+int64_t day10_part1(const char * input, int length)
+{
+  int stride = parse_stride(input, length);
+  int height = parse_height(input, length);
+  int width = stride - 1;
+
+  struct rating_score rs = find_trailheads(input, length, stride, width, height);
+
+  return rs.score;
+}
+
+int64_t day10_part2(const char * input, int length)
+{
+  int stride = parse_stride(input, length);
+  int height = parse_height(input, length);
+  int width = stride - 1;
+
+  struct rating_score rs = find_trailheads(input, length, stride, width, height);
+
+  return rs.rating;
+}
diff --git a/gen.sh b/gen.sh
index ae4f9c9..19bd70e 100755
--- a/gen.sh
+++ b/gen.sh
@@ -42,7 +42,7 @@ function gen_start_size ()
 }
 
 
-for i in day* ; do
+for i in day? day?? ; do
     make ${i}/sample1.txt.h ${i}/input.txt.h
     if [ -f ${i}/sample2.txt ]; then
         make ${i}/sample2.txt.h
@@ -52,7 +52,7 @@ done
 
 truncate -s0 input_dreamcast.inc
 
-for i in day* ; do
+for i in day? day?? ; do
     echo "#include \"${i}/sample1.txt.h\"" >> input_dreamcast.inc
     if [ -f ${i}/sample2.txt ]; then
         echo "#include \"${i}/sample2.txt.h\"" >> input_dreamcast.inc
@@ -61,7 +61,7 @@ for i in day* ; do
 done
 echo >> input_dreamcast.inc
 echo "static struct start_size sample[][2] = {" >> input_dreamcast.inc
-for i in day* ; do
+for i in day? day?? ; do
     echo "  {"  >> input_dreamcast.inc
     gen_start_size "  " "${i}" "sample1"
     if [ ! -f ${i}/sample2.txt ]; then
@@ -74,7 +74,7 @@ done
 echo "};" >> input_dreamcast.inc
 echo >> input_dreamcast.inc
 echo "static struct start_size input[] = {" >> input_dreamcast.inc
-for i in day* ; do
+for i in day? day?? ; do
     gen_start_size "" "${i}" "input"
 done
 echo "};" >> input_dreamcast.inc
@@ -82,7 +82,7 @@ echo "};" >> input_dreamcast.inc
 
 truncate -s0 solutions.mk
 echo -n "DAY_OBJ =" >> solutions.mk
-for i in day* ; do
+for i in day? day?? ; do
     echo " \\" >> solutions.mk
     echo "	${i}/sample1.txt.o \\" >> solutions.mk
     if [ -f ${i}/sample2.txt ]; then
@@ -97,7 +97,7 @@ done
 echo >> solutions.mk
 
 truncate -s0 runner.inc
-for i in day* ; do
+for i in day? day?? ; do
     echo "int64_t ${i}_part1(const char * input, int length);" >> runner.inc
     echo "int64_t ${i}_part2(const char * input, int length);" >> runner.inc
     if [ -f ${i}/render.cpp ]; then
@@ -108,7 +108,7 @@ for i in day* ; do
 done
 echo >> runner.inc
 echo "struct day_funcs solution[] = {" >> runner.inc
-for i in day* ; do
+for i in day? day?? ; do
     echo "  {" >> runner.inc
     echo "    {${i}_part1, ${i}_part2}," >> runner.inc
     if [ -f ${i}/render.cpp ]; then
diff --git a/input_dreamcast.inc b/input_dreamcast.inc
index 6891ed5..60d3f72 100644
--- a/input_dreamcast.inc
+++ b/input_dreamcast.inc
@@ -17,6 +17,8 @@
 #include "day8/input.txt.h"
 #include "day9/sample1.txt.h"
 #include "day9/input.txt.h"
+#include "day10/sample1.txt.h"
+#include "day10/input.txt.h"
 
 static struct start_size sample[][2] = {
   {
@@ -73,6 +75,12 @@ static struct start_size sample[][2] = {
     { (  char *)&_binary_day9_sample1_txt_start,
       (uint32_t)&_binary_day9_sample1_txt_size   },
   },
+  {
+    { (  char *)&_binary_day10_sample1_txt_start,
+      (uint32_t)&_binary_day10_sample1_txt_size   },
+    { (  char *)&_binary_day10_sample1_txt_start,
+      (uint32_t)&_binary_day10_sample1_txt_size   },
+  },
 };
 
 static struct start_size input[] = {
@@ -94,4 +102,6 @@ static struct start_size input[] = {
     (uint32_t)&_binary_day8_input_txt_size   },
   { (  char *)&_binary_day9_input_txt_start,
     (uint32_t)&_binary_day9_input_txt_size   },
+  { (  char *)&_binary_day10_input_txt_start,
+    (uint32_t)&_binary_day10_input_txt_size   },
 };
diff --git a/printf.c b/printf.c
index f5d589b..3f14cca 100644
--- a/printf.c
+++ b/printf.c
@@ -5,6 +5,7 @@
 #include "parse.h"
 #include "unparse.h"
 #include "printf.h"
+#include "sh7091_scif.h"
 
 enum format_type {
   FORMAT_BASE10,
@@ -62,6 +63,19 @@ static const char * parse_escape(const char * format, struct format * ft)
 
 struct output_buffer global_output_buffer = {0};
 
+void print_char(char c)
+{
+  global_output_buffer.buf[global_output_buffer.buf_ix++] = c;
+  scif_character(c);
+}
+
+static void __print_s(const char * s, int length)
+{
+  for (int i = 0; i < length; i++) {
+    scif_character(s[i]);
+  }
+}
+
 void _printf(const char * format, ...)
 {
   va_list args;
@@ -82,6 +96,7 @@ void _printf(const char * format, ...)
             int32_t num = va_arg(args, int32_t);
             char * s = &global_output_buffer.buf[global_output_buffer.buf_ix];
             int offset = unparse_base10(s, num, ft.pad_length, ft.fill_char);
+            __print_s(s, offset);
             global_output_buffer.buf_ix += offset;
           }
           break;
@@ -90,6 +105,7 @@ void _printf(const char * format, ...)
             int64_t num = va_arg(args, int64_t);
             char * s = &global_output_buffer.buf[global_output_buffer.buf_ix];
             int offset = unparse_base10_64(s, num, ft.pad_length, ft.fill_char);
+            __print_s(s, offset);
             global_output_buffer.buf_ix += offset;
           }
           break;
@@ -98,6 +114,7 @@ void _printf(const char * format, ...)
             uint32_t num = va_arg(args, uint32_t);
             char * s = &global_output_buffer.buf[global_output_buffer.buf_ix];
             int offset = unparse_base16(s, num, ft.pad_length, ft.fill_char);
+            __print_s(s, offset);
             global_output_buffer.buf_ix += offset;
           }
           break;
@@ -105,24 +122,32 @@ void _printf(const char * format, ...)
           {
             const char * s = va_arg(args, const char *);
             while (*s != 0) {
-              global_output_buffer.buf[global_output_buffer.buf_ix++] = *s++;
+              char c = *s++;
+              scif_character(c);
+              global_output_buffer.buf[global_output_buffer.buf_ix++] = c;
             }
           }
           break;
         case FORMAT_CHAR:
           {
             const int c = va_arg(args, const int);
+            scif_character((char)c);
             global_output_buffer.buf[global_output_buffer.buf_ix++] = (char)c;
           }
           break;
         case FORMAT_PERCENT:
+          scif_character('%');
           global_output_buffer.buf[global_output_buffer.buf_ix++] = '%';
           break;
         }
       }
       break;
     default:
-      global_output_buffer.buf[global_output_buffer.buf_ix++] = *format++;
+      {
+        char c = *format++;
+        scif_character(c);
+        global_output_buffer.buf[global_output_buffer.buf_ix++] = c;
+      }
       break;
     }
   }
diff --git a/runner.c b/runner.c
index 423539f..70529e5 100644
--- a/runner.c
+++ b/runner.c
@@ -32,7 +32,7 @@ bool runner_tick(struct runner_state * runner_state)
   int part = tick % 2;
   int day = tick / 2;
 
-  if (day < 8)
+  if (day < 9)
     return true;
 
   runner_state->want_render = solution[day].render != NULL;
diff --git a/runner.inc b/runner.inc
index bd1eb88..7af3b94 100644
--- a/runner.inc
+++ b/runner.inc
@@ -19,6 +19,8 @@ int64_t day8_part1(const char * input, int length);
 int64_t day8_part2(const char * input, int length);
 int64_t day9_part1(const char * input, int length);
 int64_t day9_part2(const char * input, int length);
+int64_t day10_part1(const char * input, int length);
+int64_t day10_part2(const char * input, int length);
 
 struct day_funcs solution[] = {
   {
@@ -57,4 +59,8 @@ struct day_funcs solution[] = {
     {day9_part1, day9_part2},
     NULL,
   },
+  {
+    {day10_part1, day10_part2},
+    NULL,
+  },
 };
diff --git a/runner_dreamcast.cpp b/runner_dreamcast.cpp
index 5278e9a..845fcf4 100644
--- a/runner_dreamcast.cpp
+++ b/runner_dreamcast.cpp
@@ -454,11 +454,3 @@ int main()
   serial::string("return\n");
   serial::string("return\n");
 }
-
-
-extern "C"
-void print_char(char c)
-{
-  global_output_buffer.buf[global_output_buffer.buf_ix++] = c;
-  serial::character(c);
-}
diff --git a/sh7091_scif.cpp b/sh7091_scif.cpp
new file mode 100644
index 0000000..c5cf598
--- /dev/null
+++ b/sh7091_scif.cpp
@@ -0,0 +1,15 @@
+#include "sh7091/sh7091.hpp"
+#include "sh7091/sh7091_bits.hpp"
+
+#include "sh7091_scif.h"
+
+void scif_character(const char c)
+{
+  using namespace scif;
+  // wait for transmit fifo to become partially empty
+  while ((sh7091.SCIF.SCFSR2 & scfsr2::tdfe::bit_mask) == 0);
+
+  sh7091.SCIF.SCFSR2 &= ~scfsr2::tdfe::bit_mask;
+
+  sh7091.SCIF.SCFTDR2 = static_cast<uint8_t>(c);
+}
diff --git a/sh7091_scif.h b/sh7091_scif.h
new file mode 100644
index 0000000..419a1f5
--- /dev/null
+++ b/sh7091_scif.h
@@ -0,0 +1,11 @@
+#pragma once
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+void scif_character(const char c);
+
+#ifdef __cplusplus
+}
+#endif
diff --git a/solutions.mk b/solutions.mk
index cb61e8f..f388687 100644
--- a/solutions.mk
+++ b/solutions.mk
@@ -27,4 +27,7 @@ DAY_OBJ = \
 	day8/solution.o \
 	day9/sample1.txt.o \
 	day9/input.txt.o \
-	day9/solution.o
+	day9/solution.o \
+	day10/sample1.txt.o \
+	day10/input.txt.o \
+	day10/solution.o