diff --git a/Makefile b/Makefile
index 29c1788..887cb7b 100644
--- a/Makefile
+++ b/Makefile
@@ -1,7 +1,7 @@
 CC=riscv64-unknown-elf-gcc
 AS=riscv64-unknown-elf-as
 ASFLAGS=-g -mabi=ilp32e -march=rv32ec
-CFLAGS=$(ASFLAGS)
+CFLAGS=$(ASFLAGS) -Os -Wall -std=c99
 LD=riscv64-unknown-elf-ld
 export JQ?=jaq
 export QEMU?=qemu-riscv32
@@ -57,7 +57,7 @@ tests/btohex.elf: mem.o hex.o debug.o tests/btohex.o
 tests/tohex.elf: hex.o tests/tohex.o
 	$(LD) -m elf32lriscv -T link.ld $^ -o $@
 
-tests/harness.elf: tests/harness.o
+tests/harness.elf: tests/harness.o tests/charness.o
 	$(LD) -m elf32lriscv -T link.ld $^ -o $@
 
 %.o : %.s
diff --git a/calc.h b/calc.h
new file mode 100644
index 0000000..eb78553
--- /dev/null
+++ b/calc.h
@@ -0,0 +1,11 @@
+#ifndef CALC_H
+#define CALC_H
+
+typedef unsigned char bool;
+#define true 1;
+#define false 0;
+
+typedef unsigned char u8;
+typedef unsigned int uint;
+
+#endif
diff --git a/tests/charness.c b/tests/charness.c
new file mode 100644
index 0000000..6dc0fea
--- /dev/null
+++ b/tests/charness.c
@@ -0,0 +1,43 @@
+#include "../calc.h"
+
+#define BUF_SIZE 80
+
+#define make_str(s) { sizeof(s), (const u8 *)s }
+#define make_test(name) { make_str(#name), name }
+
+extern bool streq(const u8 *, uint, const u8 *, uint);
+extern int read(u8 *, uint); // read from stdin
+
+// Tests
+extern void count_digits(void);
+extern void fmt_decimal(void);
+
+typedef struct {
+  u8 len;
+  const u8 *data;
+} str;
+
+typedef void (*testfn)(void);
+
+typedef struct {
+  str name;
+  testfn fn;
+} test;
+
+static str tests[2] = {make_test(count_digits), make_test(fmt_decimal)};
+
+void _start(void) {
+        u8 buf[BUF_SIZE];
+        int nread0 = read(buf, BUF_SIZE);
+        if (nread0 < 0) {
+                for(;;); // TODO: add abort function
+        }
+        uint nread = (uint)nread0;
+
+        for(uint i = 0; i < sizeof(tests); i++) {
+                const str s = tests[i];
+                if (streq(buf, nread, s.data, s.len)) {
+                    
+                }
+        }
+}
diff --git a/tests/harness.s b/tests/harness.s
index 37b709e..2c2f84e 100644
--- a/tests/harness.s
+++ b/tests/harness.s
@@ -3,7 +3,8 @@
 
 .org 0
 # Provide program starting address to linker
-.global _start
+.global read
+.global streq
 
 # .extern fmt_decimal
 
@@ -25,7 +26,7 @@ inbuf: .skip 80
 
 .text
 
-_start:
+_start2:
     la a0, inbuf
     li a1, 80
     jal read