diff --git a/Makefile b/Makefile
index f7c0c5f..86a9067 100644
--- a/Makefile
+++ b/Makefile
@@ -11,7 +11,7 @@ else
 debug_flags := $(debug_flags) -Og -feliminate-unused-debug-symbols
 tui_library_flags := -lncursesw
 endif
-common_source_files := field.c sim.c
+common_source_files := field.c mark.c sim.c
 tui_source_files := $(common_source_files) tui_main.c
 cli_source_files := $(common_source_files) cli_main.c
 
diff --git a/base.h b/base.h
index 903de39..777813f 100644
--- a/base.h
+++ b/base.h
@@ -8,10 +8,19 @@
 #include <string.h>
 #include <unistd.h>
 
+#if defined(__GNUC__) || defined(__clang__)
+#define ORCA_FORCE_INLINE __attribute__((always_inline)) inline
+#elif defined(_MSC_VER)
+#define ORCA_FORCE_INLINE __forceinline
+#else
+#define ORCA_FORCE_INLINE inline
+#endif
+
 #define ORCA_Y_MAX UINT16_MAX
 #define ORCA_X_MAX UINT16_MAX
 
-typedef char Glyph;
+typedef uint8_t U8;
+typedef int8_t I8;
 typedef uint16_t U16;
 typedef int16_t I16;
 typedef uint32_t U32;
@@ -21,6 +30,8 @@ typedef int64_t I64;
 typedef size_t Usz;
 typedef ssize_t Isz;
 
+typedef char Glyph;
+
 typedef struct {
   Glyph* buffer;
   U16 height;
diff --git a/mark.c b/mark.c
new file mode 100644
index 0000000..57f6806
--- /dev/null
+++ b/mark.c
@@ -0,0 +1,21 @@
+#include "mark.h"
+
+void markmap_init(Markmap* map) {
+  map->buffer = NULL;
+  map->capacity = 0;
+}
+
+void markmap_ensure_capacity(Markmap* map, Usz capacity) {
+  if (map->capacity < capacity) {
+    map->buffer = realloc(map->buffer, capacity);
+    map->capacity = capacity;
+  }
+}
+
+void markmap_clear(Markmap* map) {
+  memset(map->buffer, 0, map->capacity);
+}
+
+void markmap_deinit(Markmap* map) {
+  free(map->buffer);
+}
diff --git a/mark.h b/mark.h
new file mode 100644
index 0000000..b64da47
--- /dev/null
+++ b/mark.h
@@ -0,0 +1,49 @@
+#pragma once
+#include "base.h"
+
+typedef enum {
+  Mark_flag_none = 0,
+  Mark_flag_haste_input = 1 << 0,
+  Mark_flag_input = 1 << 1,
+  Mark_flag_lock = 1 << 2,
+  Mark_flag_output = 1 << 3,
+} Mark_flags;
+
+typedef struct {
+  U8* buffer;
+  Usz capacity;
+} Markmap;
+
+void markmap_init(Markmap* map);
+void markmap_ensure_capacity(Markmap* map, Usz capacity);
+void markmap_clear(Markmap* map);
+void markmap_deinit(Markmap* map);
+
+ORCA_FORCE_INLINE
+Mark_flags markmap_peek_relative(Markmap* map, Usz map_height, Usz map_width,
+                                 Usz y, Usz x, Isz offs_y, Isz offs_x);
+ORCA_FORCE_INLINE
+void markmap_poke_relative(Markmap* map, Usz map_height, Usz map_width, Usz y,
+                           Usz x, Isz offs_y, Isz offs_x, Mark_flags flags);
+
+// Inline implementation
+
+ORCA_FORCE_INLINE
+Mark_flags markmap_peek_relative(Markmap* map, Usz map_height, Usz map_width,
+                                 Usz y, Usz x, Isz offs_y, Isz offs_x) {
+  Isz y0 = (Isz)y + offs_y;
+  Isz x0 = (Isz)x + offs_x;
+  if (y0 >= (Isz)map_height || x0 >= (Isz)map_width || y0 < 0 || x0 < 0)
+    return Mark_flag_none;
+  return map->buffer[(Usz)y0 * map_width + (Usz)x0];
+}
+
+ORCA_FORCE_INLINE
+void markmap_poke_relative(Markmap* map, Usz map_height, Usz map_width, Usz y,
+                           Usz x, Isz offs_y, Isz offs_x, Mark_flags flags) {
+  Isz y0 = (Isz)y + offs_y;
+  Isz x0 = (Isz)x + offs_x;
+  if (y0 >= (Isz)map_height || x0 >= (Isz)map_width || y0 < 0 || x0 < 0)
+    return;
+  map->buffer[(Usz)y0 * map_width + (Usz)x0] = flags;
+}