diff --git a/field.c b/field.c
index 1ec979c..73d5c97 100644
--- a/field.c
+++ b/field.c
@@ -1,4 +1,5 @@
 #include "field.h"
+#include "gbuffer.h"
 #include <ctype.h>
 
 void field_init(Field* f) {
@@ -34,57 +35,9 @@ void field_deinit(Field* f) { free(f->buffer); }
 
 void field_copy(Field* src, Field* dest) {
   field_resize_raw_if_necessary(dest, src->height, src->width);
-  field_copy_subrect(src, dest, 0, 0, 0, 0, src->height, src->width);
-}
-
-void field_copy_subrect(Field* src, Field* dest, Usz src_y, Usz src_x,
-                        Usz dest_y, Usz dest_x, Usz height, Usz width) {
-  Usz src_height = src->height;
-  Usz src_width = src->width;
-  Usz dest_height = dest->height;
-  Usz dest_width = dest->width;
-  if (src_height <= src_y || src_width <= src_x || dest_height <= dest_y ||
-      dest_width <= dest_x)
-    return;
-  Usz ny_0 = src_height - src_y;
-  Usz ny_1 = dest_height - dest_y;
-  Usz ny = height;
-  if (ny_0 < ny)
-    ny = ny_0;
-  if (ny_1 < ny)
-    ny = ny_1;
-  if (ny == 0)
-    return;
-  Usz row_copy_0 = src_width - src_x;
-  Usz row_copy_1 = dest_width - dest_x;
-  Usz row_copy = width;
-  if (row_copy_0 < row_copy)
-    row_copy = row_copy_0;
-  if (row_copy_1 < row_copy)
-    row_copy = row_copy_1;
-  Usz copy_bytes = row_copy * sizeof(Glyph);
-  Glyph* src_p = src->buffer + src_y * src_width + src_x;
-  Glyph* dest_p = dest->buffer + dest_y * dest_width + dest_x;
-  Usz src_stride;
-  Usz dest_stride;
-  if (src_y >= dest_y) {
-    src_stride = src_width;
-    dest_stride = dest_width;
-  } else {
-    src_p += (ny - 1) * src_width;
-    dest_p += (ny - 1) * dest_width;
-    src_stride = -src_width;
-    dest_stride = -dest_width;
-  }
-  Usz iy = 0;
-  for (;;) {
-    memmove(dest_p, src_p, copy_bytes);
-    ++iy;
-    if (iy == ny)
-      break;
-    src_p += src_stride;
-    dest_p += dest_stride;
-  }
+  gbuffer_copy_subrect(src->buffer, dest->buffer, src->height, src->width,
+                       dest->height, dest->width, 0, 0, 0, 0, src->height,
+                       src->width);
 }
 
 void field_fill_subrect(Field* f, Usz y, Usz x, Usz height, Usz width,
diff --git a/field.h b/field.h
index d93930b..c169c6b 100644
--- a/field.h
+++ b/field.h
@@ -7,8 +7,6 @@ void field_resize_raw(Field* field, Usz height, Usz width);
 void field_resize_raw_if_necessary(Field* field, Usz height, Usz width);
 void field_deinit(Field* field);
 void field_copy(Field* src, Field* dest);
-void field_copy_subrect(Field* src, Field* dest, Usz src_y, Usz src_x,
-                        Usz dest_y, Usz dest_x, Usz height, Usz width);
 void field_fill_subrect(Field* field, Usz y, Usz x, Usz height, Usz width,
                         Glyph fill_char);
 Glyph field_peek(Field* field, Usz y, Usz x);
diff --git a/gbuffer.c b/gbuffer.c
new file mode 100644
index 0000000..c72a091
--- /dev/null
+++ b/gbuffer.c
@@ -0,0 +1,49 @@
+#include "gbuffer.h"
+
+void gbuffer_copy_subrect(Glyph* src, Glyph* dest, Usz src_height,
+                          Usz src_width, Usz dest_height, Usz dest_width,
+                          Usz src_y, Usz src_x, Usz dest_y, Usz dest_x,
+                          Usz height, Usz width) {
+  if (src_height <= src_y || src_width <= src_x || dest_height <= dest_y ||
+      dest_width <= dest_x)
+    return;
+  Usz ny_0 = src_height - src_y;
+  Usz ny_1 = dest_height - dest_y;
+  Usz ny = height;
+  if (ny_0 < ny)
+    ny = ny_0;
+  if (ny_1 < ny)
+    ny = ny_1;
+  if (ny == 0)
+    return;
+  Usz row_copy_0 = src_width - src_x;
+  Usz row_copy_1 = dest_width - dest_x;
+  Usz row_copy = width;
+  if (row_copy_0 < row_copy)
+    row_copy = row_copy_0;
+  if (row_copy_1 < row_copy)
+    row_copy = row_copy_1;
+  Usz copy_bytes = row_copy * sizeof(Glyph);
+  Glyph* src_p = src + src_y * src_width + src_x;
+  Glyph* dest_p = dest + dest_y * dest_width + dest_x;
+  Usz src_stride;
+  Usz dest_stride;
+  if (src_y >= dest_y) {
+    src_stride = src_width;
+    dest_stride = dest_width;
+  } else {
+    src_p += (ny - 1) * src_width;
+    dest_p += (ny - 1) * dest_width;
+    src_stride = -src_width;
+    dest_stride = -dest_width;
+  }
+  Usz iy = 0;
+  for (;;) {
+    memmove(dest_p, src_p, copy_bytes);
+    ++iy;
+    if (iy == ny)
+      break;
+    src_p += src_stride;
+    dest_p += dest_stride;
+  }
+}
diff --git a/gbuffer.h b/gbuffer.h
index 03f65bd..6836c17 100644
--- a/gbuffer.h
+++ b/gbuffer.h
@@ -34,3 +34,9 @@ static inline void gbuffer_poke_relative(Gbuffer gbuf, Usz height, Usz width,
     return;
   gbuf[(Usz)y0 * width + (Usz)x0] = g;
 }
+
+ORCA_FORCE_NO_INLINE
+void gbuffer_copy_subrect(Glyph* src, Glyph* dest, Usz src_grid_h,
+                          Usz src_grid_w, Usz dest_grid_h, Usz dest_grid_w,
+                          Usz src_y, Usz src_x, Usz dest_y, Usz dest_x,
+                          Usz height, Usz width);
diff --git a/tool b/tool
index 2e7c6fc..b997cd9 100755
--- a/tool
+++ b/tool
@@ -219,7 +219,7 @@ build_target() {
     x86_64) add cc_flags -march=nehalem;;
   esac
 
-  add source_files field.c mark.c bank.c sim.c
+  add source_files gbuffer.c field.c mark.c bank.c sim.c
   case "$2" in
     orca|cli)
       add source_files cli_main.c