diff --git a/.gitignore b/.gitignore
index dc600f9b36..70992a4388 100644
--- a/.gitignore
+++ b/.gitignore
@@ -180,6 +180,7 @@
 /test-date
 /test-delta
 /test-dump-cache-tree
+/test-dump-split-index
 /test-scrap-cache-tree
 /test-genrandom
 /test-hashmap
diff --git a/Makefile b/Makefile
index 94ad3cec7f..028749bfca 100644
--- a/Makefile
+++ b/Makefile
@@ -565,6 +565,7 @@ TEST_PROGRAMS_NEED_X += test-ctype
 TEST_PROGRAMS_NEED_X += test-date
 TEST_PROGRAMS_NEED_X += test-delta
 TEST_PROGRAMS_NEED_X += test-dump-cache-tree
+TEST_PROGRAMS_NEED_X += test-dump-split-index
 TEST_PROGRAMS_NEED_X += test-genrandom
 TEST_PROGRAMS_NEED_X += test-hashmap
 TEST_PROGRAMS_NEED_X += test-index-version
diff --git a/cache.h b/cache.h
index 7523c28ec7..c6b7770f60 100644
--- a/cache.h
+++ b/cache.h
@@ -473,6 +473,8 @@ extern int daemonize(void);
 struct lock_file;
 extern int read_index(struct index_state *);
 extern int read_index_preload(struct index_state *, const struct pathspec *pathspec);
+extern int do_read_index(struct index_state *istate, const char *path,
+			 int must_exist); /* for testting only! */
 extern int read_index_from(struct index_state *, const char *path);
 extern int is_index_unborn(struct index_state *);
 extern int read_index_unmerged(struct index_state *);
diff --git a/read-cache.c b/read-cache.c
index 12335a362b..342fe52fda 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -1463,8 +1463,7 @@ static struct cache_entry *create_from_disk(struct ondisk_cache_entry *ondisk,
 }
 
 /* remember to discard_cache() before reading a different cache! */
-static int do_read_index(struct index_state *istate, const char *path,
-			 int must_exist)
+int do_read_index(struct index_state *istate, const char *path, int must_exist)
 {
 	int fd, i;
 	struct stat st;
diff --git a/t/t1700-split-index.sh b/t/t1700-split-index.sh
new file mode 100755
index 0000000000..94fb473e7c
--- /dev/null
+++ b/t/t1700-split-index.sh
@@ -0,0 +1,194 @@
+#!/bin/sh
+
+test_description='split index mode tests'
+
+. ./test-lib.sh
+
+# We need total control of index splitting here
+sane_unset GIT_TEST_SPLIT_INDEX
+
+test_expect_success 'enable split index' '
+	git update-index --split-index &&
+	test-dump-split-index .git/index >actual &&
+	cat >expect <<EOF &&
+own 8299b0bcd1ac364e5f1d7768efb62fa2da79a339
+base 39d890139ee5356c7ef572216cebcd27aa41f9df
+replacements:
+deletions:
+EOF
+	test_cmp expect actual
+'
+
+test_expect_success 'add one file' '
+	: >one &&
+	git update-index --add one &&
+	git ls-files --stage >ls-files.actual &&
+	cat >ls-files.expect <<EOF &&
+100644 e69de29bb2d1d6434b8b29ae775ad8c2e48c5391 0	one
+EOF
+	test_cmp ls-files.expect ls-files.actual &&
+
+	test-dump-split-index .git/index | sed "/^own/d" >actual &&
+	cat >expect <<EOF &&
+base 39d890139ee5356c7ef572216cebcd27aa41f9df
+100644 e69de29bb2d1d6434b8b29ae775ad8c2e48c5391 0	one
+replacements:
+deletions:
+EOF
+	test_cmp expect actual
+'
+
+test_expect_success 'disable split index' '
+	git update-index --no-split-index &&
+	git ls-files --stage >ls-files.actual &&
+	cat >ls-files.expect <<EOF &&
+100644 e69de29bb2d1d6434b8b29ae775ad8c2e48c5391 0	one
+EOF
+	test_cmp ls-files.expect ls-files.actual &&
+
+	BASE=`test-dump-split-index .git/index | grep "^own" | sed "s/own/base/"` &&
+	test-dump-split-index .git/index | sed "/^own/d" >actual &&
+	cat >expect <<EOF &&
+not a split index
+EOF
+	test_cmp expect actual
+'
+
+test_expect_success 'enable split index again, "one" now belongs to base index"' '
+	git update-index --split-index &&
+	git ls-files --stage >ls-files.actual &&
+	cat >ls-files.expect <<EOF &&
+100644 e69de29bb2d1d6434b8b29ae775ad8c2e48c5391 0	one
+EOF
+	test_cmp ls-files.expect ls-files.actual &&
+
+	test-dump-split-index .git/index | sed "/^own/d" >actual &&
+	cat >expect <<EOF &&
+$BASE
+replacements:
+deletions:
+EOF
+	test_cmp expect actual
+'
+
+test_expect_success 'modify original file, base index untouched' '
+	echo modified >one &&
+	git update-index one &&
+	git ls-files --stage >ls-files.actual &&
+	cat >ls-files.expect <<EOF &&
+100644 2e0996000b7e9019eabcad29391bf0f5c7702f0b 0	one
+EOF
+	test_cmp ls-files.expect ls-files.actual &&
+
+	test-dump-split-index .git/index | sed "/^own/d" >actual &&
+	q_to_tab >expect <<EOF &&
+$BASE
+100644 2e0996000b7e9019eabcad29391bf0f5c7702f0b 0Q
+replacements: 0
+deletions:
+EOF
+	test_cmp expect actual
+'
+
+test_expect_success 'add another file, which stays index' '
+	: >two &&
+	git update-index --add two &&
+	git ls-files --stage >ls-files.actual &&
+	cat >ls-files.expect <<EOF &&
+100644 2e0996000b7e9019eabcad29391bf0f5c7702f0b 0	one
+100644 e69de29bb2d1d6434b8b29ae775ad8c2e48c5391 0	two
+EOF
+	test_cmp ls-files.expect ls-files.actual &&
+
+	test-dump-split-index .git/index | sed "/^own/d" >actual &&
+	q_to_tab >expect <<EOF &&
+$BASE
+100644 2e0996000b7e9019eabcad29391bf0f5c7702f0b 0Q
+100644 e69de29bb2d1d6434b8b29ae775ad8c2e48c5391 0	two
+replacements: 0
+deletions:
+EOF
+	test_cmp expect actual
+'
+
+test_expect_success 'remove file not in base index' '
+	git update-index --force-remove two &&
+	git ls-files --stage >ls-files.actual &&
+	cat >ls-files.expect <<EOF &&
+100644 2e0996000b7e9019eabcad29391bf0f5c7702f0b 0	one
+EOF
+	test_cmp ls-files.expect ls-files.actual &&
+
+	test-dump-split-index .git/index | sed "/^own/d" >actual &&
+	q_to_tab >expect <<EOF &&
+$BASE
+100644 2e0996000b7e9019eabcad29391bf0f5c7702f0b 0Q
+replacements: 0
+deletions:
+EOF
+	test_cmp expect actual
+'
+
+test_expect_success 'remove file in base index' '
+	git update-index --force-remove one &&
+	git ls-files --stage >ls-files.actual &&
+	cat >ls-files.expect <<EOF &&
+EOF
+	test_cmp ls-files.expect ls-files.actual &&
+
+	test-dump-split-index .git/index | sed "/^own/d" >actual &&
+	cat >expect <<EOF &&
+$BASE
+replacements:
+deletions: 0
+EOF
+	test_cmp expect actual
+'
+
+test_expect_success 'add original file back' '
+	: >one &&
+	git update-index --add one &&
+	git ls-files --stage >ls-files.actual &&
+	cat >ls-files.expect <<EOF &&
+100644 e69de29bb2d1d6434b8b29ae775ad8c2e48c5391 0	one
+EOF
+	test_cmp ls-files.expect ls-files.actual &&
+
+	test-dump-split-index .git/index | sed "/^own/d" >actual &&
+	cat >expect <<EOF &&
+$BASE
+100644 e69de29bb2d1d6434b8b29ae775ad8c2e48c5391 0	one
+replacements:
+deletions: 0
+EOF
+	test_cmp expect actual
+'
+
+test_expect_success 'add new file' '
+	: >two &&
+	git update-index --add two &&
+	git ls-files --stage >actual &&
+	cat >expect <<EOF &&
+100644 e69de29bb2d1d6434b8b29ae775ad8c2e48c5391 0	one
+100644 e69de29bb2d1d6434b8b29ae775ad8c2e48c5391 0	two
+EOF
+	test_cmp expect actual
+'
+
+test_expect_success 'unify index, two files remain' '
+	git update-index --no-split-index &&
+	git ls-files --stage >ls-files.actual &&
+	cat >ls-files.expect <<EOF &&
+100644 e69de29bb2d1d6434b8b29ae775ad8c2e48c5391 0	one
+100644 e69de29bb2d1d6434b8b29ae775ad8c2e48c5391 0	two
+EOF
+	test_cmp ls-files.expect ls-files.actual
+
+	test-dump-split-index .git/index | sed "/^own/d" >actual &&
+	cat >expect <<EOF &&
+not a split index
+EOF
+	test_cmp expect actual
+'
+
+test_done
diff --git a/test-dump-split-index.c b/test-dump-split-index.c
new file mode 100644
index 0000000000..9cf3112c9d
--- /dev/null
+++ b/test-dump-split-index.c
@@ -0,0 +1,34 @@
+#include "cache.h"
+#include "split-index.h"
+#include "ewah/ewok.h"
+
+static void show_bit(size_t pos, void *data)
+{
+	printf(" %d", (int)pos);
+}
+
+int main(int ac, char **av)
+{
+	struct split_index *si;
+	int i;
+
+	do_read_index(&the_index, av[1], 1);
+	printf("own %s\n", sha1_to_hex(the_index.sha1));
+	si = the_index.split_index;
+	if (!si) {
+		printf("not a split index\n");
+		return 0;
+	}
+	printf("base %s\n", sha1_to_hex(si->base_sha1));
+	for (i = 0; i < the_index.cache_nr; i++) {
+		struct cache_entry *ce = the_index.cache[i];
+		printf("%06o %s %d\t%s\n", ce->ce_mode,
+		       sha1_to_hex(ce->sha1), ce_stage(ce), ce->name);
+	}
+	printf("replacements:");
+	ewah_each_bit(si->replace_bitmap, show_bit, NULL);
+	printf("\ndeletions:");
+	ewah_each_bit(si->delete_bitmap, show_bit, NULL);
+	printf("\n");
+	return 0;
+}