diff --git a/.gitignore b/.gitignore
index c65b28e..357a912 100644
--- a/.gitignore
+++ b/.gitignore
@@ -5,6 +5,8 @@
 /build/check_makefile
 /build/check_makefile_targets
 /build/check_makefile_w_arnings
+/build/gpt_guid1
+/build/gpt_guid2
 /build/iso_files/boot/iso_boot.sys
 /build/iso_files/os.zip
 /build/makefile_built_directories
@@ -85,6 +87,11 @@
 /build/os.offsets.hex
 /build/os.reasm
 /build/os.reasm.asm
+/build/os_with_guid_0
+/build/os_with_guid_hash
+/build/os_with_guid_hash.fixed_gpt_crc
+/build/os_with_guid_hash.gpt_header_fixed_crc
+/build/os_with_guid_hash.partition_array
 /build/os.zip
 /build/os.zip.adjusted
 /build/test_pass/emu_bochs
diff --git a/Makefile.example-os b/Makefile.example-os
index da255b5..5f29008 100644
--- a/Makefile.example-os
+++ b/Makefile.example-os
@@ -1,6 +1,7 @@
 offset_names = bytes_os_size \
                bytes_mbr_start \
                bytes_mbr_end \
+               bytes_gpt_header_start \
                bytes_header_32k_start \
                bytes_header_32k_end \
                bytes_iso_start \
@@ -9,6 +10,7 @@ offset_names = bytes_os_size \
                bytes_fat12_end \
                bytes_gpt_mirror_start \
                bytes_gpt_mirror_end \
+               bytes_gpt_header_mirror_start \
                bytes_zip_start \
                bytes_zip_end
 
@@ -51,6 +53,13 @@ built_files += ${os_filename} \
                ${bld}/os.hex_with_offsets \
                ${bld}/iso_files/os.zip \
                ${bld}/iso_files/boot/iso_boot.sys \
+               ${bld}/gpt_guid1 \
+               ${bld}/gpt_guid2 \
+               ${bld}/os_with_guid_0 \
+               ${bld}/os_with_guid_hash \
+               ${bld}/os_with_guid_hash.partition_array \
+               ${bld}/os_with_guid_hash.gpt_header_fixed_crc \
+               ${bld}/os_with_guid_hash.fixed_gpt_crc \
                ${more_offset_dec} \
                ${more_offset_hex} \
 
@@ -141,6 +150,9 @@ ${eval ${call offset,bytes_os_size,     $${os_image_size_kb} * 1024,,
 ${eval ${call offset,sectors_os_size,   $${bytes_os_size}    / $${sector_size},                        bytes_os_size,}}
 ${eval ${call offset,tracks_os_size,    $${sectors_os_size}  / $${os_floppy_chs_s},                    sectors_os_size,}}
 
+# Primary GPT
+${eval ${call offset,bytes_gpt_header_start, $${sector_size},,                                         }}
+
 # round up
 ${eval ${call offset,bytes_iso_size,    $$$$(utils/file-length.sh -c ${bld}/os.iso),             ,${bld}/os.iso}}
 ${eval ${call offset,sectors_iso_size,  ${call div_round_up,$${bytes_iso_size},$${sector_size}},       bytes_iso_size,}}
@@ -176,6 +188,7 @@ ${eval ${call offset,bytes_fat12_end,        $${bytes_fat12_start} + $${bytes_fa
 # It is probably not necessary to align the GPT mirror end on a track boundary.
 ${eval ${call offset,bytes_gpt_mirror_size,  $${sectors_gpt_mirror_size} * $${sector_size},              sectors_gpt_mirror_size,}}
 ${eval ${call offset,bytes_gpt_mirror_end,   $${bytes_fat12_end} + $${bytes_gpt_mirror_size},            bytes_fat12_end bytes_gpt_mirror_size,}}
+${eval ${call offset,bytes_gpt_header_mirror_start, $${bytes_gpt_mirror_end} - $${sector_size},          bytes_gpt_mirror_end,}}
 ${eval ${call offset,bytes_gpt_mirror_start, $${bytes_gpt_mirror_end} - $${bytes_gpt_mirror_size},       bytes_gpt_mirror_end bytes_gpt_mirror_size,}}
 ${eval ${call offset,bytes_zip_end,          $${bytes_os_size},                                          bytes_os_size,}}
 
@@ -221,22 +234,27 @@ ${bld}/os.zip.adjusted: ${bld}/os.zip ${dep_bytes_zip_start} ${bld}/check_makefi
 
 gdisk_pipe_commands_slowly=while read str; do echo "$$str"; printf "\033[1;33m%s\033[m\n" "$$str" >&2; sleep 0.01; done
 
-commit_hash_as_guid=$$((git log -1 --pretty=format:%H || echo 0000000000000000000000000000000000000000) | sed -e 's/^\(.\{8\}\)\(.\{4\}\)\(.\{4\}\)\(.\{4\}\)\(.\{12\}\).*$$/\1-\2-\3-\4-\5/' | tr '[:lower:]' '[:upper:]')
-git_dirty=test -n "$$(git diff --shortstat)"
-gpt_disk_guid=${commit_hash_as_guid}$$(if $$git_dirty; then printf '0'; else printf '2'; fi)
-gpt_partition_guid=${commit_hash_as_guid}$$(if $$git_dirty; then printf '1'; else printf '3'; fi)
+# The last digit of guid1 is 1, for guid2 it's 2, and so on.
+# This allows us to have a disk GUID which is different from the partition GUID
+${bld}/gpt_guid%: ${bld}/os_with_guid_0 ${bld}/check_makefile
+	  sha256sum $< \
+	| sed -e 's/^\(.\{8\}\)\(.\{4\}\)\(.\{4\}\)\(.\{4\}\)\(.\{11\}\).*$$/\1-\2-\3-\4-\5$*/' \
+	| tr '[:lower:]' '[:upper:]' \
+	> $@
 
-${os_filename}: ${bld}/os.32k ${bld}/os.iso ${bld}/os.fat12 ${bld}/os.zip.adjusted \
-                ${dep_bytes_header_32k_start} \
-                ${dep_bytes_header_32k_size} \
-                ${dep_bytes_fat12_start} \
-                ${dep_bytes_fat12_size} \
-                ${dep_bytes_gpt_mirror_start} \
-                ${dep_bytes_gpt_mirror_end} \
-                ${dep_sectors_fat12_start} \
-                ${dep_sectors_fat12_size} \
-                ${dep_bytes_zip_start} \
-                ${bld}/check_makefile
+gpt_disk_guid=${bld}/gpt_guid1
+gpt_partition_guid=${bld}/gpt_guid2
+
+${bld}/os_with_guid_0: ${bld}/os.32k ${bld}/os.iso ${bld}/os.fat12 \
+                       ${dep_bytes_header_32k_start} \
+                       ${dep_bytes_header_32k_size} \
+                       ${dep_bytes_fat12_start} \
+                       ${dep_bytes_fat12_size} \
+                       ${dep_bytes_gpt_mirror_start} \
+                       ${dep_bytes_gpt_mirror_end} \
+                       ${dep_sectors_fat12_start} \
+                       ${dep_sectors_fat12_size} \
+                       ${bld}/check_makefile
 	rm -f $@
 # start with the .iso
 	cp ${bld}/os.iso $@
@@ -266,8 +284,23 @@ ${os_filename}: ${bld}/os.32k ${bld}/os.iso ${bld}/os.fat12 ${bld}/os.zip.adjust
 	 printf "1\nN\n"; \
 	 printf "01\nY\nN\n"; \
 	 printf "x\n"; \
-	 printf "g\n${gpt_disk_guid}\n"; \
-	 printf "c\n${gpt_partition_guid}\n"; \
+	 printf "g\n00000000-0000-0000-0000-000000000000\n"; \
+	 printf "c\n00000000-0000-0000-0000-000000000000\n"; \
+	 printf "p\no\nw\nY\n") | ${gdisk_pipe_commands_slowly} | gdisk $@
+
+${bld}/os_with_guid_hash: ${bld}/os_with_guid_0 \
+                          ${gpt_disk_guid} ${gpt_partition_guid} \
+                          ${bld}/check_makefile
+	cp $< $@
+#       gdisk commands:
+#         * eXpert mode,
+#           * Change partition GUID, the GUID itself,
+#           * change disk GUID, the GUID itself,
+#           * Print GPT, print prOtective MBR, Write, Proceed.
+	(if test "$$(uname -o)" = "Cygwin"; then printf "Y\n"; fi; \
+	 printf "x\n"; \
+	 printf "g\n$$(cat ${gpt_disk_guid})\n"; \
+	 printf "c\n$$(cat ${gpt_partition_guid})\n"; \
 	 printf "p\no\nw\nY\n") | ${gdisk_pipe_commands_slowly} | gdisk $@
 # Inject MS-DOS newlines (CR+LF) and comments (":: ") in the GUID field of unused partition table entries,
 # so that the part that is to be skipped by MS-DOS does not form a line longer than the MS-DOS maximum
@@ -277,6 +310,49 @@ ${os_filename}: ${bld}/os.32k ${bld}/os.iso ${bld}/os.fat12 ${bld}/os.zip.adjust
 	  printf "\r\n:: %02x" $$i | dd bs=1 seek=$$(( 1024 + ( ($$i) - 1) * 128 + 16)) count=7 conv=notrunc of=$@; \
 	  printf "\r\n:: %02x" $$i | dd bs=1 seek=$$(( ${bytes_gpt_mirror_start} + ( ($$i) - 1) * 128 + 16)) count=7 conv=notrunc of=$@; \
 	done
+
+%.partition_array: % ${bld}/check_makefile
+# Extract the partition array header
+	dd if=$< bs=512 skip=2 count=32 of=$@
+
+# The GPT header of the given disk image, with its CRC adjusted to the actual
+# data. It is used to fix the CRC after injecting MS-DOS comments in ignored
+# parts of the partition array.
+%.gpt_header_fixed_crc: % %.partition_array % ${bld}/check_makefile
+# Extract the GPT header
+	dd if=$< bs=512 skip=1 count=1 of=$@
+# * Compute the CRC32 of the particion array
+# * Convert endianness
+# * Convert to binary
+# * Inject into the GPT header
+	  crc32 $*.partition_array \
+	| sed -e 's/^\(..\)\(..\)\(..\)\(..\)$$/\4\3\2\1/' \
+	| xxd -r -ps \
+	| dd bs=1 seek=88 count=4 conv=notrunc of=$@
+# Zero-out the CRC of the header itself
+	printf '00000000' | xxd -r -ps | dd bs=1 seek=16 count=4 conv=notrunc of=$@
+# * Compute the CRC32 of the GPT header (with the CRC field itself zeroed out)
+# * Convert endianness
+# * Convert to binary
+# * Inject into the GPT header
+	  crc32 $@ \
+	| sed -e 's/^\(..\)\(..\)\(..\)\(..\)$$/\4\3\2\1/' \
+	| xxd -r -ps \
+	| dd bs=1 seek=16 count=4 conv=notrunc of=$@
+
+%.fixed_gpt_crc: % %.gpt_header_fixed_crc \
+                 ${dep_bytes_gpt_header_start} ${dep_bytes_gpt_header_mirror_start} \
+                 ${bld}/check_makefile
+	cp $< $@
+#	Inject the fixed GPT header
+	dd if=$*.gpt_header_fixed_crc bs=1 seek=${bytes_gpt_header_start}        count=${sector_size} conv=notrunc of=$@
+#	Inject the fixed GPT header in the mirror
+	dd if=$*.gpt_header_fixed_crc bs=1 seek=${bytes_gpt_header_mirror_start} count=${sector_size} conv=notrunc of=$@
+
+${os_filename}: ${bld}/os_with_guid_hash.fixed_gpt_crc ${bld}/os.zip.adjusted \
+                ${dep_bytes_zip_start} \
+                ${bld}/check_makefile
+	cp -f $< $@
 # splice in zip at the end
 	set -x; dd skip=${bytes_zip_start} seek=${bytes_zip_start} bs=1 conv=notrunc if=${bld}/os.zip.adjusted of=$@
 	chmod a+x-w $@
diff --git a/guix.scm b/guix.scm
index 898b5de..0625496 100644
--- a/guix.scm
+++ b/guix.scm
@@ -31,7 +31,9 @@
              (gnu packages cdrom)
              (gnu packages compression)
              (gnu packages disk)
-             (gnu packages linux))
+             (gnu packages vim)
+             (gnu packages linux)
+             (gnu packages perl))
 
 ;; For faketime
 (use-modules (guix git-download))
@@ -106,7 +108,10 @@
         ("zip" ,zip)
         ("faketime" ,faketime)
         ("gdisk" ,gptfdisk)
-        ("column" ,util-linux)))
+        ("xxd" ,xxd)
+        ("column" ,util-linux)
+        ;; perl is needed as an extra dependency to get crc32 to work.
+        ("perl" ,perl) ("crc32" ,perl-archive-zip)))
      (description "Test framework to run an OS in multiple emulators, as a guest graphical / text shell on linux, and so on.")
      (home-page "https://github.com/jsmaniac/os-test-framework")
      (license "CC0-1.0")