diff --git a/meta-phosphor/recipes-phosphor/mboxd/mboxd/mboxd.service b/meta-phosphor/recipes-phosphor/mboxd/mboxd/mboxd.service
index c491b18..df6a59a 100644
--- a/meta-phosphor/recipes-phosphor/mboxd/mboxd/mboxd.service
+++ b/meta-phosphor/recipes-phosphor/mboxd/mboxd/mboxd.service
@@ -7,7 +7,7 @@
 
 [Service]
 Restart=always
-ExecStart=/usr/bin/env mboxd --flash {FLASH_SIZE} --window-size 1M
+ExecStart=/usr/bin/env mboxd --flash {FLASH_SIZE} --window-size 1M --window-num {WINDOW_NUM}
 SyslogIdentifier=mboxd
 
 [Install]
diff --git a/meta-phosphor/recipes-phosphor/mboxd/mboxd_git.bb b/meta-phosphor/recipes-phosphor/mboxd/mboxd_git.bb
index 13f6886..164f5d1 100644
--- a/meta-phosphor/recipes-phosphor/mboxd/mboxd_git.bb
+++ b/meta-phosphor/recipes-phosphor/mboxd/mboxd_git.bb
@@ -18,6 +18,7 @@
 
 S = "${WORKDIR}/git"
 SYSTEMD_SUBSTITUTIONS += "FLASH_SIZE:${MBOXD_FLASH_SIZE}:${PN}.service"
+SYSTEMD_SUBSTITUTIONS += "WINDOW_NUM:${MBOXD_WINDOW_NUM}:${PN}.service"
 SYSTEMD_SERVICE:${PN} += "mboxd.service"
 SYSTEMD_SERVICE:${PN} += "mboxd-reload@.service"
 SYSTEMD_LINK:${PN} += "${@compose_list(d, 'FMT', 'OBMC_HOST_INSTANCES')}"
@@ -31,6 +32,8 @@
 }
 
 MBOXD_FLASH_SIZE ??= "32M"
+# When 0, code will default to use the entire reserved memory region
+MBOXD_WINDOW_NUM ??= "0"
 TMPL = "mboxd-reload@.service"
 TGTFMT = "obmc-host-startmin@{0}.target"
 INSTFMT = "mboxd-reload@{0}.service"