diff --git a/sys/boot/pc98/btx/btx/Makefile b/sys/boot/pc98/btx/btx/Makefile
index 229963228898..de14daade2a1 100644
--- a/sys/boot/pc98/btx/btx/Makefile
+++ b/sys/boot/pc98/btx/btx/Makefile
@@ -1,37 +1,46 @@
 # $FreeBSD$
 
 M4?=	m4
 
 .if defined(PAGING)
 M4FLAGS+=	-DPAGING
 .endif
 
 .if defined(BOOT_BTX_NOHANG)
 BOOT_BTX_FLAGS=0x1
 .else
 BOOT_BTX_FLAGS=0x0
 .endif
 
 AFLAGS+=	--defsym BTX_FLAGS=${BOOT_BTX_FLAGS}
 AFLAGS+=	--defsym PC98=1
 
+.if defined(BTX_SERIAL)
+BOOT_COMCONSOLE_PORT?= 0x238
+BOOT_COMCONSOLE_SPEED?= 9600
+B2SIOFMT?=	0x3
+
+M4FLAGS+=	-DBTX_SERIAL -DSIOPRT=${BOOT_COMCONSOLE_PORT} \
+		-DSIOFMT=${B2SIOFMT} -DSIOSPD=${BOOT_COMCONSOLE_SPEED}
+.endif
+
 ORG=	0x9000
 
 all: btx
 
 btx: btx.o
 .if ${OBJFORMAT} == aout
 	${LD} -nostdlib -N -s -T ${ORG} -o btx.out btx.o
 	dd if=btx.out of=${.TARGET} ibs=32 skip=1
 .else
 	${LD} -N -e start -Ttext ${ORG} -o btx.out btx.o
 	objcopy -S -O binary btx.out ${.TARGET}
 .endif
 
 btx.o: btx.s
 	(cd ${.CURDIR}; ${M4} ${M4FLAGS} btx.s) | \
 	   ${AS} ${AFLAGS} -o ${.TARGET}
 
 CLEANFILES+= btx btx.out btx.o
 
 .include <bsd.prog.mk>