diff --git a/bios/bios-rom.S b/bios/bios-rom.S
index 6dec953..8a53dcd 100644
--- a/bios/bios-rom.S
+++ b/bios/bios-rom.S
@@ -77,7 +77,7 @@ ENTRY(bios_int15)
 	pushl	%ebx
 	pushl	%eax
 
-	movl	%esp, %eax
+	movl	%esp, %eax	# it's bioscall case
 	call	e820_query_map
 
 	popl	%eax
diff --git a/bios/e820.c b/bios/e820.c
index 7f3f86f..baa2c5b 100644
--- a/bios/e820.c
+++ b/bios/e820.c
@@ -18,7 +18,7 @@ static inline uint8_t rdfs8(unsigned long addr)
 	return v;
 }
 
-void e820_query_map(struct e820_query *query)
+bioscall void e820_query_map(struct e820_query *query)
 {
 	uint8_t map_size;
 	uint16_t fs_seg;
diff --git a/include/kvm/bios.h b/include/kvm/bios.h
index 1e77770..dd70c44 100644
--- a/include/kvm/bios.h
+++ b/include/kvm/bios.h
@@ -62,4 +62,10 @@
 #define BIOS_NEXT_IRQ_ADDR(addr, size)	\
 	ALIGN((addr + size + 1), 16)
 
+/*
+ * When interfere with assembler code we need to be sure how
+ * arguments are passed in real mode.
+ */
+#define bioscall __attribute__((regparm(3)))
+
 #endif /* BIOS_H_ */