diff --git a/detect.c b/detect.c
index f229d70..9954865 100644
--- a/detect.c
+++ b/detect.c
@@ -59,7 +59,7 @@ unsigned int init_pci(unsigned char bus) {
 	int mem = open("/dev/mem", O_RDONLY);
 	if (mem < 0) die(_("Can't open /dev/mem, are you root?"));
 
-	area = mmap(NULL, 4, PROT_READ, MAP_PRIVATE, mem,
+	area = mmap(NULL, MMAP_SIZE, PROT_READ, MAP_PRIVATE, mem,
 			dev->regions[2].base_addr + 0x8000);
 	if (area == MAP_FAILED) die(_("mmap failed"));
 
diff --git a/radeontop.c b/radeontop.c
index cbac6e6..58b1074 100644
--- a/radeontop.c
+++ b/radeontop.c
@@ -109,6 +109,6 @@ int main(int argc, char **argv) {
 
 	present(ticks, cardname, color);
 
-	munmap((void *) area, 4);
+	munmap((void *) area, MMAP_SIZE);
 	return 0;
 }
diff --git a/radeontop.h b/radeontop.h
index 630f219..b7edfd0 100644
--- a/radeontop.h
+++ b/radeontop.h
@@ -32,6 +32,7 @@
 #include <string.h>
 
 #define GRBM_STATUS 0x8010
+#define MMAP_SIZE 0x14
 
 // radeontop.c
 void die(const char *why);