diff --git a/Makefile b/Makefile
index a26f3db..0893966 100644
--- a/Makefile
+++ b/Makefile
@@ -151,6 +151,7 @@ UPROGS=\
 	$U/_forphan\
 	$U/_dorphan\
 	$U/_sync\
+	$U/_readcount\
 
 fs.img: mkfs/mkfs README $(UPROGS)
 	mkfs/mkfs fs.img README $(UPROGS)
diff --git a/kernel/syscall.c b/kernel/syscall.c
index 1800f20..45cd5e2 100644
--- a/kernel/syscall.c
+++ b/kernel/syscall.c
@@ -103,6 +103,7 @@ extern uint64 sys_link(void);
 extern uint64 sys_mkdir(void);
 extern uint64 sys_close(void);
 extern uint64 sys_sync(void);
+extern uint64 sys_getreadcount(void);
 
 // An array mapping syscall numbers from syscall.h
 // to the function that handles the system call.
@@ -130,6 +131,7 @@ static uint64 (*syscalls[])(void) = {
   [SYS_mkdir]   = sys_mkdir,
   [SYS_close]   = sys_close,
   [SYS_sync]    = sys_sync,
+  [SYS_getreadcount] = sys_getreadcount,
   // clang-format on
 };
 
diff --git a/kernel/syscall.h b/kernel/syscall.h
index 5a4a39f..b337d29 100644
--- a/kernel/syscall.h
+++ b/kernel/syscall.h
@@ -21,3 +21,4 @@
 #define SYS_mkdir  20
 #define SYS_close  21
 #define SYS_sync   22
+#define SYS_getreadcount 23
diff --git a/kernel/sysfile.c b/kernel/sysfile.c
index 7781287..7390195 100644
--- a/kernel/sysfile.c
+++ b/kernel/sysfile.c
@@ -65,6 +65,9 @@ sys_dup(void)
   return fd;
 }
 
+// Llamadas a read() desde el arranque.
+uint64 readcount = 0;
+
 uint64
 sys_read(void)
 {
@@ -72,6 +75,9 @@ sys_read(void)
   int n;
   uint64 p;
 
+  // Varios CPUs a la vez: suma atómica.
+  __atomic_fetch_add(&readcount, 1,
+                     __ATOMIC_RELAXED);
   argaddr(1, &p);
   argint(2, &n);
   if (argfd(0, 0, &f) < 0)
diff --git a/kernel/sysproc.c b/kernel/sysproc.c
index cd25557..3a9f5da 100644
--- a/kernel/sysproc.c
+++ b/kernel/sysproc.c
@@ -22,6 +22,15 @@ sys_getpid(void)
   return myproc()->pid;
 }
 
+extern uint64 readcount; // en kernel/sysfile.c
+
+uint64
+sys_getreadcount(void)
+{
+  // Un solo ld de 64 bits: no se corta.
+  return readcount;
+}
+
 uint64
 sys_fork(void)
 {
diff --git a/user/readcount.c b/user/readcount.c
new file mode 100644
index 0000000..b775e6b
--- /dev/null
+++ b/user/readcount.c
@@ -0,0 +1,17 @@
+#include "kernel/types.h"
+#include "kernel/fcntl.h"
+#include "user/user.h"
+
+int
+main(void)
+{
+  char buf[64];
+  int before = getreadcount();
+  int fd = open("README", O_RDONLY);
+  while (read(fd, buf, sizeof(buf)) > 0)
+    ;
+  close(fd);
+  int after = getreadcount();
+  printf("antes %d, despues %d\n", before, after);
+  exit(0);
+}
diff --git a/user/user.h b/user/user.h
index 29aad59..501d3bb 100644
--- a/user/user.h
+++ b/user/user.h
@@ -25,6 +25,7 @@ char *sys_sbrk(int, int);
 int pause(int);
 int uptime(void);
 int sync(void);
+int getreadcount(void);
 
 // ulib.c
 int stat(const char *, struct stat *);
diff --git a/user/usys.pl b/user/usys.pl
index 4d054e8..f5d3cd7 100755
--- a/user/usys.pl
+++ b/user/usys.pl
@@ -43,3 +43,4 @@ entry("sbrk");
 entry("pause");
 entry("uptime");
 entry("sync");
+entry("getreadcount");