diff --git a/target/arm/arm-semi.c b/target/arm/arm-semi.c
index 86e6ec48c2..53e807ab72 100644
--- a/target/arm/arm-semi.c
+++ b/target/arm/arm-semi.c
@@ -331,13 +331,15 @@ target_ulong do_arm_semihosting(CPUARMState *env)
         } else {
             s = lock_user(VERIFY_READ, arg1, len, 1);
             if (!s) {
-                /* FIXME - should this error code be -TARGET_EFAULT ? */
-                return (uint32_t)-1;
+                /* Return bytes not written on error */
+                return len;
             }
             ret = set_swi_errno(ts, write(arg0, s, len));
             unlock_user(s, arg1, 0);
-            if (ret == (uint32_t)-1)
-                return -1;
+            if (ret == (uint32_t)-1) {
+                ret = 0;
+            }
+            /* Return bytes not written */
             return len - ret;
         }
     case TARGET_SYS_READ:
@@ -352,15 +354,17 @@ target_ulong do_arm_semihosting(CPUARMState *env)
         } else {
             s = lock_user(VERIFY_WRITE, arg1, len, 0);
             if (!s) {
-                /* FIXME - should this error code be -TARGET_EFAULT ? */
-                return (uint32_t)-1;
+                /* return bytes not read */
+                return len;
             }
             do {
                 ret = set_swi_errno(ts, read(arg0, s, len));
             } while (ret == -1 && errno == EINTR);
             unlock_user(s, arg1, len);
-            if (ret == (uint32_t)-1)
-                return -1;
+            if (ret == (uint32_t)-1) {
+                ret = 0;
+            }
+            /* Return bytes not read */
             return len - ret;
         }
     case TARGET_SYS_READC: