diff --git a/PDP10/pdp10_sys.c b/PDP10/pdp10_sys.c
index 3ff943bb..ed2d207f 100644
--- a/PDP10/pdp10_sys.c
+++ b/PDP10/pdp10_sys.c
@@ -189,12 +189,12 @@ extern d10 rot (d10 val, a10 ea);
 data = getrimw (fileref);                               /* get first word */
 if ((data < 0) || ((data & AMASK) != 0))                /* error? SA != 0? */
     return SCPE_FMT;
-ldrc = (-((int32) LRZ (data))) & 0777777;               /* get loader count */
+ldrc = 01000000 - ((int32) (LRZ (data)));               /* get loader count */
 if (ldrc == 016)                                        /* 16? RIM10B */
     its_rim = FALSE;
 else if (ldrc == 017)                                   /* 17? ITS RIM */
     its_rim = TRUE;
-else SCPE_FMT;                                          /* unknown */
+else return SCPE_FMT;                                   /* unknown */
 
 for (i = 0; i < ldrc; i++) {                            /* skip the loader */
     data = getrimw (fileref);