diff --git a/scp.c b/scp.c
index c436e984..38153645 100644
--- a/scp.c
+++ b/scp.c
@@ -1845,6 +1845,18 @@ return 0;
 }
 #endif
 
+/* Convert switch letter to bit mask */
+
+int32 sim_swmask (int switch_ch)
+{
+static char *letters = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
+char *index = strchr (letters, switch_ch);
+
+if (index)
+    return (1u << (int)(index - letters));
+return 0;                   /* shouldn't happen */
+}
+
 t_stat process_stdin_commands (t_stat stat, char *argv[]);
 
 /* Main command loop */
diff --git a/scp.h b/scp.h
index a205335e..23d526c7 100644
--- a/scp.h
+++ b/scp.h
@@ -116,6 +116,7 @@ t_stat sim_run_boot_prep (void);
 double sim_gtime (void);
 uint32 sim_grtime (void);
 int32 sim_qcount (void);
+int32 sim_swmask (int switch_ch);
 t_stat attach_unit (UNIT *uptr, char *cptr);
 t_stat detach_unit (UNIT *uptr);
 t_stat assign_device (DEVICE *dptr, char *cptr);
diff --git a/sim_defs.h b/sim_defs.h
index 4e5e28f0..8c6e6a7d 100644
--- a/sim_defs.h
+++ b/sim_defs.h
@@ -278,6 +278,10 @@ typedef uint32          t_addr;
 #define SIM_SW_REG      (1u << 28)                      /* register value */
 #define SIM_SW_STOP     (1u << 29)                      /* stop message */
 
+/* Convert switch letter to bit mask */
+
+#define SWMASK(x) sim_swmask ((int)x)
+
 /* Simulator status codes
 
    0                    ok
@@ -358,10 +362,6 @@ typedef uint32          t_addr;
 #define KBD_LIM_WAIT(x) (((x) > KBD_MAX_WAIT)? KBD_MAX_WAIT: (x))
 #define KBD_WAIT(w,s)   ((w)? w: KBD_LIM_WAIT (s))
 
-/* Convert switch letter to bit mask */
-
-#define SWMASK(x) (1u << (((int) (x)) - ((int) 'A')))
-
 /* String match - at least one character required */
 
 #define MATCH_CMD(ptr,cmd) ((NULL == (ptr)) || (!*(ptr)) || strncmp ((ptr), (cmd), strlen (ptr)))