diff --git a/VAX/vax860_defs.h b/VAX/vax860_defs.h
index 61e6dbf5..58dc69f4 100644
--- a/VAX/vax860_defs.h
+++ b/VAX/vax860_defs.h
@@ -179,7 +179,7 @@
 #define MAXMEMWIDTH     25                              /* max mem, 4MB boards */
 #define MAXMEMSIZE      (1 << MAXMEMWIDTH)
 #define MAXMEMWIDTH_X   28                              /* max mem, 64MB boards */
-#define MAXMEMSIZE_X    (1 << MAXMEMWIDTH_X)
+#define MAXMEMSIZE_X    ((1 << MAXMEMWIDTH_X) + (1 << 22)) /* 8 64MB + 1 4MB */
 #define INITMEMSIZE     (1 << MAXMEMWIDTH)              /* initial memory size */
 #define MEMSIZE         (cpu_unit.capac)
 #define ADDR_IS_MEM(x)  (((uint32) (x)) < MEMSIZE)
@@ -190,7 +190,7 @@
                         { UNIT_MSIZE, (1u << 26), NULL, "64M", &cpu_set_size, NULL, NULL, "Set Memory to 64M bytes" },              \
                         { UNIT_MSIZE, (1u << 27), NULL, "128M", &cpu_set_size, NULL, NULL, "Set Memory to 128M bytes" },            \
                         { UNIT_MSIZE, (1u << 28), NULL, "256M", &cpu_set_size, NULL, NULL, "Set Memory to 256M bytes" },            \
-                        { UNIT_MSIZE, (1u << 27) + (1u << 22), NULL, "260M", &cpu_set_size, NULL, NULL, "Set Memory to 260M bytes" }
+                        { UNIT_MSIZE, (1u << 28) + (1u << 22), NULL, "260M", &cpu_set_size, NULL, NULL, "Set Memory to 260M bytes" }
 #define CPU_MODEL_MODIFIERS                                                                     \
                         { MTAB_XTD|MTAB_VDV, 0, "MODEL", NULL,                                  \
                               NULL, &cpu_show_model, NULL, "Display the simulator CPU Model" }