diff --git a/hw/display/sm501.c b/hw/display/sm501.c
index d5e165daef..cacb6c87ec 100644
--- a/hw/display/sm501.c
+++ b/hw/display/sm501.c
@@ -571,6 +571,25 @@ static uint32_t get_local_mem_size_index(uint32_t size)
     return index;
 }
 
+static void set_new_local_mem_size_index(SM501State *s, uint32_t idx)
+{
+    /*
+     * Update local_mem_size_index on guest write. We don't allow this
+     * to be set to larger than the actual RAM size. (The guest will
+     * still read back the SYSTEM_CONTROL.Size bits that it wrote.)
+     */
+    if (idx < ARRAY_SIZE(sm501_mem_local_size) &&
+        sm501_mem_local_size[idx] <= memory_region_size(&s->local_mem_region)) {
+        s->local_mem_size_index = idx;
+        return;
+    }
+    qemu_log_mask(LOG_GUEST_ERROR,
+                  "sm501: Guest set DRAM_CONTROL.Size to 0x%x but "
+                  "local memory is not that large\n",
+                  idx);
+    /* Don't change the effective size, leave it as whatever it was */
+}
+
 static ram_addr_t get_fb_addr(SM501State *s, int crt)
 {
     return (crt ? s->dc_crt_fb_addr : s->dc_panel_fb_addr) & 0x3FFFFF0;
@@ -990,7 +1009,7 @@ static uint64_t sm501_system_config_read(void *opaque, hwaddr addr,
         ret = 0x050100A0;
         break;
     case SM501_DRAM_CONTROL:
-        ret = (s->dram_control & 0x07F107C0) | s->local_mem_size_index << 13;
+        ret = (s->dram_control & 0x07F1E7C0);
         break;
     case SM501_ARBTRTN_CONTROL:
         ret = s->arbitration_control;
@@ -1049,8 +1068,7 @@ static void sm501_system_config_write(void *opaque, hwaddr addr,
         s->gpio_63_32_control = value & 0xFF80FFFF;
         break;
     case SM501_DRAM_CONTROL:
-        s->local_mem_size_index = (value >> 13) & 0x7;
-        /* TODO : check validity of size change */
+        set_new_local_mem_size_index(s, (value >> 13) & 0x7);
         s->dram_control &= 0x80000000;
         s->dram_control |= value & 0x7FFFFFC3;
         break;