Diff: README.md

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+# Example for no XRAM configuration for targets with XRAM support
+
+This is a simple example to remove external SRAM (XRAM) support for a target which has it.
+Some customers may wish their final products not attached with XRAM for cost consideration.
+The no-XRAM configuration is provided for their evaluation.
+
+Currently, only the *NuMaker-PFM-NUC472* board (*NUMAKER_PFM_NUC472* target) has XRAM support, and it is taken as an example for explanation.
+
+## No XRAM configuration
+By default, XRAM support is enabled in the *NUMAKER_PFM_NUC472* target.
+To remove it, create a mbed configuration file `mbed_app.json` (see the [mbed configuration system](https://docs.mbed.com/docs/mbed-os-handbook/en/5.3/advanced/config_system/))
+in the root path of your application with the following content:
+
+```
+{
+    "target_overrides": {
+        "NUMAKER_PFM_NUC472": {
+            "target.extra_labels_remove": ["NU_XRAM_SUPPORTED"],
+            "target.extra_labels_add": ["NU_XRAM_UNSUPPORTED"]
+        }
+    }
+}
+```
+
+The *NUMAKER_PFM_NUC472* target supports `LWIP`. You could meet OOM issue on compiling `LWIP` examples because XRAM support has been removed.
+To avoid confusion, you could also remove the `LWIP` support:
+
+```
+{
+    "target_overrides": {
+        "NUMAKER_PFM_NUC472": {
+            "target.extra_labels_remove": ["NU_XRAM_SUPPORTED"],
+            "target.extra_labels_add": ["NU_XRAM_UNSUPPORTED"],
+            "target.features_remove": ["LWIP"]
+        }
+    }
+}
+```