diff --git a/Tiny-RISCV-SoC-for-RFU.md b/Tiny-RISCV-SoC-for-RFU.md
index 42042a4..af7aa13 100644
--- a/Tiny-RISCV-SoC-for-RFU.md
+++ b/Tiny-RISCV-SoC-for-RFU.md
@@ -11,6 +11,10 @@ ZephyrOS seems to also include some RSA implementations, so that's useful.
 * Build SERVANT for CMOD A7: `fusesoc run --tool=vivado --target=cmod_a7_35t servant`
 * For faster RISCV-IM add multiply divide from `fusesoc library add mdu https://github.com/zeeshanrafique23/mdu`
 
+## Building ZephyrOS
+
+SERVs demo drivers and stuff are old. Really old. ZephyrOS 2.4.0 old, which also requires an old compiler (old binutils). sdk-ng v0.14.2 works well: https://github.com/zephyrproject-rtos/sdk-ng/releases/tag/v0.14.2
+
 ## Links
 
 * https://github.com/olofk/serv