Add notes about compressed instructions

README.md

@@ -18,8 +18,24 @@ In u-boot use the following to boot:
 => booti 0x84000000 - ${fdtcontroladdr} 
 ```
 
-## Notes
+## Use more than 1 CPU
 
-- With only 1 CPU some thigs are soo slow to work. For example, the ttyS0 device
+With only 1 CPU some thigs are soo slow to work. For example, the ttyS0 device
 doesn't arrive on time and the login fails. Or the SSH daemon doesn't start
 until the third try. With -smp 32 seems to work ok.
+
+## Compressed instructions
+
+To see if a binary has compressed instructions or not, you can use objdump:
+
+    hut% nix shell -vL nixpkgs#pkgsCross.riscv64.bintools
+    hut% objdump -M no-aliases -D result/u-boot | grep 'c\.' | head -5
+        80200000:   822a                    c.mv    tp,a0
+        80200002:   84ae                    c.mv    s1,a1
+        80200018:   52c1                    c.li    t0,-16
+        8020001e:   0356                    c.slli  t1,0x15
+        80200024:   850a                    c.mv    a0,sp
+
+To dump a ROM without the ELF header:
+
+    hut% objdump -b binary -m riscv:rv64 -M no-aliases -D result/u-boot.bin