For Cortex-M3 devices, the verify command is a two-step
sequence for each page to be verified, as follows:
1. Send the 0x80000000 value in the value field and the last
four bytes of the page in the data bytes field.
2. Send the page address in the value field and the result of
the SIGN command of the page in the data bytes field.
After receiving these two packets, the loader checks whether the
value received in Step 1 is correct and then obtains the SIGN value
for the page and compares it to the value from Step 2. If both values
are correct, an ACK is subsequently returned. An error in either
value results in a BEL. For details on the SIGN command, see the
appropriate device data sheet or hardware reference manual.