**Not mandatory.** Only needed if a VSCP boot-loader algorithm is used.

Begin transfer of data for a block of memory. This event has no meaning for any node that is not in boot mode and should be disregarded.

 | Data byte | Description | 
 | :---------: | ----------- | 
 | 0         | MSB of block number. | 
 | 1         | Block number. | 
 | 2         | Block number. | 
 | 3         | LSB of block number. | 
 | 4         | (optional) Type of Memory we want to write. See table below | 
 | 5         | (optional) Bank/Image to be written Used together with byte 4 to specify either separate Flash or EEPROM/MRAM spaces. If absent or set to zero normally, means first memory from the view of the node creator, e.g. internal Flash, internal EEPROM etc. Useful for projects that have internal as well as external EEPROMs so the external one could be addressed with byte5=1. Also with byte4=0 and byte5=1 an SD-Card as well as a second firmware image inside the flash could be addressed. |

**Type of memory to write (byte 4)**


 | Memory type | Description | 
 | :-----------: | ----------- | 
 | 0 or byte absent | PROGRAM Flash (status quo for old nodes) | 
 | 1 | DATA (EEPROM, MRAM, FRAM) | 
 | 2 | CONFIG (Fuses, CPU configuration) | 
 | 3 | RAM | 
 | 4-255  | Currently undefined - send a NACK as response | 

Response can be 

   [CLASS1.PROTOCOL, Type=50 (Start block data transfer ACK)](./class1.protocol.md#type50) 
   
   or 
   
   [CLASS1.PROTOCOL, Type=51 (Start block data transfer NACK)](./class1.protocol.md#type51).

