avr: started working on base system vs. main system.

base system will be a base system which can be used to flash and start
the main system.

avr/com.asm

@@ -6,6 +6,7 @@
 
 .equ COM_MAXWAIT                   = 200  ; maximum loop count when waiting for rising/falling clock (TODO: Make frequency-dependant)
 
+.equ COM_MAINTENANCE_ADDR         = 0xf1
 
 .equ COM_BUFFER_FLAGS_DONE        = 0x80
 .equ COM_BUFFER_FLAGS_RECEIVED    = 0x40
@@ -36,6 +37,9 @@
 .equ COM_MSG_OFFS_DESTADDR        = 0
 .equ COM_MSG_OFFS_MSGLEN          = 1
 .equ COM_MSG_OFFS_MSGDATA         = 2
+.equ COM_MSG_OFFS_CMD             = 2 ; first at COM_MSG_OFFS_MSGDATA
+.equ COM_MSG_OFFS_SRCADDR         = 3
+.equ COM_MSG_OFFS_PAYLOAD         = 4 ; payload for the cmd follows here
 
 .equ COM_ERR_NOTFORME             = 1
 .equ COM_ERR_CHECKSUM             = 2
@@ -581,6 +585,8 @@ comReceivePacketToSram_acceptAddr:
   brcc comReceivePacketToSram_error
   eor r1, r16
   st X+, r16
+  cpi r16, (COM_BUFFER_SIZE-3-COM_BUFFER_OFFS_DATA)+1
+  brcc comReceivePacketToSram_error            ; packet too long (TODO: count overruns)
   tst r16
   breq comReceivePacketToSram_readXOR
   mov r17, r16