One of the functions performed in a couple of places in the sandbox program is to display the contents of the network Rx data block on the LCD, taking into account the number of bytes of available data, as indicated by bRxLength. This is done by the following subroutine:

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;----- Display contents (up to 30 bytes) of Xwire network Rx data block
RxDCount:               defBYTE                 ;local loop counter
RxDisplay:
        OBLCD_Cls
        Recall          bRxLength               ;Get the byte count
        Push                                    ;Duplicate
        GoIfNZ          RxD1                    ;Test there's something there
        OBLCD_Text      "Nothing frm srvr"      ;Oops!
        Return
RxD1:
        Store           RxDCount                ;Save the loop counter
        LoadI           0                       ;Index into the Rx data block
RxDispLoop:
        iRecall         abRxData                ;Get the next byte
        OBLCD_CharX                             ;Display
        IncI                                    ;Increment the read index to the next character
        DMGNZ           RxDCount,RxDispLoop     ;Decrement and test the loop counter
        Return                                  ;All done

The main trick here is using the index register I to access a range of data bytes, while using a memory location (RxDCount) to count down the bytes being moved.