; ECHO -- copy each day's uplink into the downlink buffer.
;
        .equ    RX      0               ; uplink buffer
        .equ    TX      1024            ; downlink buffer
        .equ    P_UPNEW 0x70            ; uplink-arrived flag / acknowledge
        .equ    P_UPLEN 0x71            ; uplink length in bytes

wait:   in      r1, P_UPNEW
        ldi     r2, 0
        beq     r1, r2, sleep           ; nothing new today
        in      r4, P_UPLEN
        ldi     r5, 0                   ; r5 = byte index
copy:   bge     r5, r4, done
        ldb     r6, [r5+RX]
        stb     r6, [r5+TX]
        addi    r5, 1
        jmp     copy
done:   out     P_UPNEW, r1             ; acknowledge
sleep:  yield
        jmp     wait