( ACIA INT handler, read into ACIAW> )
( Set RST 38 jump ) PC ORG @ 0x39 + !
    AF PUSH,
    ACIA_CTL INAi, 0x01 ANDi, ( is ACIA rcv buf full? )
    IFZ, ( no, abort ) AF POP, EI, RETI, THEN,
    HL PUSH,
    HL ACIAW> LDdi, A (HL) LDrr,
    HL DECd, (HL) CPr, ( W> == R> ? )
    IFNZ, ( buffer not full )
        ( get wr ptr ) HL ACIA( LDd(i),
        L ADDr, IFC, H INCr, THEN, L A LDrr,
        ( fetch/write ) ACIA_IO INAi, (HL) A LDrr,
        ( advance W> ) ACIAW> LDA(i), A INCr,
        ACIA_BUFSZ 1- ANDi, ACIAW> LD(i)A,
    THEN,
    HL POP, AF POP, EI, RETI,