collapseos/drv/acia.z80

( Save ACIA conf )
ACIA_CTL
: ACIA_CTL [ LITN ] ;
ACIA_IO
: ACIA_IO [ LITN ] ;
( Points to ACIA buf )
: ACIA( [ ACIA_MEM 4 + LITN ] ;
( Points to ACIA buf end )
: ACIA) [ ACIA_MEM 6 + LITN ] ;
( Read buf pointer. Pre-inc )
: ACIAR> [ ACIA_MEM LITN ] ;
( Write buf pointer. Post-inc )
: ACIAW> [ ACIA_MEM 2 + LITN ] ;
( This means that if W> == R>, buffer is full.
  If R>+1 == W>, buffer is empty. )

(entry) ~ACIA
    AF PUSHqq,
    HL PUSHqq,
    DE PUSHqq,

	( Read our character from ACIA into our BUFIDX )
    ACIA_CTL INAn,
    0x01 ANDn,   ( is ACIA rcv buf full? )
    JRZ, L2 FWR ( end, no, wrong interrupt cause. )

    ACIAW> LDHL(nn),
    ( is it == to ACIAR>? )
    DE ACIAR> LDdd(nn),
    ( carry cleared from ANDn above )
    DE SBCHLss,
    JRZ, L3 FWR ( end, buffer full )

    DE ADDHLss, ( restore ACIAW> )
    ( buffer not full, let's write )
    ACIA_IO INAn,
    (HL) A LDrr,

    ( advance W> )
    HL INCss,
    ACIAW> LD(nn)HL,
    DE ACIA) @ LDdd(nn),
    DE SUBHLss,
    JRNZ, L4 FWR ( skip )
    ( end of buffer reached )
    ACIA( @ LDHL(nn),
    ACIAW> LD(nn)HL,
L4 FSET ( skip )
L3 FSET L2 FSET ( end )

    DE POPqq,
    HL POPqq,
    AF POPqq,
    EI,
    RETI,