( INT handler. Set RST 38 jump ) PC ORG @ 0x39 + !
AF PUSH, BEGIN,
SIO_ACTL INAi, ( RR0 ) 0x01 ANDi, ( is recv buf full? )
IFZ, ( nope, exit ) A 0x20 ( CMD 4 ) LDri, SIO_ACTL OUTiA,
    AF POP, EI, RETI, THEN,
HL PUSH,
HL SIOW> LDdi, A (HL) LDrr,
HL DECd, (HL) CPr, ( W> == R> ? )
IFNZ, ( buffer not full )
    ( get wr ptr ) HL SIO( LDd(i),
    L ADDr, IFC, H INCr, THEN, L A LDrr,
    ( fetch/write ) SIO_ADATA INAi, (HL) A LDrr,
    ( advance W> ) SIOW> LDA(i), A INCr,
    SIO_BUFSZ 1- ANDi, SIOW> LD(i)A,
THEN, HL POP, JR, AGAIN,