hi,

except purely formal - push ACC, pop ACC

It depends on compiler you use. For example, A.D. 2500 requires exactly "push a".

why are you using External interrupts, why not Serial?

Why not? As I can see from that example, he sends byte(s) after external interrupt comes on. So INT1_ISR is good place for that.

I always use Serial for Receive (or test RI), but I disable all interrupts for Transmit, just wait to have byte sent (jnb TI,$).

Then you are happy guy. I cannot allow myself to waste time for dummy wait loops like you. There are many other [useful] things my MCU must do at these microseconds. Try to calculate how much commands may be done during one byte is sent via UART even at 115200 bitrate and you understand me.

Regards,
Oleg