Let's forget about interrupts for a moment.

Here is code trying to count number of bytes UART received:

#include <reg52.h>
#define uchar unsigned char

void Init (void)
{
TMOD = 0x20;
TL1 = 0xff;
TH1 = 0xff;
TCON = 0x50;
SCON = 0x70;
TR1 =1;
}

void main (void)
{
uchar i=0;
Init();
P2=0;
while(1)
{
if(RI)
{
i++;
RI=0;
P2=i;
}
}
}

I am sending pairs NoteOn NoteOff from MIDIOX (6 bytes) with large pause (few seconds) between the pairs.

After first pair i=1; (?? 6 bytes should be received)
After next pair i=4
then i=7
then i=11, ETC

Note that i do not change on single NoteOn or single NoteOff

Strange isn't it, unless i am missing something obvious?

Thanks