Could you amplify this a little bit ?

From what I figure out , in between every 1 serial interrupt ( i.e. in between two character received / sent ) you can process 50 instructions. Am I right ?
How do you arrive at this ? Any rough n ready calculator ?

115k in round figures is 1 character every 100usec

At 12 MHz a 12 clock '51 complete 1 instruction cycle every microsecond

Instructions take 1-4 instruction cycles to complete so a fair average is 2 cycles

so, without going into datails and exactness, this gives 50 instructions per byte recieved.

For a 6 clocker (such as P89C51Rx2 - P89C66x) running at 18,xxx MHz it would be 150 instructions per byte recieved.

Since you have no idea where in the main the processor is between interrupts, the abovs means of estimating is as goood as any. A note: "instruction" refer to assembler NOT C.

Erik