I agree that Andy is right: I tried the POP SBUF on a TI MSC1210. Your mileage may vary on other derivatives.

What isn't so clear is if you perform a read-modify-write instruction, where the read data comes from.

I also agree with Andrew. From the documents that have been cited in this thread it doesn't really look like there is much question that any instruction that uses SBUF as the destination register will cause data to be sent. I think, as Andrew does, that the main question is where/what data will be used in a read-modify-write instruction such as ORL.

I just tried some more tests on the MSC1210:

1. INC SBUF will write out a character one greater than the last character received. If the last character received was 'B' an INC SBUF will write out 'C'.

2. ORL SBUF will OR the value with the last character received and write it out. So, interestingly, you could use ORL SBUF,#00h to echo back the last character received. :)

Again, your mileage may vary with other derivatives but this worked with the TI MSC1210 since its the SBC I have on my desk right now. Perhaps next week I'll try the same code on the ADuC812 SBC and a Cygnal SBC.

Craig Steiner