Misemulation of RTI

[EtchedPixels]

The code as implemented looks at the current E flag in order to decide whether it is doing a fast or slow interrupt return. The processor pops cc then looks at E to decide what the rest of the frame is.

So it should be something like

        /* rti */
        case 0x3b:
                reg_cc = pull8(sp);
                cycles += 1;
                if (get_cc (FLAG_E)) {
                        inst_pul (0xfe, &reg_s, &reg_u, &cycles);
                } else {
                        inst_pul (0x80, &reg_s, &reg_u, &cycles);
                }
                cycles += 3;
                break;

Found because I've been using the e6809 code for another project and hit a case where it mattered.

[gtoal]

Appears to be correct according to https://www.maddes.net/m6809pm/appendix_a.htm#RTI - hard to believe that in all these years no-one has emulated any code that uses RTI, or at least hasn't noticed a problem with it, but that does appear to be the case. This is going to cause a lot of frantic editing as there are many emulators out there all based on the underlying code from vecx!

Thank you for finding this - well spotted!