Just from memory the cases mentioned:
So far as I can see, the 6809 lacks instructions that the 6800 has, such as aba, eorb
ABA becomes PSHS B; ADDA ,S+ - a nifty use of the autoincrement feature :)
EORB is still the same on a '09.
the transfer instructions (tba, tap and the like).
The 6809 uses a unified instruction, 'TFR' so
`TBA` becomes `TFR B,A` (1F 98) `TAP` becomes `TFR A,CC` (1F 8A) and so on. Here's a full table taken from a Motorola manual:
6800 6809 Replacement ABA PSHS B; ADDA ,S + CBA PSHS B; CMPA ,S + CLC ANDCC #$FE CLI ANDCC #$EF CLV ANDCC #$FD CPX CMPX P DES LEAS -1,5 DEX LEAX -1 ,X INS LEAS 1.-S INX LEAX 1.X LDAA LDA LDAB LDB ORAA ORA ORAB ORB PSHA PSHS A PSHB PSHS B PULA PULS A PULB PULS B SBA PSHS B; SUBA ,S+ SEC ORCC #$01 SEI ORCC #$10 SEV ORCC #$02 STAA STA STAB STB TAB TFR A,B; TST A TAP TFR A,CC TBA TFR B,A; TST A TPA TFR CC,A TSX TFR S,X TXS TFR X,S WAI CWAI #$FF (hope I didn't add any typo or screw replacements)