Let's have a look on a quite sophisticated example for the usage of SMC. It not only modifies code, but also the modification of the code is modified - allowing reuse of some instructions.
The code is from my 'memset()'implementation:
1: ...
2: SMC_StoreAddress StoreAccuFirstSection
3:
4: StoreToFirstSection:
5: SMC StoreAccuFirstSection, { sta SMC_AbsAdr, Y }
6: ...
7: RestoreCodeBranchBaseAdr:
8: SMC FirstIncHighByte, { SMC_OperateOnHighByte inc, StoreAccuFirstSection } ; code will be overwritten to 'beq RestoreCode' (*)
9: ...
10: SMC_TransferOpcode FirstIncHighByte, OPC_BEQ , x ; change code marked above with (*)
11: SMC_TransferValue FirstIncHighByte, #(restoreCode - RestoreCodeBranchBaseAdr-2), x ; set relative adress to 'RestoreCode'
12: ...
13: restoreCode:
14: SMC_TransferOpcode FirstIncHighByte, OPC_INC_abs , x ; restore original code...
15: SMC_TransferValue FirstIncHighByte, #(<(StoreToFirstSection+2)), x ; (second byte of inc contained low-byte of adress)
16: ...
Some explanation:
Line 2: The register pair A/X contains an address, which is stored on the address location of a SMC line called 'StoreAccuFirstSection'. According to cc65's calling convention, the low-byte is in accu while the high-byte is in the X-register.
Line 5: The (modified) address is accessed.
Line 8: We have a line here, which is about to be modified (it begins with SMC), but itself modifies code. Please note: Contrary to the rest of SMC-line modifying macros, the 'OperateOn'-macros just expand their given arguments into a single instruction line. These can be changed of course too.
Line 10,11: These lines construct a branch operation for line 8: The X-register will be used to change it from 'inc StoreAccuFirstSection+2' (high-byte operation) to 'beq restoreCode'. Please note: To calculate the relaive branch offset, we introduced a second label ('RestoreCodeBranchBaseAdr') for to calculate it. Some could also use the internal name of the SMC label, but you should abstain to do so - it may be changed in the future...
Line 14,15: The original code from line 8 is reestablished.