Skip to navigation

Revs on the BBC Micro

Extra tracks: ModifyGameCode (Part 1 of 4)

Name: ModifyGameCode (Part 1 of 4) [Show more] Type: Subroutine Category: Extra tracks Summary: Modify the game code to support the extra track data
Context: See this subroutine in context in the source code References: This subroutine is called as follows: * CallTrackHook calls ModifyGameCode

The code modifications are done in four parts. The (modifyAddressHi modifyAddressLo) table contains the locations in the main game code that we want to modify. The (newContentHi newContentLo) table contains the new two-byte addresses that we want to poke into the main game code at the modify locations. This part also does a couple of single-byte modifications. This is also where the xTrackSegmentO table is built, once the modifications have been done. The routine is padded out to be exactly 40 bytes long, so there's one byte for each inner segment x-coordinate.
.ModifyGameCode LDX #18 \ We are about to modify 19 two-byte addresses in the \ main game code, so set a counter in X .mods1 LDA modifyAddressHi,X \ Set (U T) = the X-th entry in the (modifyAddressHi STA U \ modifyAddressLo) table, which contains the location LDA modifyAddressLo,X \ of the code to modify in the main game code STA T LDY #0 \ We now modify two bytes, so set an index in Y LDA newContentLo,X \ We want to modify the two-byte address at location \ (U T), setting it to the new address in the \ (newContentHi newContentLo) table, so set A to the \ low byte of the X-th entry from the table, i.e. to \ the low byte of the new address STA (T),Y \ Modify the byte at (U T) to the low byte of the new \ address in A INY \ Increment Y to point to the next byte LDA newContentHi,X \ Set A to the high byte of the X-th entry from the \ table, i.e. to the high byte of the new address STA (T),Y \ Modify the byte at (U T) + 1 to the high byte of the \ new address in A DEX \ Decrement the loop counter to move on to the next \ address to modify BPL mods1 \ Loop back until we have modified all 19 addresses LDA #&4C \ ?&261A = &4C (opcode for a JMP &xxxx instruction) STA &261A STA &248B \ ?&248B = &4C (opcode for a JMP &xxxx instruction) JMP mods2 \ Jump to part 2 EQUB &C8 \ This byte pads the routine out to exactly 40 bytes