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Elite on the BBC Micro

Equipment: refund (Disc version)

Name: refund [View in context] Type: Subroutine [Compare versions] Category: Equipment Summary: Install a new laser, processing a refund if applicable
Arguments: A The power of the new laser to be fitted X The view number for fitting the new laser Returns: A A is preserved X X is preserved
IF _STH_DISC NOP \ In the first version of disc Elite, there was a nasty NOP \ bug where buying a laser that you already owned NOP \ affected your credit balance, so if you were clever, NOP \ you could keep doing this to get as many credits as NOP \ you liked. This was quickly fixed by replacing the NOP \ incorrect code with NOPs, which is what we have here NOP NOP NOP .refund STA T1 \ Store A in T1 so we can retrieve it later LDA LASER,X \ If there is no laser in view X (i.e. the laser power BEQ ref3 \ is zero), jump to ref3 to skip the refund code ELIF _IB_DISC \ In the first version of disc Elite, there was a nasty \ bug where buying a laser that you already owned \ affected your credit balance. This was quickly fixed \ by replacing the incorrect code with NOPs, but the \ version on Ian Bell's website contains this bug, and \ this is the section responsible for the problem .ref2 LDY #187 \ Print out the error: "LASER PRESENT" and refund the JMP pres \ value of the laser, returning from the subroutine \ using a tail call. This is the cause of the refund \ bug, as pres is called with the laser power in A \ rather than the item number, so the prx routine \ fetches a refund price from a location well outside \ the prxs table. This means that depending on which \ type of laser you are attempting to buy, your credit \ level will go up or down, when it shouldn't change at \ all .refund STA T1 \ Store A in T1 so we can retrieve it later LDA LASER,X \ If there is no laser in view X (i.e. the laser power BEQ ref3 \ is zero), jump to ref3 to skip the refund code CMP T1 \ If we are trying to replace a laser with one of the BEQ ref2 \ same type, jump up ref2 above ENDIF LDY #4 \ If the current laser has power #POW (pulse laser), CMP #POW \ jump to ref1 with Y = 4 (the item number of a pulse BEQ ref1 \ laser in the table at PRXS) LDY #5 \ If the current laser has power #POW+128 (beam laser), CMP #POW+128 \ jump to ref1 with Y = 5 (the item number of a beam BEQ ref1 \ laser in the table at PRXS) LDY #12 \ If the current laser has power #Armlas (military CMP #Armlas \ laser), jump to ref1 with Y = 12 (the item number of a BEQ ref1 \ military laser in the table at PRXS) LDY #13 \ Otherwise this is a mining laser, so fall through into \ ref1 with Y = 13 (the item number of a mining laser in \ the table at PRXS) .ref1 \ We now want to refund the laser of type Y that we are \ exchanging for the new laser STX ZZ \ Store the view number in ZZnso we can retrieve it \ later TYA \ Copy the laser type to be refunded from Y to A JSR prx \ Call prx to set (Y X) to the price of equipment item \ number A JSR MCASH \ Call MCASH to add (Y X) to the cash pot LDX ZZ \ Retrieve the view number from ZZ .ref3 \ Finally, we install the new laser LDA T1 \ Retrieve the new laser's power from T1 into A STA LASER,X \ Set the laser view to the new laser's power RTS \ Return from the subroutine