Equipment: refund [BBC Micro disc version, Docked]

       Name: refund                                                  [Show more]
       Type: Subroutine
   Category: Equipment
    Summary: Install a new laser, processing a refund if applicable
    Context: See this subroutine in context in the source code
 Variations: See code variations for this subroutine in the different versions
 References: This subroutine is called as follows:
             * EQSHP calls refund


 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