Market: var

       Name: var                                                     [Show more]
       Type: Subroutine
   Category: Market
    Summary: Calculate QQ19+3 = economy * |economic_factor|
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * GVL calls var
             * TT151 calls var


 Set QQ19+3 = economy * |economic_factor|, given byte #1 of the market prices
 table for an item. Also sets the availability of alien items to 0.

 This routine forms part of the calculations for market item prices (TT151)
 and availability (GVL).


 Arguments:

   QQ19+1               Byte #1 of the market prices table for this market item
                        (which contains the economic_factor in bits 0-5, and the
                        sign of the economic_factor in bit 7)


.var

 LDA QQ19+1             \ Extract bits 0-5 from QQ19+1 into A, to get the
 AND #31                \ economic_factor without its sign, in other words:
                        \
                        \   A = |economic_factor|

 LDY QQ28               \ Set Y to the economy byte of the current system

 STA QQ19+2             \ Store A in QQ19+2

 CLC                    \ Clear the C flag so we can do additions below

 LDA #0                 \ Set AVL+16 (availability of alien items) to 0,
 STA AVL+16             \ setting A to 0 in the process

.TT153

                        \ We now do the multiplication by doing a series of
                        \ additions in a loop, building the result in A. Each
                        \ loop adds QQ19+2 (|economic_factor|) to A, and it
                        \ loops the number of times given by the economy byte;
                        \ in other words, because A starts at 0, this sets:
                        \
                        \   A = economy * |economic_factor|

 DEY                    \ Decrement the economy in Y, exiting the loop when it
 BMI TT154              \ becomes negative

 ADC QQ19+2             \ Add QQ19+2 to A

 JMP TT153              \ Loop back to TT153 to do another addition

.TT154

 STA QQ19+3             \ Store the result in QQ19+3

 RTS                    \ Return from the subroutine