Universe: GVL

       Name: GVL                                                     [Show more]
       Type: Subroutine
   Category: Universe
    Summary: Calculate the availability of market items
  Deep dive: Market item prices and availability
             Galaxy and system seeds
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * stay_here calls via stay_quit


 Calculate the availability for each market item and store it in AVL. This is
 called on arrival in a new system.


 Other entry points:

   stay_quit            Go to the docking bay (i.e. show the Status Mode
                        screen)


.GVL

 JSR DORND              \ Set A and X to random numbers

 STA QQ26               \ Set QQ26 to the random byte that's used in the market
                        \ calculations

 LDX #0                 \ We are now going to loop through the market item
 STX XX4                \ availability table in AVL, so set a counter in XX4
                        \ (and X) for the market item number, starting with 0

.hy9

 LDA QQ23+1,X           \ Fetch byte #1 from the market prices table (units and
 STA QQ19+1             \ economic_factor) for item number X and store it in
                        \ QQ19+1

 JSR var                \ Call var to set QQ19+3 = economy * |economic_factor|
                        \ (and set the availability of alien items to 0)

 LDA QQ23+3,X           \ Fetch byte #3 from the market prices table (mask) and
 AND QQ26               \ AND with the random number for this system visit
                        \ to give:
                        \
                        \   A = random AND mask

 CLC                    \ Add byte #2 from the market prices table
 ADC QQ23+2,X           \ (base_quantity) so we now have:
                        \
                        \   A = base_quantity + (random AND mask)

 LDY QQ19+1             \ Fetch the byte #1 that we stored above and jump to
 BMI TT157              \ TT157 if it is negative (i.e. if the economic_factor
                        \ is negative)

 SEC                    \ Set A = A - QQ19+3
 SBC QQ19+3             \
                        \       = base_quantity + (random AND mask)
                        \         - (economy * |economic_factor|)
                        \
                        \ which is the result we want, as the economic_factor
                        \ is positive

 JMP TT158              \ Jump to TT158 to skip TT157

.TT157

 CLC                    \ Set A = A + QQ19+3
 ADC QQ19+3             \
                        \       = base_quantity + (random AND mask)
                        \         + (economy * |economic_factor|)
                        \
                        \ which is the result we want, as the economic_factor
                        \ is negative

.TT158

 BPL TT159              \ If A < 0, then set A = 0, so we don't have negative
 LDA #0                 \ availability

.TT159

 LDY XX4                \ Fetch the counter (the market item number) into Y

 AND #%00111111         \ Take bits 0-5 of A, i.e. A mod 64, and store this as
 STA AVL,Y              \ this item's availability in the Y=th byte of AVL, so
                        \ each item has a maximum availability of 63t

 INY                    \ Increment the counter into XX44, Y and A
 TYA
 STA XX4

 ASL A                  \ Set X = counter * 4, so that X points to the next
 ASL A                  \ item's entry in the four-byte market prices table,
 TAX                    \ ready for the next loop

 CMP #63                \ If A < 63, jump back up to hy9 to set the availability
 BCC hy9                \ for the next market item

                        \ --- Mod: Code removed for Elite-A: ------------------>

\.hyR
\
\RTS                    \ Return from the subroutine

                        \ --- And replaced by: -------------------------------->

.stay_quit

 JMP BAY                \ Go to the docking bay (i.e. show the Status Mode
                        \ screen)

                        \ --- End of replacement ------------------------------>