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⌂ ∞ ▽ Elite on the 6502

Market: TT151

[Commodore 64 version]

       Name: TT151                                                   [Show more]
       Type: Subroutine
   Category: Market
    Summary: Print the name, price and availability of a market item
  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:
             * TT167 calls TT151
             * TT210 calls TT151
             * TT219 calls TT151




 Arguments:

   A                    The number of the market item to print, 0-16 (see QQ23
                        for details of item numbers)



 Returns:

   QQ19+1               Byte #1 from the market prices table for this item

   QQ24                 The item's price / 4

   QQ25                 The item's availability



.TT151q

                        ; We jump here from below if we are in witchspace

 PLA                    ; Restore the item number from the stack

 RTS                    ; Return from the subroutine

.TT151

 PHA                    ; Store the item number on the stack and in QQ19+4
 STA QQ19+4

 ASL A                  ; Store the item number * 4 in QQ19, so this will act as
 ASL A                  ; an index into the market prices table at QQ23 for this
 STA QQ19               ; item (as there are four bytes per item in the table)

 LDA MJ                 ; If we are in witchspace, we can't trade items, so jump
 BNE TT151q             ; up to TT151q to return from the subroutine

 LDA #1                 ; Move the text cursor to column 1, for the item's name
 JSR DOXC

 PLA                    ; Restore the item number

 ADC #208               ; Print recursive token 48 + A, which will be in the
 JSR TT27               ; range 48 ("FOOD") to 64 ("ALIEN ITEMS"), so this
                        ; prints the item's name

 LDA #14                ; Move the text cursor to column 14, for the price
 JSR DOXC

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

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

 CLC                    ; Add byte #0 from the market prices table (base_price),
 ADC QQ23,X             ; so we now have:
 STA QQ24               ;
                        ;   A = base_price + (random AND mask)

 JSR TT152              ; Call TT152 to print the item's unit ("t", "kg" or
                        ; "g"), padded to a width of two characters

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

 LDA QQ19+1             ; Fetch the byte #1 that we stored above and jump to
 BMI TT155              ; TT155 if it is negative (i.e. if the economic_factor
                        ; is negative)

 LDA QQ24               ; Set A = QQ24 + QQ19+3
 ADC QQ19+3             ;
                        ;       = base_price + (random AND mask)
                        ;         + (economy * |economic_factor|)
                        ;
                        ; which is the result we want, as the economic_factor
                        ; is positive

 JMP TT156              ; Jump to TT156 to multiply the result by 4

.TT155

 LDA QQ24               ; Set A = QQ24 - QQ19+3
 SEC                    ;
 SBC QQ19+3             ;       = base_price + (random AND mask)
                        ;         - (economy * |economic_factor|)
                        ;
                        ; which is the result we want, as economic_factor
                        ; is negative

.TT156

 STA QQ24               ; Store the result in QQ24 and P
 STA P

 LDA #0                 ; Set A = 0 and call GC2 to calculate (Y X) = (A P) * 4,
 JSR GC2                ; which is the same as (Y X) = P * 4 because A = 0

 SEC                    ; We now have our final price, * 10, so we can call pr5
 JSR pr5                ; to print (Y X) to 5 digits, including a decimal
                        ; point, as the C flag is set

 LDY QQ19+4             ; We now move on to availability, so fetch the market
                        ; item number that we stored in QQ19+4 at the start

 LDA #5                 ; Set A to 5 so we can print the availability to 5
                        ; digits (right-padded with spaces)

 LDX AVL,Y              ; Set X to the item's availability, which is given in
                        ; the AVL table

 STX QQ25               ; Store the availability in QQ25

 CLC                    ; Clear the C flag

 BEQ TT172              ; If none are available, jump to TT172 to print a tab
                        ; and a "-"

 JSR pr2+2              ; Otherwise print the 8-bit number in X to 5 digits,
                        ; right-aligned with spaces. This works because we set
                        ; A to 5 above, and we jump into the pr2 routine just
                        ; after the first instruction, which would normally
                        ; set the number of digits to 3

 JMP TT152              ; Print the unit ("t", "kg" or "g") for the market item,
                        ; with a following space if required to make it two
                        ; characters long, and return from the subroutine using
                        ; a tail call

.TT172

 LDA #25                ; Move the text cursor to column 25
 JSR DOXC

 LDA #'-'               ; Print a "-" character by jumping to TT162+2, which
 BNE TT162+2            ; contains JMP TT27 (this BNE is effectively a JMP as A
                        ; will never be zero), and return from the subroutine
                        ; using a tail call