Version analysis of TT151 - Elite on the BBC Micro and NES

This code appears in the following versions (click to see it in the source code):

Cassette, Disc (flight), Disc (docked), 6502 Second Processor, Master and Electron

Code variations between these versions are shown below.

       Name: TT151
       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


 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

Code variation 1 of 5Related to an advanced feature

In the advanced versions, the Market Price screen doesn't list any prices when you're in witchspace, while the other versions still show the prices from the system you jumped from.

See below for more variations related to this code.

This variation is blank in the Cassette, Disc (flight), Disc (docked) and Electron versions.

6502SP, Master
.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)

Code variation 2 of 5Related to an advanced feature

See variation 1 above for details.

This variation is blank in the Cassette, Disc (flight), Disc (docked) and Electron versions.

6502SP, Master
 LDA MJ                 \ If we are in witchspace, we can't trade items, so jump
 BNE TT151q             \ up to TT151q to return from the subroutine

Code variation 3 of 5Related to Elite's use of the Tube

Tap on a block to expand it, and tap it again to revert.

Cassette, Flight, Docked, Electron
6502SP, Master
 LDA #1                 \ Move the text cursor to column 1, for the item's name
 STA XC
 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

Code variation 4 of 5Related to Elite's use of the Tube

Tap on a block to expand it, and tap it again to revert.

Cassette, Flight, Docked, Master, Electron
6502SP
 LDA #14                \ Move the text cursor to column 14, for the price
 STA XC
 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

Code variation 5 of 5Related to Elite's use of the Tube

Tap on a block to expand it, and tap it again to revert.

Cassette, Flight, Docked, Electron
6502SP, Master
 LDA XC                 \ Move the text cursor in XC to the right by 4 columns,
 ADC #4                 \ so the cursor is where the last digit would be if we
 STA XC                 \ were printing a 5-digit availability number
 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