Market: gnum

       Name: gnum                                                    [Show more]
       Type: Subroutine
   Category: Market
    Summary: Get a number from the keyboard
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * menu calls gnum


 Get a number from the keyboard, up to the maximum number in QQ25, for the
 buying and selling of cargo and equipment.

 Pressing "Y" will return the maximum number (i.e. buy/sell all items), while
 pressing "N" will abort the sale and return a 0.

 Pressing a key with an ASCII code less than ASCII "0" will return a 0 in A (so
 that includes pressing Space or Return), while pressing a key with an ASCII
 code greater than ASCII "9" will jump to the Inventory screen (so that
 includes all letters and most punctuation).


 Arguments:

   QQ25                 The maximum number allowed


 Returns:

   A                    The number entered

   R                    Also contains the number entered

   C flag               Set if the number is too large (> QQ25), clear otherwise


.gnum

 LDX #0                 \ We will build the number entered in R, so initialise
 STX R                  \ it with 0

 LDX #12                \ We will check for up to 12 key presses, so set a
 STX T1                 \ counter in T1

.TT223

 JSR TT217              \ Scan the keyboard until a key is pressed, and return
                        \ the key's ASCII code in A (and X)

 LDX R                  \ If R is non-zero then skip to NWDAV2, as we are
 BNE NWDAV2             \ already building a number

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

\ CMP #'y'              \ If "Y" was pressed, jump to NWDAV1 to return the
\ BEQ NWDAV1            \ maximum number allowed (i.e. buy/sell the whole stock)
\
\ CMP #'n'              \ If "N" was pressed, jump to NWDAV3 to return from the
\ BEQ NWDAV3            \ subroutine with a result of 0 (i.e. abort transaction)

                        \ --- End of removed code ----------------------------->

.NWDAV2

 STA Q                  \ Store the key pressed in Q

 SEC                    \ Subtract ASCII "0" from the key pressed, to leave the
 SBC #'0'               \ numeric value of the key in A (if it was a number key)

 BCC OUT                \ If A < 0, jump to OUT to load the current number and
                        \ return from the subroutine, as the key pressed was
                        \ RETURN (or some other ncharacter with a value less
                        \ than ASCII "0")

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

\ CMP #10               \ If A >= 10, jump to BAY2 to display the Inventory
\ BCS BAY2              \ screen, as the key pressed was a letter or other
\                       \ non-digit and is greater than ASCII "9"

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

 CMP #10                \ If A >= 10, jump to buy_invnt to decide which screen
 BCS buy_invnt          \ to display, as the key pressed was a letter or other
                        \ non-digit and is greater than ASCII "9" (so it could
                        \ be a red function key, for example)

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

 STA S                  \ Store the numeric value of the key pressed in S

 LDA R                  \ Fetch the result so far into A

 CMP #26                \ If A >= 26, where A is the number entered so far, then
 BCS OUT                \ adding a further digit will make it bigger than 256,
                        \ so jump to OUT to return from the subroutine with the
                        \ result in R (i.e. ignore the last key press)

 ASL A                  \ Set A = (A * 2) + (A * 8) = A * 10
 STA T
 ASL A
 ASL A
 ADC T

 ADC S                  \ Add the pressed digit to A and store in R, so R now
 STA R                  \ contains its previous value with the new key press
                        \ tacked onto the end

 CMP QQ25               \ If the result in R = the maximum allowed in QQ25, jump
 BEQ TT226              \ to TT226 to print the key press and keep looping (the
                        \ BEQ is needed because the BCS below would jump to OUT
                        \ if R >= QQ25, which we don't want)

 BCS OUT                \ If the result in R > QQ25, jump to OUT to return from
                        \ the subroutine with the result in R

.TT226

 LDA Q                  \ Print the character in Q (i.e. the key that was
 JSR TT26               \ pressed, as we stored the ASCII value in Q earlier)

 DEC T1                 \ Decrement the loop counter

 BNE TT223              \ Loop back to TT223 until we have checked for 12 digits

.OUT

 LDA R                  \ Set A to the result we have been building in R

 RTS                    \ Return from the subroutine

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

\.NWDAV1
\
\                       \ If we get here then "Y" was pressed, so we return the
\                       \ maximum number allowed, which is in QQ25
\
\ JSR TT26              \ Print the character for the key that was pressed
\
\ LDA QQ25              \ Set R = QQ25, so we return the maximum value allowed
\ STA R
\
\ RTS                   \ Return from the subroutine
\
\.NWDAV3
\
\                       \ If we get here then "N" was pressed, so we return 0
\
\ JSR TT26              \ Print the character for the key that was pressed
\
\ LDA #0                \ Set R = 0, so we return 0
\ STA R
\
\ RTS                   \ Return from the subroutine

                        \ --- End of removed code ----------------------------->